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15.18: Misconceptions of Evolution - Biology

15.18: Misconceptions of Evolution - Biology


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Although the theory of evolution generated some controversy when it was first proposed, it was almost universally accepted by biologists, particularly younger biologists, within 20 years after publication of On theOrigin of Species. Nevertheless, the theory of evolution is a difficult concept and misconceptions about how it works abound.

This site addresses some of the main misconceptions associated with the theory of evolution.

Evolution Is Just a Theory

Critics of the theory of evolution dismiss its importance by purposefully confounding the everyday usage of the word “theory” with the way scientists use the word. In science, a “theory” is understood to be a body of thoroughly tested and verified explanations for a set of observations of the natural world. Scientists have a theory of the atom, a theory of gravity, and the theory of relativity, each of which describes understood facts about the world. In the same way, the theory of evolution describes facts about the living world. As such, a theory in science has survived significant efforts to discredit it by scientists. In contrast, a “theory” in common vernacular is a word meaning a guess or suggested explanation; this meaning is more akin to the scientific concept of “hypothesis.” When critics of evolution say evolution is “just a theory,” they are implying that there is little evidence supporting it and that it is still in the process of being rigorously tested. This is a mischaracterization.

Individuals Evolve

Evolution is the change in genetic composition of a population over time, specifically over generations, resulting from differential reproduction of individuals with certain alleles. Individuals do change over their lifetime, obviously, but this is called development and involves changes programmed by the set of genes the individual acquired at birth in coordination with the individual’s environment. When thinking about the evolution of a characteristic, it is probably best to think about the change of the average value of the characteristic in the population over time. For example, when natural selection leads to bill-size change in medium-ground finches in the Galápagos, this does not mean that individual bills on the finches are changing. If one measures the average bill size among all individuals in the population at one time and then measures the average bill size in the population several years later, this average value will be different as a result of evolution. Although some individuals may survive from the first time to the second, they will still have the same bill size; however, there will be many new individuals that contribute to the shift in average bill size.

Organisms Evolve on Purpose

Statements such as “organisms evolve in response to a change in an environment” are quite common, but such statements can lead to two types of misunderstandings. First, the statement must not be understood to mean that individual organisms evolve. The statement is shorthand for “a population evolves in response to a changing environment.” However, a second misunderstanding may arise by interpreting the statement to mean that the evolution is somehow intentional. A changed environment results in some individuals in the population, those with particular phenotypes, benefiting and therefore producing proportionately more offspring than other phenotypes. This results in change in the population if the characteristics are genetically determined.

It is also important to understand that the variation that natural selection works on is already in a population and does not arise in response to an environmental change. For example, applying antibiotics to a population of bacteria will, over time, select a population of bacteria that are resistant to antibiotics. The resistance, which is caused by a gene, did not arise by mutation because of the application of the antibiotic. The gene for resistance was already present in the gene pool of the bacteria, likely at a low frequency. The antibiotic, which kills the bacterial cells without the resistance gene, strongly selects individuals that are resistant, since these would be the only ones that survived and divided. Experiments have demonstrated that mutations for antibiotic resistance do not arise as a result of antibiotic.

In a larger sense, evolution is not goal directed. Species do not become “better” over time; they simply track their changing environment with adaptations that maximize their reproduction in a particular environment at a particular time. Evolution has no goal of making faster, bigger, more complex, or even smarter species, despite the commonness of this kind of language in popular discourse. What characteristics evolve in a species are a function of the variation present and the environment, both of which are constantly changing in a non-directional way. What trait is fit in one environment at one time may well be fatal at some point in the future. This holds equally well for a species of insect as it does the human species.

Evolution Explains the Origin of Life

It is a common misunderstanding that evolution includes an explanation of life’s origins. Conversely, some of the theory’s critics believe that it cannot explain the origin of life. The theory does not try to explain the origin of life. The theory of evolution explains how populations change over time and how life diversifies the origin of species. It does not shed light on the beginnings of life including the origins of the first cells, which is how life is defined. The mechanisms of the origin of life on Earth are a particularly difficult problem because it occurred a very long time ago, and presumably it just occurred once. Importantly, biologists believe that the presence of life on Earth precludes the possibility that the events that led to life on Earth can be repeated because the intermediate stages would immediately become food for existing living things.

However, once a mechanism of inheritance was in place in the form of a molecule like DNA either within a cell or pre-cell, these entities would be subject to the principle of natural selection. More effective reproducers would increase in frequency at the expense of inefficient reproducers. So while evolution does not explain the origin of life, it may have something to say about some of the processes operating once pre-living entities acquired certain properties.

Learning Objectives

Many misconceptions exist about the theory of evolution—including some perpetuated by critics of the theory. First, evolution as a scientific theory means that it has years of observation and accumulated data supporting it. It is not “just a theory” as a person may say in common vernacular.

Another misconception is that individuals evolve, though in fact it is populations that evolve over time. Individuals simply carry mutations. Furthermore, these mutations neither arise on purpose nor do they arise in response to an environmental pressure. Instead, mutations in DNA happen spontaneously and are already present in individuals of a population when a selective pressure occurs. Once the environment begins to favor a particular trait, then those individuals already carrying that mutation will have a selective advantage and are likely to survive better and outproduce others without the adaptation.

Finally, the theory of evolution does not in fact address the origins of life on this planet. Scientists believe that we cannot, in fact, repeat the circumstances that led to life on Earth because at this time life already exists. The presence of life has so dramatically changed the environment that the origins cannot be totally produced for study.


15.18: Misconceptions of Evolution - Biology

Five Major Misconceptions about Evolution
Copyright © 1995-1997 by Mark Isaak
[Last Update: October 1, 2003]

Other Links: A Creationist Rebuts this FAQ Creationist Tim Wallace has written a rebuttal of each of the points made in this FAQ. (Despite its pilfered masthead, Wallace's web page is not a part of the Talk.Origins Archive.) A Critique of Wallace Evolutionist Wayne Duck responds to Tim Wallace's rebuttal.

large part of the reason why Creationist arguments against evolution can sound so persuasive is because they don't address evolution, but rather argue against a set of misunderstandings that people are right to consider ludicrous. The Creationists wrongly believe that their understanding of evolution is what the theory of evolution really says, and declare evolution banished. In fact, they haven't even addressed the topic of evolution. (The situation isn't helped by poor science education generally. Even most beginning college biology students don't understand the theory of evolution.)

  • Evolution has never been observed.
  • Evolution violates the 2nd law of thermodynamics.
  • There are no transitional fossils.
  • The theory of evolution says that life originated, and evolution proceeds, by random chance.
  • Evolution is only a theory it hasn't been proved.

Biologists define evolution as a change in the gene pool of a population over time. One example is insects developing a resistance to pesticides over the period of a few years. Even most Creationists recognize that evolution at this level is a fact. What they don't appreciate is that this rate of evolution is all that is required to produce the diversity of all living things from a common ancestor.

The origin of new species by evolution has also been observed, both in the laboratory and in the wild. See, for example, (Weinberg, J.R., V.R. Starczak, and D. Jorg, 1992, "Evidence for rapid speciation following a founder event in the laboratory." Evolution 46: 1214-1220). Nature . 230:289-292). --> The "Observed Instances of Speciation" FAQ in the talk.origins archives gives several additional examples.

Even without these direct observations, it would be wrong to say that evolution hasn't been observed. Evidence isn't limited to seeing something happen before your eyes. Evolution makes predictions about what we would expect to see in the fossil record, comparative anatomy, genetic sequences, geographical distribution of species, etc., and these predictions have been verified many times over. The number of observations supporting evolution is overwhelming.

What hasn't been observed is one animal abruptly changing into a radically different one, such as a frog changing into a cow. This is not a problem for evolution because evolution doesn't propose occurrences even remotely like that. In fact, if we ever observed a frog turn into a cow, it would be very strong evidence against evolution.

This shows more a misconception about thermodynamics than about evolution. The second law of thermodynamics says, "No process is possible in which the sole result is the transfer of energy from a cooler to a hotter body." [Atkins, 1984, The Second Law , pg. 25] Now you may be scratching your head wondering what this has to do with evolution. The confusion arises when the 2nd law is phrased in another equivalent way, "The entropy of a closed system cannot decrease." Entropy is an indication of unusable energy and often (but not always!) corresponds to intuitive notions of disorder or randomness. Creationists thus misinterpret the 2nd law to say that things invariably progress from order to disorder.

However, they neglect the fact that life is not a closed system. The sun provides more than enough energy to drive things. If a mature tomato plant can have more usable energy than the seed it grew from, why should anyone expect that the next generation of tomatoes can't have more usable energy still? Creationists sometimes try to get around this by claiming that the information carried by living things lets them create order. However, not only is life irrelevant to the 2nd law, but order from disorder is common in nonliving systems, too. Snowflakes, sand dunes, tornadoes, stalactites, graded river beds, and lightning are just a few examples of order coming from disorder in nature none require an intelligent program to achieve that order. In any nontrivial system with lots of energy flowing through it, you are almost certain to find order arising somewhere in the system. If order from disorder is supposed to violate the 2nd law of thermodynamics, why is it ubiquitous in nature?

The thermodynamics argument against evolution displays a misconception about evolution as well as about thermodynamics, since a clear understanding of how evolution works should reveal major flaws in the argument. Evolution says that organisms reproduce with only small changes between generations (after their own kind, so to speak). For example, animals might have appendages which are longer or shorter, thicker or flatter, lighter or darker than their parents. Occasionally, a change might be on the order of having four or six fingers instead of five. Once the differences appear, the theory of evolution calls for differential reproductive success. For example, maybe the animals with longer appendages survive to have more offspring than short-appendaged ones. All of these processes can be observed today. They obviously don't violate any physical laws.

A transitional fossil is one that looks like it's from an organism intermediate between two lineages, meaning it has some characteristics of lineage A, some characteristics of lineage B, and probably some characteristics part way between the two. Transitional fossils can occur between groups of any taxonomic level, such as between species, between orders, etc. Ideally, the transitional fossil should be found stratigraphically between the first occurrence of the ancestral lineage and the first occurrence of the descendent lineage, but evolution also predicts the occurrence of some fossils with transitional morphology that occur after both lineages. There's nothing in the theory of evolution which says an intermediate form (or any organism, for that matter) can have only one line of descendents, or that the intermediate form itself has to go extinct when a line of descendents evolves.

To say there are no transitional fossils is simply false. Paleontology has progressed a bit since Origin of Species was published, uncovering thousands of transitional fossils, by both the temporally restrictive and the less restrictive definitions. The fossil record is still spotty and always will be erosion and the rarity of conditions favorable to fossilization make that inevitable. Also, transitions may occur in a small population, in a small area, and/or in a relatively short amount of time when any of these conditions hold, the chances of finding the transitional fossils goes down. Still, there are still many instances where excellent sequences of transitional fossils exist. Some notable examples are the transitions from reptile to mammal, from land animal to early whale, and from early ape to human. For many more examples, see the transitional fossils FAQ in the talk.origins archive, and see http://www.geo.ucalgary.ca/

macrae/talk_origins.html for sample images for some invertebrate groups.

The misconception about the lack of transitional fossils is perpetuated in part by a common way of thinking about categories. When people think about a category like "dog" or "ant," they often subconsciously believe that there is a well-defined boundary around the category, or that there is some eternal ideal form (for philosophers, the Platonic idea) which defines the category. This kind of thinking leads people to declare that Archaeopteryx is "100% bird," when it is clearly a mix of bird and reptile features (with more reptile than bird features, in fact). In truth, categories are man-made and artificial. Nature is not constrained to follow them, and it doesn't.

Some Creationists claim that the hypothesis of punctuated equilibrium was proposed (by Eldredge and Gould) to explain gaps in the fossil record. Actually, it was proposed to explain the relative rarity of transitional forms, not their total absence, and to explain why speciation appears to happen relatively quickly in some cases, gradually in others, and not at all during some periods for some species. In no way does it deny that transitional sequences exist. In fact, both Gould and Eldredge are outspoken opponents of Creationism.

There is probably no other statement which is a better indication that the arguer doesn't understand evolution. Chance certainly plays a large part in evolution, but this argument completely ignores the fundamental role of natural selection, and selection is the very opposite of chance. Chance, in the form of mutations, provides genetic variation, which is the raw material that natural selection has to work with. From there, natural selection sorts out certain variations. Those variations which give greater reproductive success to their possessors (and chance ensures that such beneficial mutations will be inevitable) are retained, and less successful variations are weeded out. When the environment changes, or when organisms move to a different environment, different variations are selected, leading eventually to different species. Harmful mutations usually die out quickly, so they don't interfere with the process of beneficial mutations accumulating.

Nor is abiogenesis (the origin of the first life) due purely to chance. Atoms and molecules arrange themselves not purely randomly, but according to their chemical properties. In the case of carbon atoms especially, this means complex molecules are sure to form spontaneously, and these complex molecules can influence each other to create even more complex molecules. Once a molecule forms that is approximately self-replicating, natural selection will guide the formation of ever more efficient replicators. The first self-replicating object didn't need to be as complex as a modern cell or even a strand of DNA. Some self-replicating molecules are not really all that complex (as organic molecules go).

Some people still argue that it is wildly improbable for a given self-replicating molecule to form at a given point (although they usually don't state the "givens," but leave them implicit in their calculations). This is true, but there were oceans of molecules working on the problem, and no one knows how many possible self-replicating molecules could have served as the first one. A calculation of the odds of abiogenesis is worthless unless it recognizes the immense range of starting materials that the first replicator might have formed from, the probably innumerable different forms that the first replicator might have taken, and the fact that much of the construction of the replicating molecule would have been non-random to start with.

(One should also note that the theory of evolution doesn't depend on how the first life began. The truth or falsity of any theory of abiogenesis wouldn't affect evolution in the least.)

First, we should clarify what "evolution" means. Like so many other words, it has more than one meaning. Its strict biological definition is "a change in allele frequencies over time." By that definition, evolution is an indisputable fact. Most people seem to associate the word "evolution" mainly with common descent, the theory that all life arose from one common ancestor. Many people believe that there is enough evidence to call this a fact, too. However, common descent is still not the theory of evolution, but just a fraction of it (and a part of several quite different theories as well). The theory of evolution not only says that life evolved, it also includes mechanisms, like mutations, natural selection, and genetic drift, which go a long way towards explaining how life evolved.

Calling the theory of evolution "only a theory" is, strictly speaking, true, but the idea it tries to convey is completely wrong. The argument rests on a confusion between what "theory" means in informal usage and in a scientific context. A theory, in the scientific sense, is "a coherent group of general propositions used as principles of explanation for a class of phenomena" [Random House American College Dictionary]. The term does not imply tentativeness or lack of certainty. Generally speaking, scientific theories differ from scientific laws only in that laws can be expressed more tersely. Being a theory implies self-consistency, agreement with observations, and usefulness. (Creationism fails to be a theory mainly because of the last point it makes few or no specific claims about what we would expect to find, so it can't be used for anything. When it does make falsifiable predictions, they prove to be false.)

Lack of proof isn't a weakness, either. On the contrary, claiming infallibility for one's conclusions is a sign of hubris. Nothing in the real world has ever been rigorously proved, or ever will be. Proof, in the mathematical sense, is possible only if you have the luxury of defining the universe you're operating in. In the real world, we must deal with levels of certainty based on observed evidence. The more and better evidence we have for something, the more certainty we assign to it when there is enough evidence, we label the something a fact, even though it still isn't 100% certain.

What evolution has is what any good scientific claim has--evidence, and lots of it. Evolution is supported by a wide range of observations throughout the fields of genetics, anatomy, ecology, animal behavior, paleontology, and others. If you wish to challenge the theory of evolution, you must address that evidence. You must show that the evidence is either wrong or irrelevant or that it fits another theory better. Of course, to do this, you must know both the theory and the evidence.

These are not the only misconceptions about evolution by any means. Other common misunderstandings include how geological dating techniques work, implications to morality and religion, the meaning of "uniformitarianism," and many more. To address all these objections here would be impossible.

But consider: About a hundred years ago, scientists, who were then mostly creationists, looked at the world to figure out how God did things. These creationists came to the conclusions of an old earth and species originating by evolution. Since then, thousands of scientists have been studying evolution with increasingly more sophisticated tools. Many of these scientists have excellent understandings of the laws of thermodynamics, how fossil finds are interpreted, etc., and finding a better alternative to evolution would win them fame and fortune. Sometimes their work has changed our understanding of significant details of how evolution operates, but the theory of evolution still has essentially unanimous agreement from the people who work on it.


Survival of the "Fittest"

Most likely, most of the misconceptions about natural selection come from this single phrase that has become synonymous with it. "Survival of the fittest" is how most people with only a superficial understanding of the process would describe it. While technically, this is a correct statement, the common definition of "fittest" is what seems to create the most problems for understanding the true nature of natural selection.

Although Charles Darwin did use this phrase in a revised edition of his book On the Origin of Species, it was not intended to create confusion. In Darwin's writings, he intended for the word "fittest" to mean those who were most suited to their immediate environment. However, in the modern use of language, "fittest" often means strongest or in best physical condition. This is not necessarily how it works in the natural world when describing natural selection. In fact, the "fittest" individual may actually be much weaker or smaller than others in the population. If the environment favored smaller and weaker individuals, then they would be considered more fit than their stronger and larger counterparts.


Why biology students have misconceptions about science

/>January 10, 2013 - Cognitive psychology Professor John Coley worked with Novartis pharmaceutical researchers to determine how chemists make decisions on what molecules to move forward in drug development

Zebras developed stripes to avoid predators.

No, that statement wasn’t ripped from the annals of Who Wants to Be a Millionaire? It’s an example of a “misconception”—a term biology-education researchers use to describe a scientifically inaccurate idea held by biology students, even majors in the field.

In fact, new research by Northeastern associate professor John Coley and his team has found that both biology and non-biology majors are equally prone to agreeing with common scientific misconceptions. The difference is that biology majors give more systematic reasons for why they agree or disagree with the inaccurate ideas presented to them—a finding that points to the way they are taught science.

The findings, published earlier this year in CBE-Life Sciences Education, could change the way instructors teach science—and improve how students learn it.

Misconceptions come from intuitive thinking
In the study, Coley and his team surveyed Northeastern University students, both biology majors and non-biology majors, about whether or not they agreed with several scientific ideas—which unbeknownst to the students were inaccurate. Their study yielded some startling results, namely that biology majors agreed with common scientific misconceptions nearly as frequently as non-biology majors. But interestingly, biology majors were much more systematic in their reasoning for agreeing or disagreeing with these ideas—which the researchers say indicates that biology education itself is reinforcing these intuitive ways of thinking.

“A misconception is not just a factual error,” says Coley, a psychologist in the College of Science who studies cognition. “It’s a belief that, while contrary to how scientists understand a phenomenon, arises from our intuitive ways of organizing knowledge.”

A study co-authored by Northeastern associate professor John Coley could change the way instructors teach science—and improve how students learn it. Photo via Istock

From evolution to cell biology, biology and non-biology majors agreed nearly to the same degree, differed on reasons
To dive deeply into the minds of biology students, Coley teamed up with Kimberly Tanner, a neurobiologist at San Francisco State University trained in science-education research. The study, which represents a breakthrough in interdisciplinary research, examines the thought processes driving students’ misconceptions across biological disciplines, from evolution to ecology to cell biology.

The authors hypothesized that seemingly unrelated biological misconceptions—about cellular respiration, say, or plant nutrition—sprang not from the complexity of the material but from our intuitive ways of understanding the world. They posited three types of intuitive thinking: cause-effect driven (“zebras developed stripes for protection”), conflating internal properties with external features (“different cells have different DNA”), and imbuing nonhuman species with human characteristics (“plants get food from the soil”).

To test their hypothesis, they asked 137 Northeastern undergraduates—69 biology majors with AP biology credit and 68 non-majors with non-science AP credit, to show comparable accomplishment—to indicate their level of agreement with six biological misconceptions, each linked to a type of intuitive thinking. They also asked the students to write down their reasoning.

The results were astonishing. The difference between how frequently both biology and non-biology majors agreed with misconceptions was “surprisingly small,” says Coley, with 93 percent of biology majors and 98 percent of non-majors agreeing with at least one misconception. And both groups employed varied types of intuitive thinking. Remarkable—“amazing to me!” exclaims Tanner—was the tight correlation only among the biology majors between the type of reasoning they employed (say, cause-effect driven) and the type of misconception they agreed with (“zebras developed stripes to avoid predators”).

The non-biology majors were “kind of promiscuous,” notes Tanner, while the biology majors were far more systematic. “That suggests that biology education itself—the way students learn the subject—is reinforcing these intuitive ways of thinking and, potentially, reinforcing the misconceptions as well.”

A mis­con­cep­tion is not just a fac­tual error. It’s a belief that, while con­trary to how sci­en­tists under­stand a phe­nom­enon, arises from our intu­itive ways of orga­nizing knowledge.

These are not isolated misunderstandings
Next, Coley and Tanner will look at students as they advance through their biological studies and at how biology teachers present information in the classroom. “Our work shows that these are not isolated misunderstandings, which is how they’ve been viewed,” says Coley, “but rather that there are systems of misconceptions—all generated from underlying intuitive ways of thinking.”

One way to counteract those systems, says Coley, would be to make students “explicitly aware,” in the first week of an introductory class, of basic principles of cognitive science. “Intuitive ways of thinking are deeply embedded in our cognitive systems, and they’re useful in everyday contexts,” says Coley. “But they are not appropriate for explaining scientific phenomena.

“We need to help students think hard about how cognition works, not just in terms of how we memorize material, but in terms of how we organize knowledge in different domains.”

So about those zebras
Thinking that zebras got stripes to dodge predators, Coley says, is an example of a misconception arising from a particular type of intuitive thinking: Our minds automatically attribute cause and effect to phenomena or events, even when there might be none.

But evolution doesn’t involve “forward thinking,” or intention—ancestral zebras didn’t sprout stripes to blend in with their surroundings. Rather, given a population of zebra-like animals varying in stripedness, those with abundant verticals had a selective advantage over their plainer relatives: Hence, they were more successful at reproducing, and over time, the stripes prevailed.


Evolution: 24 myths and misconceptions

It will soon be 200 years since the birth of Charles Darwin and 150 years since the publication of On the Origin of Species, arguably the most important book ever written. In it, Darwin outlined an idea that many still find shocking – that all life on Earth, including human life, evolved through natural selection.

Darwin presented compelling evidence for evolution in On the Origin and, since his time, the case has become overwhelming. Countless fossil discoveries allow us to trace the evolution of today’s organisms from earlier forms. DNA sequencing has confirmed beyond any doubt that all living creatures share a common origin. Innumerable examples of evolution in action can be seen all around us, from the pollution-matching peppered moth to fast-changing viruses such as HIV and H5N1 bird flu. Evolution is as firmly established a scientific fact as the roundness of the Earth.

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And yet despite an ever-growing mountain of evidence, most people around the world are not taught the truth about evolution, if they are taught about it at all. Even in the UK, the birthplace of Darwin with an educated and increasingly secular population, one recent poll suggests less than half the population accepts evolution.

For those who have never had the opportunity to find out about biology or science, claims made by those who believe in supernatural alternatives to evolutionary theory can appear convincing. Meanwhile, even among those who accept evolution, misconceptions abound.

Most of us are happy to admit that we do not understand, say, string theory in physics, yet we are all convinced we understand evolution. In fact, as biologists are discovering, its consequences can be stranger than we ever imagined. Evolution must be the best-known yet worst-understood of all scientific theories.

So here is New Scientist’s guide to some of the most common myths and misconceptions about evolution.

There are already several good and comprehensive guides out there. But there can’t be too many.


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NOVA’s Evolution Lab Worksheets and Interactive Lessons
Explore the evidence of evolution with these worksheets based off of NOVA’s Evolution Lab. These worksheets contain questions for each level and video of the Evolution Lab and provide assessment on topics like cladograms, fossil evidence of evolution, DNA and genetics, biogeography, applications of phylogeny to health, and human evolution. The worksheets are divided into missions and are designed to be completed while playing through the game. The worksheets feature multiple choice questions, short response questions, and cladogram drawings.

The interactive lessons are digital versions of the worksheets and contain the same content from the worksheets.

NCSEteach
The National Center for Science Education is the only national organization devoted to defending the teaching of evolution in public schools. NCSE’s Evolution Primers are written by NCSE’s scientific staff to explain key concepts or findings in evolution that are frequently misrepresented by creationists. NCSEteach is a network that brings science teachers together, allows educators to connect with one another (and NCSE staff), guides them to good-quality and well-vetted resources, shares stories of how teachers have dealt with challenges to science education and also connects them to early career scientists as a resource.

Misconception Monday
Stephanie Keep’s blog posts on the NCSE website cover common misconceptions about evolution that appear everywhere from textbooks to science news articles.

Understanding Evolution
Understanding Evolution is your one-stop shop for teaching and learning about evolution from kindergarten through college. Get friendly, clear background information as well as animations, comics, interactive investigations, news briefs, research profiles, and a database of free, vetted lessons for your classroom.

HHMI BioInteractive
The evolution collection contains short films, interactives, and classroom activities that cover mechanisms of evolution, human evolution, phylogeny, and more.

Teaching Evolution through Human Examples
This NSF-funded project from the Smithsonian contains 4 curriculum units for AP Biology classes that use human case studies to teach core evolutionary principles. The curriculum units include Adaptation to Altitude, Malaria, Evolution of Human Skin Color, What Does It Mean To Be Human, and a unit called Cultural and Religious Sensitivity.

Shape of Life
Shape of Life is a series of short classroom videos that depict evolution of the animal kingdom on Earth. Shape of Life focuses on biodiversity, adaptability, body structure, design, behaviors, and the innovative scientists who explore these creatures. Shape of Life includes videos, lesson plans, readings aligned with the Common Core, illustrations, and relevant resources.

Birds-of-Paradise Project
The birds-of-paradise are among the most beautiful creatures on earth—and an extraordinary example of evolutionary adaptation. On this site you can find what few have witnessed in the wild: the displays of color, sound, and motion that make these birds so remarkable. Then you can delve deeper, examining the principles that guided their evolution and the epic adventure it took to bring us all 39 species. There are also free lesson plans that explore the topics of the scientific process, natural and sexual selection, behavior, and heritability through hands-on activities and lively discussions: http://www.birdsleuth.org/paradise/.

All About Fancy Males
All About Fancy Males is an eight section online interactive developed to accompany one of the most respected introductory evolution courses in the country—Cornell University’s Evolution and Biology and Diversity. This interactive allows students and the general public to develop a solid understanding of fundamental concepts in evolution while exploring rare behavioral clips and engaging animations.

Have questions about these resources or suggestions for additional resources we should add to the collection? Let us know in the comments!


18.1 Understanding Evolution

By the end of this section, you will be able to do the following:

  • Describe how scientists developed the present-day theory of evolution
  • Define adaptation
  • Explain convergent and divergent evolution
  • Describe homologous and vestigial structures
  • Discuss misconceptions about the theory of evolution

Evolution by natural selection describes a mechanism for how species change over time. Scientists, philosophers, researchers, and others had made suggestions and debated this topic well before Darwin began to explore this idea. Classical Greek philosopher Plato emphasized in his writings that species were static and unchanging, yet there were also ancient Greeks who expressed evolutionary ideas. In the eighteenth century, naturalist Georges-Louis Leclerc Comte de Buffon reintroduced ideas about the evolution of animals and observed that various geographic regions have different plant and animal populations, even when the environments are similar. Some at this time also accepted that there were extinct species.

Also during the eighteenth century, James Hutton, a Scottish geologist and naturalist, proposed that geological change occurred gradually by accumulating small changes from processes operating like they are today over long periods of time. This contrasted with the predominant view that the planet's geology was a consequence of catastrophic events occurring during a relatively brief past. Nineteenth century geologist Charles Lyell popularized Hutton's view. A friend to Darwin. Lyell’s ideas were influential on Darwin’s thinking: Lyell’s notion of the greater age of Earth gave more time for gradual change in species, and the process of change provided an analogy for this change. In the early nineteenth century, Jean-Baptiste Lamarck published a book that detailed a mechanism for evolutionary change. We now refer to this mechanism as an inheritance of acquired characteristics by which the environment causes modifications in an individual, or offspring could use or disuse of a structure during its lifetime, and thus bring about change in a species. While many discredited this mechanism for evolutionary change, Lamarck’s ideas were an important influence on evolutionary thought.

Charles Darwin and Natural Selection

In the mid-nineteenth century, two naturalists, Charles Darwin and Alfred Russel Wallace, independently conceived and described the actual mechanism for evolution. Importantly, each naturalist spent time exploring the natural world on expeditions to the tropics. From 1831 to 1836, Darwin traveled around the world on H.M.S. Beagle, including stops in South America, Australia, and the southern tip of Africa. Wallace traveled to Brazil to collect insects in the Amazon rainforest from 1848 to 1852 and to the Malay Archipelago from 1854 to 1862. Darwin’s journey, like Wallace’s later journeys to the Malay Archipelago, included stops at several island chains, the last being the Galápagos Islands west of Ecuador. On these islands, Darwin observed species of organisms on different islands that were clearly similar, yet had distinct differences. For example, the ground finches inhabiting the Galápagos Islands comprised several species with a unique beak shape (Figure 18.2). The species on the islands had a graded series of beak sizes and shapes with very small differences between the most similar. He observed that these finches closely resembled another finch species on the South American mainland. Darwin imagined that the island species might be species modified from one of the original mainland species. Upon further study, he realized that each finch's varied beaks helped the birds acquire a specific type of food. For example, seed-eating finches had stronger, thicker beaks for breaking seeds, and insect-eating finches had spear-like beaks for stabbing their prey.

Wallace and Darwin both observed similar patterns in other organisms and they independently developed the same explanation for how and why such changes could take place. Darwin called this mechanism natural selection. Natural selection , or “survival of the fittest,” is the more prolific reproduction of individuals with favorable traits that survive environmental change because of those traits. This leads to evolutionary change.

For example, Darwin observed a population of giant tortoises in the Galápagos Archipelago to have longer necks than those that lived on other islands with dry lowlands. These tortoises were “selected” because they could reach more leaves and access more food than those with short necks. In times of drought when fewer leaves would be available, those that could reach more leaves had a better chance to eat and survive than those that couldn’t reach the food source. Consequently, long-necked tortoises would be more likely to be reproductively successful and pass the long-necked trait to their offspring. Over time, only long-necked tortoises would be present in the population.

Natural selection, Darwin argued, was an inevitable outcome of three principles that operated in nature. First, most characteristics of organisms are inherited, or passed from parent to offspring. Although no one, including Darwin and Wallace, knew how this happened at the time, it was a common understanding. Second, more offspring are produced than are able to survive, so resources for survival and reproduction are limited. The capacity for reproduction in all organisms outstrips the availability of resources to support their numbers. Thus, there is competition for those resources in each generation. Both Darwin and Wallace’s understanding of this principle came from reading economist Thomas Malthus' essay that explained this principle in relation to human populations. Third, offspring vary among each other in regard to their characteristics and those variations are inherited. Darwin and Wallace reasoned that offspring with inherited characteristics which allow them to best compete for limited resources will survive and have more offspring than those individuals with variations that are less able to compete. Because characteristics are inherited, these traits will be better represented in the next generation. This will lead to change in populations over generations in a process that Darwin called descent with modification. Ultimately, natural selection leads to greater adaptation of the population to its local environment. It is the only mechanism known for adaptive evolution.

In 1858, Darwin and Wallace (Figure 18.3) presented papers at the Linnean Society in London that discussed the idea of natural selection. The following year Darwin’s book, On the Origin of Species, was published. His book outlined in considerable detail his arguments for evolution by natural selection.

It is difficult and time-consuming to document and present examples of evolution by natural selection. The Galápagos finches are an excellent example. Peter and Rosemary Grant and their colleagues have studied Galápagos finch populations every year since 1976 and have provided important evidence of natural selection. The Grants found changes from one generation to the next in beak shape distribution with the medium ground finch on the Galápagos island of Daphne Major. The birds have inherited a variation in their bill shape with some having wide deep bills and others having thinner bills. During a period in which rainfall was higher than normal because of an El Niño, there was a lack of large hard seeds of which the large-billed birds ate however, there was an abundance of the small soft seeds which the small-billed birds ate. Therefore, the small-billed birds were able to survive and reproduce. In the years following this El Niño, the Grants measured beak sizes in the population and found that the average bill size was smaller. Since bill size is an inherited trait, parents with smaller bills had more offspring and the bill evolved into a much smaller size. As conditions improved in 1987 and larger seeds became more available, the trend toward smaller average bill size ceased.

Career Connection

Field Biologist

Many people hike, explore caves, scuba dive, or climb mountains for recreation. People often participate in these activities hoping to see wildlife. Experiencing the outdoors can be incredibly enjoyable and invigorating. What if your job entailed working in the wilderness? Field biologists by definition work outdoors in the “field.” The term field in this case refers to any location outdoors, even under water. A field biologist typically focuses research on a certain species, group of organisms, or a single habitat (Figure 18.4).

One objective of many field biologists includes discovering new, unrecorded species. Not only do such findings expand our understanding of the natural world, but they also lead to important innovations in fields such as medicine and agriculture. Plant and microbial species, in particular, can reveal new medicinal and nutritive knowledge. Other organisms can play key roles in ecosystems or if rare require protection. When discovered, researchers can use these important species as evidence for environmental regulations and laws.

Processes and Patterns of Evolution

Natural selection can only take place if there is variation , or differences, among individuals in a population. Importantly, these differences must have some genetic basis otherwise, the selection will not lead to change in the next generation. This is critical because nongenetic reasons can cause variation among individuals such as an individual's height because of better nutrition rather than different genes.

Genetic diversity in a population comes from two main mechanisms: mutation and sexual reproduction. Mutation, a change in DNA, is the ultimate source of new alleles, or new genetic variation in any population. The genetic changes that mutation causes can have one of three outcomes on the phenotype. A mutation affects the organism's phenotype in a way that gives it reduced fitness—lower likelihood of survival or fewer offspring. A mutation may produce a phenotype with a beneficial effect on fitness. Many mutations will also have no effect on the phenotype's fitness. We call these neutral mutations. Mutations may also have a whole range of effect sizes on the organism's fitness that expresses them in their phenotype, from a small effect to a great effect. Sexual reproduction also leads to genetic diversity: when two parents reproduce, unique combinations of alleles assemble to produce the unique genotypes and thus phenotypes in each offspring.

We call a heritable trait that helps an organism's survival and reproduction in its present environment an adaptation . Scientists describe groups of organisms adapting to their environment when a genetic variation occurs over time that increases or maintains the population's “fit” to its environment. A platypus's webbed feet are an adaptation for swimming. A snow leopard's thick fur is an adaptation for living in the cold. A cheetah's fast speed is an adaptation for catching prey.

Whether or not a trait is favorable depends on the current environmental conditions. The same traits are not always selected because environmental conditions can change. For example, consider a plant species that grew in a moist climate and did not need to conserve water. Large leaves were selected because they allowed the plant to obtain more energy from the sun. Large leaves require more water to maintain than small leaves, and the moist environment provided favorable conditions to support large leaves. After thousands of years, the climate changed, and the area no longer had excess water. The direction of natural selection shifted so that plants with small leaves were selected because those populations were able to conserve water to survive the new environmental conditions.

The evolution of species has resulted in enormous variation in form and function. Sometimes, evolution gives rise to groups of organisms that become tremendously different from each other. We call two species that evolve in diverse directions from a common point divergent evolution . We can see such divergent evolution in the forms of the reproductive organs of flowering plants which share the same basic anatomies however, they can look very different as a result of selection in different physical environments and adaptation to different kinds of pollinators (Figure 18.5).

In other cases, similar phenotypes evolve independently in distantly related species. For example, flight has evolved in both bats and insects, and they both have structures we refer to as wings, which are adaptations to flight. However, bat and insect wings have evolved from very different original structures. We call this phenomenon convergent evolution , where similar traits evolve independently in species that do not share a common ancestry. The two species came to the same function, flying, but did so separately from each other.

These physical changes occur over enormous time spans and help explain how evolution occurs. Natural selection acts on individual organisms, which can then shape an entire species. Although natural selection may work in a single generation on an individual, it can take thousands or even millions of years for an entire species' genotype to evolve. It is over these large time spans that life on earth has changed and continues to change.

Evidence of Evolution

The evidence for evolution is compelling and extensive. Looking at every level of organization in living systems, biologists see the signature of past and present evolution. Darwin dedicated a large portion of his book, On the Origin of Species, to identifying patterns in nature that were consistent with evolution, and since Darwin, our understanding has become clearer and broader.

Fossils

Fossils provide solid evidence that organisms from the past are not the same as those today, and fossils show a progression of evolution. Scientists determine the age of fossils and categorize them from all over the world to determine when the organisms lived relative to each other. The resulting fossil record tells the story of the past and shows the evolution of form over millions of years (Figure 18.6). For example, scientists have recovered highly detailed records showing the evolution of humans and horses (Figure 18.6). The whale flipper shares a similar morphology to bird and mammal appendages (Figure 18.7) indicating that these species share a common ancestor.

Anatomy and Embryology

Another type of evidence for evolution is the presence of structures in organisms that share the same basic form. For example, the bones in human, dog, bird, and whale appendages all share the same overall construction (Figure 18.7) resulting from their origin in a common ancestor's appendages. Over time, evolution led to changes in the bones' shapes and sizes different species, but they have maintained the same overall layout. Scientists call these synonymous parts homologous structures .

Some structures exist in organisms that have no apparent function at all, and appear to be residual parts from a past common ancestor. We call these unused structures without function vestigial structures . Other examples of vestigial structures are wings on flightless birds, leaves on some cacti, and hind leg bones in whales. Not all similarities represent homologous structures. As explained in Determining Evolutionary Relationships, when similar characteristics occur because of environmental constraints and not due to a close evolutionary relationship, it is an analogy or homoplasy. For example, insects use wings to fly like bats and birds, but the wing structure and embryonic origin are completely different. These are analogous structures (Figure 20.8).

Link to Learning

Watch this video exploring the bones in the human body.

Another evidence of evolution is the convergence of form in organisms that share similar environments. For example, species of unrelated animals, such as the arctic fox and ptarmigan, living in the arctic region have been selected for seasonal white phenotypes during winter to blend with the snow and ice (Figure 18.8). These similarities occur not because of common ancestry, but because of similar selection pressures—the benefits of predators not seeing them.

Embryology, the study of the anatomy of an organism's development to its adult form, also provides evidence of relatedness between now widely divergent groups of organisms. Mutational tweaking in the embryo can have such magnified consequences in the adult that tends to conserve embryo formation. As a result, structures that are absent in some groups often appear in their embryonic forms and disappear when they reach the adult or juvenile form. For example, all vertebrate embryos, including humans, exhibit gill slits and tails at some point in their early development. These disappear in the adults of terrestrial groups but adult forms of aquatic groups such as fish and some amphibians maintain them. Great ape embryos, including humans, have a tail structure during their development that they lose when they are born.

Biogeography

The geographic distribution of organisms on the planet follows patterns that we can explain best by evolution in conjunction with tectonic plate movement over geological time. Broad groups that evolved before the supercontinent Pangaea broke up (about 200 million years ago) are distributed worldwide. Groups that evolved since the breakup appear uniquely in regions of the planet, such as the unique flora and fauna of northern continents that formed from the supercontinent Laurasia and of the southern continents that formed from the supercontinent Gondwana. The presence of members of the plant family Proteaceae in Australia, southern Africa, and South America was most predominant prior to the southern supercontinent Gondwana breaking up.

Marsupial diversification in Australia and the absence of other mammals reflect Australia’s long isolation. Australia has an abundance of endemic species—species found nowhere else—which is typical of islands whose isolation by expanses of water prevents species to migrate. Over time, these species diverge evolutionarily into new species that look very different from their ancestors that may exist on the mainland. Australia's marsupials, the Galápagos' finches, and many species on the Hawaiian Islands are all unique to their one point of origin, yet they display distant relationships to ancestral species on mainlands.

Molecular Biology

Like anatomical structures, the molecular structures of life reflect descent with modification. DNA's universality reflects evidence of a common ancestor for all of life. Fundamental divisions in life between the genetic code, DNA replication, and expression are reflected in major structural differences in otherwise conservative structures such as ribosome components and membrane structures. In general, the relatedness of groups of organisms is reflected in the similarity of their DNA sequences—exactly the pattern that we would expect from descent and diversification from a common ancestor.

DNA sequences have also shed light on some of the mechanisms of evolution. For example, it is clear that the evolution of new functions for proteins commonly occurs after gene duplication events that allow freely modifying one copy by mutation, selection, or drift (changes in a population’s gene pool resulting from chance), while the second copy continues to produce a functional protein.

Misconceptions of Evolution

Although the theory of evolution generated some controversy when Darwin first proposed it, biologists almost universally accepted it, particularly younger biologists, within 20 years after publication of On the Origin of Species. Nevertheless, the theory of evolution is a difficult concept and misconceptions about how it works abound.

Link to Learning

This site addresses some of the main misconceptions associated with the theory of evolution.

Evolution Is Just a Theory

Critics of the theory of evolution dismiss its importance by purposefully confounding the everyday usage of the word “theory” with the way scientists use the word. In science, we understand a “theory” to be a body of thoroughly tested and verified explanations for a set of observations of the natural world. Scientists have a theory of the atom, a theory of gravity, and the theory of relativity, each which describes understood facts about the world. In the same way, the theory of evolution describes facts about the living world. As such, a theory in science has survived significant efforts to discredit it by scientists. In contrast, a “theory” in common vernacular is a word meaning a guess or suggested explanation. This meaning is more akin to the scientific concept of “hypothesis.” When critics of evolution say it is “just a theory,” they are implying that there is little evidence supporting it and that it is still in the process of rigorous testing. This is a mischaracterization.

Individuals Evolve

Evolution is the change in a population's genetic composition over time, specifically over generations, resulting from differential reproduction of individuals with certain alleles. Individuals do change over their lifetime, obviously, but this is development and involves changes programmed by the set of genes the individual acquired at birth in coordination with the individual’s environment. When thinking about the evolution of a characteristic, it is probably best to think about the change of the average value of the characteristic in the population over time. For example, when natural selection leads to bill-size change in medium ground finches in the Galápagos, this does not mean that individual bills on the finches are changing. If one measures the average bill size among all individuals in the population at one time and then measures them in the population several years later, this average value will be different as a result of evolution. Although some individuals may survive from the first time to the second, they will still have the same bill size however, there will be many new individuals who contribute to the shift in average bill size.

Evolution Explains the Origin of Life

It is a common misunderstanding that evolution includes an explanation of life’s origins. Conversely, some of the theory’s critics believe that it cannot explain the origin of life. The theory does not try to explain the origin of life. The theory of evolution explains how populations change over time and how life diversifies the origin of species. It does not shed light on the beginnings of life including the origins of the first cells, which define life. Importantly, biologists believe that the presence of life on Earth precludes the possibility that the events that led to life on Earth can repeat themselves because the intermediate stages would immediately become food for existing living things.

However, once a mechanism of inheritance was in place in the form of a molecule like DNA either within a cell or pre-cell, these entities would be subject to the principle of natural selection. More effective reproducers would increase in frequency at the expense of inefficient reproducers. While evolution does not explain the origin of life, it may have something to say about some of the processes operating once pre-living entities acquired certain properties.

Organisms Evolve on Purpose

Statements such as “organisms evolve in response to a change in an environment” are quite common, but such statements can lead to two types of misunderstandings. First, do not interpret the statement to mean that individual organisms evolve. The statement is shorthand for “a population evolves in response to a changing environment.” However, a second misunderstanding may arise by interpreting the statement to mean that the evolution is somehow intentional. A changed environment results in some individuals in the population, those with particular phenotypes, benefiting and therefore producing proportionately more offspring than other phenotypes. This results in change in the population if the characteristics are genetically determined.

It is also important to understand that the variation that natural selection works on is already in a population and does not arise in response to an environmental change. For example, applying antibiotics to a population of bacteria will, over time, select a population of bacteria that are resistant to antibiotics. The resistance, which a gene causes, did not arise by mutation because of applying the antibiotic. The gene for resistance was already present in the bacteria's gene pool, likely at a low frequency. The antibiotic, which kills the bacterial cells without the resistance gene, strongly selects individuals that are resistant, since these would be the only ones that survived and divided. Experiments have demonstrated that mutations for antibiotic resistance do not arise as a result of antibiotic.

In a larger sense, evolution is not goal directed. Species do not become “better” over time. They simply track their changing environment with adaptations that maximize their reproduction in a particular environment at a particular time. Evolution has no goal of making faster, bigger, more complex, or even smarter species, despite the commonness of this kind of language in popular discourse. What characteristics evolve in a species are a function of the variation present and the environment, both of which are constantly changing in a nondirectional way. A trait that fits in one environment at one time may well be fatal at some point in the future. This holds equally well for insect and human species.


A Catalog of Common Misconceptions

Whereas the causes of cognitive barriers to understanding remain to be determined, their consequences are well documented. It is clear from many studies that complex but accurate explanations of biological adaptation typically yield to naïve intuitions based on common experience (Fig. 2 Tables 2 and 3). As a result, each of the fundamental components of natural selection may be overlooked or misunderstood when it comes time to consider them in combination, even if individually they appear relatively straightforward. The following sections provide an overview of the various, non-mutually exclusive, and often correlated misconceptions that have been found to be most common. All readers are encouraged to consider these conceptual pitfalls carefully in order that they may be avoided. Teachers, in particular, are urged to familiarize themselves with these errors so that they may identify and address them among their students.

Teleology and the “Function Compunction”

Much of the human experience involves overcoming obstacles, achieving goals, and fulfilling needs. Not surprisingly, human psychology includes a powerful bias toward thoughts about the “purpose” or “function” of objects and behaviors—what Kelemen and Rosset (2009) dub the “human function compunction.” This bias is particularly strong in children, who are apt to see most of the world in terms of purpose for example, even suggesting that “rocks are pointy to keep animals from sitting on them” (Kelemen 1999a, b Kelemen and Rosset 2009). This tendency toward explanations based on purpose (“teleology”) runs very deep and persists throughout high school (Southerland et al. 2001) and even into postsecondary education (Kelemen and Rosset 2009). In fact, it has been argued that the default mode of teleological thinking is, at best, suppressed rather than supplanted by introductory scientific education. It therefore reappears easily even in those with some basic scientific training for example, in descriptions of ecological balance (“fungi grow in forests to help decomposition”) or species survival (“finches diversified in order to survive” Kelemen and Rosset 2009).

Teleological explanations for biological features date back to Aristotle and remain very common in naïve interpretations of adaptation (e.g., Tamir and Zohar 1991 Pedersen and Halldén 1992 Southerland et al. 2001 Sinatra et al. 2008 Table 2). On the one hand, teleological reasoning may preclude any consideration of mechanisms altogether if simply identifying a current function for an organ or behavior is taken as sufficient to explain its existence (e.g., Bishop and Anderson 1990). On the other hand, when mechanisms are considered by teleologically oriented thinkers, they are often framed in terms of change occurring in response to a particular need (Table 2). Obviously, this contrasts starkly with a two-step process involving undirected mutations followed by natural selection (see Fig. 2 and Table 3).

Anthropomorphism and Intentionality

A related conceptual bias to teleology is anthropomorphism, in which human-like conscious intent is ascribed either to the objects of natural selection or to the process itself (see below). In this sense, anthropomorphic misconceptions can be characterized as either internal (attributing adaptive change to the intentional actions of organisms) or external (conceiving of natural selection or “Nature” as a conscious agent e.g., Kampourakis and Zogza 2008 Sinatra et al. 2008).

Internal anthropomorphism or “intentionality” is intimately tied to the misconception that individual organisms evolve in response to challenges imposed by the environment (rather than recognizing evolution as a population-level process). Gould (1980) described the obvious appeal of such intuitive notions as follows:

Since the living world is a product of evolution, why not suppose that it arose in the simplest and most direct way? Why not argue that organisms improve themselves by their own efforts and pass these advantages to their offspring in the form of altered genes—a process that has long been called, in technical parlance, the “inheritance of acquired characters.” This idea appeals to common sense not only for its simplicity but perhaps even more for its happy implication that evolution travels an inherently progressive path, propelled by the hard work of organisms themselves.

The penchant for seeing conscious intent is often sufficiently strong that it is applied not only to non-human vertebrates (in which consciousness, though certainly not knowledge of genetics and Darwinian fitness, may actually occur), but also to plants and even to single-celled organisms. Thus, adaptations in any taxon may be described as “innovations,” “inventions,” or “solutions” (sometimes “ingenious” ones, no less). Even the evolution of antibiotic resistance is characterized as a process whereby bacteria “learn” to “outsmart” antibiotics with frustrating regularity. Anthropomorphism with an emphasis on forethought is also behind the common misconception that organisms behave as they do in order to enhance the long-term well-being of their species. Once again, a consideration of the actual mechanics of natural selection should reveal why this is fallacious.

All too often, an anthropomorphic view of evolution is reinforced with sloppy descriptions by trusted authorities (Jungwirth 1975a, b, 1977 Moore et al. 2002). Consider this particularly egregious example from a website maintained by the National Institutes of Health Footnote 10 :

As microbes evolve, they adapt to their environment. If something stops them from growing and spreading—such as an antimicrobial—they evolve new mechanisms to resist the antimicrobials by changing their genetic structure. Changing the genetic structure ensures that the offspring of the resistant microbes are also resistant.

Fundamentally inaccurate descriptions such as this are alarmingly common. As a corrective, it is a useful exercise to translate such faulty characterizations into accurate language Footnote 11 . For example, this could read:

Bacteria that cause disease exist in large populations, and not all individuals are alike. If some individuals happen to possess genetic features that make them resistant to antibiotics, these individuals will survive the treatment while the rest gradually are killed off. As a result of their greater survival, the resistant individuals will leave more offspring than susceptible individuals, such that the proportion of resistant individuals will increase each time a new generation is produced. When only the descendants of the resistant individuals are left, the population of bacteria can be said to have evolved resistance to the antibiotics.

Use and Disuse

Many students who manage to avoid teleological and anthropomorphic pitfalls nonetheless conceive of evolution as involving change due to use or disuse of organs. This view, which was developed explicitly by Jean-Baptiste Lamarck but was also invoked to an extent by Darwin (1859), emphasizes changes to individual organisms that occur as they use particular features more or less. For example, Darwin (1859) invoked natural selection to explain the loss of sight in some subterranean rodents, but instead favored disuse alone as the explanation for loss of eyes in blind, cave-dwelling animals: “As it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness, I attribute their loss wholly to disuse.” This sort of intuition remains common in naïve explanations for why unnecessary organs become vestigial or eventually disappear. Modern evolutionary theory recognizes several reasons that may account for the loss of complex features (e.g., Jeffery 2005 Espinasa and Espinasa 2008), some of which involve direct natural selection, but none of which is based simply on disuse.

Soft Inheritance

Evolution involving changes in individual organisms, whether based on conscious choice or use and disuse, would require that characteristics acquired during the lifetime of an individual be passed on to offspring Footnote 12 , a process often termed “soft inheritance.” The notion that acquired traits can be transmitted to offspring remained a common assumption among thinkers for more than 2,000 years, including into Darwin's time (Zirkle 1946). As is now understood, inheritance is actually “hard,” meaning that physical changes that occur during an organism's lifetime are not passed to offspring. This is because the cells that are involved in reproduction (the germline) are distinct from those that make up the rest of the body (the somatic line) only changes that affect the germline can be passed on. New genetic variants arise through mutation and recombination during replication and will often only exert their effects in offspring and not in the parents in whose reproductive cells they occur (though they could also arise very early in development and appear later in the adult offspring). Correct and incorrect interpretations of inheritance are contrasted in Fig. 3.

A summary of correct (left) and incorrect (right) conceptions of heredity as it pertains to adaptive evolutionary change. The panels on the left display the operation of “hard inheritance”, whereas those on the right illustrate naïve mechanisms of “soft inheritance”. In all diagrams, a set of nine squares represents an individual multicellular organism and each square represents a type of cell of which the organisms are constructed. In the left panels, the organisms include two kinds of cells: those that produce gametes (the germline, black) and those that make up the rest of the body (the somatic line, white). In the top left panel, all cells in a parent organism initially contain a gene that specifies white coloration marked W (A). A random mutation occurs in the germline, changing the gene from one that specifies white to one that specifies gray marked G (B). This mutant gene is passed to the egg (C), which then develops into an offspring exhibiting gray coloration (D). The mutation in this case occurred in the parent (specifically, in the germline) but its effects did not become apparent until the next generation. In the bottom left panel, a parent once again begins with white coloration and the white gene in all of its cells (H). During its lifetime, the parent comes to acquire a gray coloration due to exposure to particular environmental conditions (I). However, because this does not involve any change to the genes in the germline, the original white gene is passed into the egg (J), and the offspring exhibits none of the gray coloration that was acquired by its parent (K). In the top right panel, the distinction between germline and somatic line is not understood. In this case, a parent that initially exhibits white coloration (P) changes during its lifetime to become gray (Q). Under incorrect views of soft inheritance, this altered coloration is passed on to the egg (R), and the offspring is born with the gray color acquired by its parent (S). In the bottom right panel, a more sophisticated but still incorrect view of inheritance is shown. Here, traits are understood to be specified by genes, but no distinction is recognized between the germline and somatic line. In this situation, a parent begins with white coloration and white-specifying genes in all its cells (W). A mutation occurs in one type of body cells to change those cells to gray (X). A mixture of white and gray genes is passed on to the egg (Y), and the offspring develops white coloration in most cells but gray coloration in the cells where gray-inducing mutations arose in the parent (Z). Intuitive ideas regarding soft inheritance underlie many misconceptions of how adaptive evolution takes place (see Fig. 2)

Studies have indicated that belief in soft inheritance arises early in youth as part of a naïve model of heredity (e.g., Deadman and Kelly 1978 Kargbo et al. 1980 Lawson and Thompson 1988 Wood-Robinson 1994). That it seems intuitive probably explains why the idea of soft inheritance persisted so long among prominent thinkers and why it is so resistant to correction among modern students. Unfortunately, a failure to abandon this belief is fundamentally incompatible with an appreciation of evolution by natural selection as a two-step process in which the origin of new variation and its relevance to survival in a particular environment are independent considerations.

Nature as a Selecting Agent

Thirty years ago, widely respected broadcaster Sir David Attenborough (1979) aptly described the challenge of avoiding anthropomorphic shorthand in descriptions of adaptation:

Darwin demonstrated that the driving force of [adaptive] evolution comes from the accumulation, over countless generations, of chance genetical changes sifted by the rigors of natural selection. In describing the consequences of this process it is only too easy to use a form of words that suggests that the animals themselves were striving to bring about change in a purposeful way–that fish wanted to climb onto dry land, and to modify their fins into legs, that reptiles wished to fly, strove to change their scales into feathers and so ultimately became birds.

Unlike many authors, Attenborough (1979) admirably endeavored to not use such misleading terminology. However, this quote inadvertently highlights an additional challenge in describing natural selection without loaded language. In it, natural selection is described as a “driving force” that rigorously “sifts” genetic variation, which could be misunderstood to imply that it takes an active role in prompting evolutionary change. Much more seriously, one often encounters descriptions of natural selection as a processes that “chooses” among “preferred” variants or “experiments with” or “explores” different options. Some expressions, such as “favored” and “selected for” are used commonly as shorthand in evolutionary biology and are not meant to impart consciousness to natural selection however, these too may be misinterpreted in the vernacular sense by non-experts and must be clarified.

Darwin (1859) himself could not resist slipping into the language of agency at times:

It may be said that natural selection is daily and hourly scrutinizing, throughout the world, every variation, even the slightest rejecting that which is bad, preserving and adding up all that is good silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages, and then so imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they formerly were.

Perhaps recognizing the ease with which such language can be misconstrued, Darwin (1868) later wrote that “The term ‘Natural Selection’ is in some respects a bad one, as it seems to imply conscious choice but this will be disregarded after a little familiarity.” Unfortunately, more than “a little familiarity” seems necessary to abandon the notion of Nature as an active decision maker.

Being, as it is, the simple outcome of differences in reproductive success due to heritable traits, natural selection cannot have plans, goals, or intentions, nor can it cause changes in response to need. For this reason, Jungwirth (1975a, b, 1977) bemoaned the tendency for authors and instructors to invoke teleological and anthropomorphic descriptions of the process and argued that this served to reinforce misconceptions among students (see also Bishop and Anderson 1990 Alters and Nelson 2002 Moore et al. 2002 Sinatra et al. 2008). That said, a study of high school students by Tamir and Zohar (1991) suggested that older students can recognize the distinction between an anthropomorphic or teleological formulation (i.e., merely a convenient description) versus an anthropomorphic/teleological explanation (i.e., involving conscious intent or goal-oriented mechanisms as causal factors see also Bartov 1978, 1981). Moore et al. (2002), by contrast, concluded from their study of undergraduates that “students fail to distinguish between the relatively concrete register of genetics and the more figurative language of the specialist shorthand needed to condense the long view of evolutionary processes” (see also Jungwirth 1975a, 1977). Some authors have argued that teleological wording can have some value as shorthand for describing complex phenomena in a simple way precisely because it corresponds to normal thinking patterns, and that contrasting this explicitly with accurate language can be a useful exercise during instruction (Zohar and Ginossar 1998). In any case, biologists and instructors should be cognizant of the risk that linguistic shortcuts may send students off track.

Source Versus Sorting of Variation

Intuitive models of evolution based on soft inheritance are one-step models of adaptation: Traits are modified in one generation and appear in their altered form in the next. This is in conflict with the actual two-step process of adaptation involving the independent processes of mutation and natural selection. Unfortunately, many students who eschew soft inheritance nevertheless fail to distinguish natural selection from the origin of new variation (e.g., Greene 1990 Creedy 1993 Moore et al. 2002). Whereas an accurate understanding recognizes that most new mutations are neutral or harmful in a given environment, such naïve interpretations assume that mutations occur as a response to environmental challenges and therefore are always beneficial (Fig. 2). For example, many students may believe that exposure to antibiotics directly causes bacteria to become resistant, rather than simply changing the relative frequencies of resistant versus non-resistant individuals by killing off the latter Footnote 13 . Again, natural selection itself does not create new variation, it merely influences the proportion of existing variants. Most forms of selection reduce the amount of genetic variation within populations, which may be counteracted by the continual emergence of new variation via undirected mutation and recombination.

Typological, Essentialist, and Transformationist Thinking

Misunderstandings about how variation arises are problematic, but a common failure to recognize that it plays a role at all represents an even a deeper concern. Since Darwin (1859), evolutionary theory has been based strongly on “population” thinking that emphasizes differences among individuals. By contrast, many naïve interpretations of evolution remain rooted in the “typological” or “essentialist” thinking that has existed since the ancient Greeks (Mayr 1982, 2001 Sinatra et al. 2008). In this case, species are conceived of as exhibiting a single “type” or a common “essence,” with variation among individuals representing anomalous and largely unimportant deviations from the type or essence. As Shtulman (2006) notes, “human beings tend to essentialize biological kinds and essentialism is incompatible with natural selection.” As with many other conceptual biases, the tendency to essentialize seems to arise early in childhood and remains the default for most individuals (Strevens 2000 Gelman 2004 Evans et al. 2005 Shtulman 2006).

The incorrect belief that species are uniform leads to “transformationist” views of adaptation in which an entire population transforms as a whole as it adapts (Alters 2005 Shtulman 2006 Bardapurkar 2008). This contrasts with the correct, “variational” understanding of natural selection in which it is the proportion of traits within populations that changes (Fig. 2). Not surprisingly, transformationist models of adaptation usually include a tacit assumption of soft inheritance and one-step change in response to challenges. Indeed, Shtulman (2006) found that transformationists appeal to “need” as a cause of evolutionary change three times more often than do variationists.

Events and Absolutes Versus Processes and Probabilities

A proper understanding of natural selection recognizes it as a process that occurs within populations over the course of many generations. It does so through cumulative, statistical effects on the proportion of traits differing in their consequences for reproductive success. This contrasts with two major errors that are commonly incorporated into naïve conceptions of the process:

Natural selection is mistakenly seen as an event rather than as a process (Ferrari and Chi 1998 Sinatra et al. 2008). Events generally have a beginning and end, occur in a specific sequential order, consist of distinct actions, and may be goal-oriented. By contrast, natural selection actually occurs continually and simultaneously within entire populations and is not goal-oriented (Ferrari and Chi 1998). Misconstruing selection as an event may contribute to transformationist thinking as adaptive changes are thought to occur in the entire population simultaneously. Viewing natural selection as a single event can also lead to incorrect “saltationist” assumptions in which complex adaptive features are imagined to appear suddenly in a single generation (see Gregory 2008b for an overview of the evolution of complex organs).

Natural selection is incorrectly conceived as being “all or nothing,” with all unfit individuals dying and all fit individuals surviving. In actuality, it is a probabilistic process in which some traits make it more likely—but do not guarantee—that organisms possessing them will successfully reproduce. Moreover, the statistical nature of the process is such that even a small difference in reproductive success (say, 1%) is enough to produce a gradual increase in the frequency of a trait over many generations.


Why biology students have misconceptions about science

Northeastern associate professor and cognitive scientist John Coley has helped unlock why misconceptions persist in science education—research that could change the way instructors teach science and improve how students learn it. Credit: Brooks Canaday/Northeastern University

Zebras developed stripes to avoid predators.

No, that statement wasn't ripped from the annals of "Who Wants to Be a Millionaire?" It's an example of a "misconception"—a term biology-education researchers use to describe a scientifically inaccurate idea held by biology students, even majors in the field.

In fact, new research by Northeastern associate professor John Coley and his team has found that both biology and non-biology majors are equally prone to agreeing with common scientific misconceptions. The difference is that biology majors give more systematic reasons for why they agree or disagree with the inaccurate ideas presented to them—a finding that points to the way they are taught science.

The findings, published earlier this year in CBE-Life Sciences Education, could change the way instructors teach science—and improve how students learn it.

Misconceptions come from intuitive thinking

In the study, Coley and his team surveyed Northeastern University students, both biology majors and non-biology majors, about whether or not they agreed with several scientific ideas—which unbeknownst to the students were inaccurate. Their study yielded some startling results, namely that biology majors agreed with common scientific misconceptions nearly as frequently as non-biology majors. But interestingly, biology majors were much more systematic in their reasoning for agreeing or disagreeing with these ideas—which the researchers say indicates that biology education itself is reinforcing these intuitive ways of thinking.

"A misconception is not just a factual error," says Coley, a psychologist in the College of Science who studies cognition. "It's a belief that, while contrary to how scientists understand a phenomenon, arises from our intuitive ways of organizing knowledge."

From evolution to cell biology, biology and non-biology majors agreed nearly to the same degree, differed on reasons

To dive deeply into the minds of biology students, Coley teamed up with Kimberly Tanner, a neurobiologist at San Francisco State University trained in science-education research. The study, which represents a breakthrough in interdisciplinary research, examines the thought processes driving students' misconceptions across biological disciplines, from evolution to ecology to cell biology.

The authors hypothesized that seemingly unrelated biological misconceptions—about cellular respiration, say, or plant nutrition—sprang not from the complexity of the material but from our intuitive ways of understanding the world. They posited three types of intuitive thinking: cause-effect driven ("zebras developed stripes for protection"), conflating internal properties with external features ("different cells have different DNA"), and imbuing nonhuman species with human characteristics ("plants get food from the soil").

To test their hypothesis, they asked 137 Northeastern undergraduates—69 biology majors with AP biology credit and 68 non-majors with non-science AP credit, to show comparable accomplishment—to indicate their level of agreement with six biological misconceptions, each linked to a type of intuitive thinking. They also asked the students to write down their reasoning.

The results were astonishing. The difference between how frequently both biology and non-biology majors agreed with misconceptions was "surprisingly small," says Coley, with 93 percent of biology majors and 98 percent of non-majors agreeing with at least one misconception. And both groups employed varied types of intuitive thinking. Remarkable—"amazing to me!" exclaims Tanner—was the tight correlation only among the biology majors between the type of reasoning they employed (say, cause-effect driven) and the type of misconception they agreed with ("zebras developed stripes to avoid predators").

The non-biology majors were "kind of promiscuous," notes Tanner, while the biology majors were far more systematic. "That suggests that biology education itself—the way students learn the subject—is reinforcing these intuitive ways of thinking and, potentially, reinforcing the misconceptions as well."

These are not isolated misunderstandings

Next, Coley and Tanner will look at students as they advance through their biological studies and at how biology teachers present information in the classroom. "Our work shows that these are not isolated misunderstandings, which is how they've been viewed," says Coley, "but rather that there are systems of misconceptions—all generated from underlying intuitive ways of thinking."

One way to counteract those systems, says Coley, would be to make students "explicitly aware," in the first week of an introductory class, of basic principles of cognitive science. "Intuitive ways of thinking are deeply embedded in our cognitive systems, and they're useful in everyday contexts," says Coley. "But they are not appropriate for explaining scientific phenomena.

"We need to help students think hard about how cognition works, not just in terms of how we memorize material, but in terms of how we organize knowledge in different domains."

So about those zebras

Thinking that zebras got stripes to dodge predators, Coley says, is an example of a misconception arising from a particular type of intuitive thinking: Our minds automatically attribute cause and effect to phenomena or events, even when there might be none.

But evolution doesn't involve "forward thinking," or intention—ancestral zebras didn't sprout stripes to blend in with their surroundings. Rather, given a population of zebra-like animals varying in stripedness, those with abundant verticals had a selective advantage over their plainer relatives: Hence, they were more successful at reproducing, and over time, the stripes prevailed.


Teaching About Evolution and the Nature of Science (1998)

Why is it so important to teach evolution? After all, many questions in biology can be answered without mentioning evolution: How do birds fly? How can certain plants grow in the desert? Why do children resemble their parents? Each of these questions has an immediate answer involving aerodynamics, the storage and use of water by plants, or the mechanisms of heredity. Students ask about such things all the time.

The answers to these questions often raise deeper questions that are sometimes asked by students: How did things come to be that way? What is the advantage to birds of flying? How did desert plants come to differ from others? How did an individual organism come to have its particular genetic endowment? Answering questions like these requires a historical context&mdasha framework of understanding that recognizes change through time.

People who study nature closely have always asked these kinds of questions. Over time, two observations have proved to be especially perplexing. The older of these has to do with the diversity of life: Why are there so many different kinds of plants and animals? The more we explore the world, the more impressed we are with the multiplicity of kinds of organisms. In the mid-nineteenth century, when Charles Darwin was writing On the Origin of Species, naturalists recognized several tens of thousands of different plant and animal species. By the middle of the twentieth century, biologists had paid more attention to less conspicuous forms of life, from insects to microorganisms, and the estimate was up to 1 or 2 million. Since then, investigations in tropical rain forests&mdashthe center of much of the world's biological diversity&mdashhave multiplied those estimates at least tenfold. What process has created this extraordinary variety of life?

The second question involves the inverse of life's diversity. How can the similarities among organisms be explained? Humans have always noticed the similarities among closely related species, but it gradually became apparent that even distantly related species share many anatomical and functional characteristics. The bones in a whale's front flippers are arranged in much the same way as the bones in our own arms. As organisms grow from fertilized egg cells into embryos, they pass through many similar developmental stages. Furthermore, as paleontologists studied the fossil record, they discovered countless extinct species that are clearly related in various ways to organisms living today.

This question has emerged with even greater force as modern experimental biology has focused on processes at the cellular and molecular level. From bacteria to yeast to mice to humans, all living things use the same biochemical machinery to carry out the basic processes of life. Many of the proteins that make up cells and catalyze chemical reactions in the body are virtually identical across species. Certain human genes that code for proteins differ little from the corresponding genes in fruit flies,

Investigations of forest ecosystems have helped reveal the incredible diversity of earth's living things.

mice, and primates. All living things use the same biochemical system to pass genetic information from one generation to another.

From a scientific standpoint, there is one compelling answer to questions about life's commonalities. Different kinds of organisms share so many characteristics of structure and function because they are related to one another. But how?

Solving the Puzzle

The concept of biological evolution addresses both of these fundamental questions. It accounts for the relatedness among organisms by explaining that the millions of different species of plants, animals, and microorganisms that live on earth today are related by descent from common ancestors&mdashlike distant cousins. Organisms in nature typically produce more offspring than can survive and reproduce given the constraints of food, space, and other resources in the environment. These offspring often differ from one another in ways that are heritable&mdashthat is, they can pass on the differences genetically to their own offspring. If competing offspring have traits that are advantageous in a given environment, they will survive and pass on those traits. As differences continue to accumulate over generations, populations of organisms diverge from their ancestors.

This straightforward process, which is a natural consequence of biologically reproducing organisms competing for limited resources, is responsible for one of the most magnificent chronicles known to science. Over billions of years, it has led the earliest organisms on earth to diversify into all of the plants, animals, and microorganisms that exist today. Though humans, fish, and bacteria would seem to be so different as to defy comparison, they all share some of the characteristics of their common ancestors.

Evolution also explains the great diversity of modern species. Populations of organisms

Living fish and fossil fish share many similarities, but the fossil fish clearly belongs to a different species that no longer exists. The progression of species found in the fossil record provides powerful evidence for evolution.

with characteristics enabling them to occupy ecological niches not occupied by similar organisms have a greater chance of surviving. Over time&mdashas the next chapter discusses in more detail&mdashspecies have diversified and have occupied more and more ecological niches to take advantage of new resources.

Evolution explains something else as well. During the billions of years that life has been on earth, it has played an increasingly important role in altering the planet's physical environment. For example, the composition of our atmosphere is partly a consequence of living systems. During photosynthesis, which is a product of evolution, green plants absorb carbon dioxide and water, produce organic compounds, and release oxygen. This process has created and continues to maintain an atmosphere rich in oxygen. Living communities also profoundly affect weather and the movement of water among the oceans, atmosphere, and land. Much of the rainfall in the forests of the western Amazon basin consists of water that has already made one or more recent trips through a living plant. In addition, plants and soil microorganisms exert important controls over global temperature by absorbing or emitting ''greenhouse gases" (such as carbon dioxide and methane) that increase the earth's capacity to retain heat.

In short, biological evolution accounts for three of the most fundamental features of the world around us: the similarities among living things, the diversity of life, and many features of the physical world we inhabit. Explanations of these phenomena in terms of evolution draw on results from physics, chemistry, geology, many areas of biology, and other sciences. Thus, evolution is the central organizing principle that biologists use to understand the world. To teach biology without explaining evolution deprives students of a powerful concept that brings great order and coherence to our understanding of life.

The teaching of evolution also has great practical value for students. Directly or indirectly, evolutionary biology has made many contributions to society. Evolution explains why many human pathogens have been developing resistance to formerly effective drugs and suggests ways of confronting this increasingly serious problem (this issue is discussed in greater detail in Chapter 2). Evolutionary biology has also

Living things have altered the earth's oceans, land surfaces, and atmosphere. For example, photosynthetic organisms are responsible for the oxygen that makes up about a fifth of the earth's atmosphere. The rapid accumulation of atmospheric oxygen about 2 billion years ago led to the evolution of more structured eucaryotic cells, which in turn gave rise to multicellular plants and animals.

contributed to many important agricultural advances by explaining the relationships among wild and domesticated plants and animals and their natural enemies. An understanding of evolution has been essential in finding and using natural resources, such as fossil fuels, and it will be indispensable as human societies strive to establish sustainable relationships with the natural environment.

Such examples can be multiplied many times. Evolutionary research is one of the most active fields of biology today, and discoveries with important practical applications occur on a regular basis.

Those who oppose the teaching of evolution in public schools sometimes ask that teachers present "the evidence against evolution." However, there is no debate within the scientific community over whether evolution occurred, and there is no evidence that evolution has not occurred. Some of the details of how evolution occurs are still being investigated. But scientists continue to debate only the particular mechanisms that result in evolution, not the overall accuracy of evolution as the explanation of life's history.

Evolution and the Nature of Science

Teaching about evolution has another important function. Because some people see evolution as conflicting with widely held beliefs, the teaching of evolution offers educators a superb opportunity to illuminate the nature of science and to differentiate science from other forms of human endeavor and understanding.

Chapter 3 describes the nature of science in detail. However, it is important from the outset to understand how the meanings of certain key words in science differ from the way that those words are used in everyday life.

Think, for example, of how people usually use the word "theory." Someone might refer to an idea and then add, "But that's only a theory." Or someone might preface a remark by saying, "My theory is &hellip." In common usage, theory often means "guess" or ''hunch."

In science, the word "theory" means something quite different. It refers to an overarching explanation that has been well substantiated. Science has many other powerful theories besides evolution. Cell theory says that all living things are composed of

cells. The heliocentric theory says that the earth revolves around the sun rather than vice versa. Such concepts are supported by such abundant observational and experimental evidence that they are no longer questioned in science.

Sometimes scientists themselves use the word "theory" loosely and apply it to tentative explanations that lack well-established evidence. But it is important to distinguish these casual uses of the word "theory" with its use to describe concepts such as evolution that are supported by overwhelming evidence. Scientists might wish that they had a word other than "theory" to apply to such enduring explanations of the natural world, but the term is too deeply engrained in science to be discarded.

As with all scientific knowledge, a theory can be refined or even replaced by an alternative theory in light of new and compelling evidence. For example, Chapter 3 describes how the geocentric theory that the sun revolves around the earth was replaced by the heliocentric theory of the earth's rotation on its axis and revolution around the sun. However, ideas are not referred to as "theories" in science unless they are supported by bodies of evidence that make their subsequent abandonment very unlikely. When a theory is supported by as much evidence as evolution, it is held with a very high degree of confidence.

In science, the word "hypothesis" conveys the tentativeness inherent in the common use of the word "theory." A hypothesis is a testable statement about the natural world. Through experiment and observation, hypotheses can be supported or rejected. As the earliest level of understanding, hypotheses can be used to construct more complex inferences and explanations.

Like "theory," the word "fact" has a different meaning in science than it does in common usage. A scientific fact is an observation that has been confirmed over and over. However, observations are gathered by our senses, which can never be trusted entirely. Observations also can change with better technologies or with better ways of looking at data. For example, it was held as a scientific fact for many years that human cells have 24 pairs of chromosomes, until improved techniques of microscopy revealed that they actually have 23. Ironically, facts in science often are more susceptible to change than theories&mdashwhich is one reason why the word "fact" is not much used in science.

Finally, "laws" in science are typically descriptions of how the physical world behaves under certain circumstances. For example, the laws of motion describe how objects move when subjected to certain forces. These laws can be very useful in supporting hypotheses and theories, but like all elements of science they can be altered with new information and observations.

Glossary of Terms Used in Teaching About the Nature of Science

Fact: In science, an observation that has been repeatedly confirmed.

Law: A descriptive generalization about how some aspect of the natural world behaves under stated circumstances.

Hypothesis: A testable statement about the natural world that can be used to build more complex inferences and explanations.

Theory: In science, a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypot


Watch the video: Ch. 21 Evolution misconceptions (October 2022).