How can we decide at which level does natural selection apply?

How can we decide at which level does natural selection apply?

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Here is a passage from The Selfish Gene

If selection goes on between groups within species, and between species, why should it not also go on between larger groupings? Species are grouped into genera, genera into orders and orders into classes. Lions and antelopes are both members of class Mammalia, as are we. Should we then not expect lions to refrain from killing antelopes, 'for the good of the mammals'? Surely they should hunt birds or reptiles instead, in order to prevent the extinction of the class. But then, what of the need to perpetuate the whole phylum of vertebrates?

So shouldn't it eat the other animals? Also in the next line, it says about vertebrates that since reptiles and birds are vertebrates, the lion should go for invertebrates.

It sounds illogical, but what stops lion from eating his "relatives", or from sparing a deer? And till what extent is the term "relatives" valid?

Is the question is too broad, please give a source where I can get an answer.

Natural selection favors genotypes that produce phenotypes that survive to reproduce. To the extent that natural selection drives the instincts that drive predation, the (essentially tautological) answer is that, "It has proven selectively beneficial for that particular genotype to eat whatever it's eating."

Indeed, nature has shown us a variety of solutions to these selective pressures. Cannibalism is not hard to find. In the extreme case, mothers from diverse species are known to kill and eat their own offspring, which at first glance may seem like the height of failure when it comes to propagating one's genes. And yet we can hypothesize a number of selective advantages of that behavior. (E.g., the mother can produce large litters when food is plentiful, and then adjust for shortages to shore up a smaller litter when food is scarce.)

We also know that there are selective pressures against eating similar animals. For example, pathogens that kill an animal are likely to afflict its genetic relatives, and so that could drive the aversion of some carnivores (including humans) to eating close relatives.

There are plenty of examples of evolved symbiotic organisms. Alligators don't eat plovers, and sharks don't eat remoras.

But it is hard to imagine selective pressure for altruistic behaviors that don't benefit a particular genotype. E.g., if there are mammals to be eaten, the lions that eat them will survive and the ones that don't will die. If the entire class Mammalia is under pressure, population dynamics trump evolved behavior: If prey become scarce, predators will become scarce. But if any predators survive, it will be the greedy ones.

Group selection

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Group selection, in biology, a type of natural selection that acts collectively on all members of a given group. Group selection may also be defined as selection in which traits evolve according to the fitness (survival and reproductive success) of groups or, mathematically, as selection in which overall group fitness is higher or lower than the mean of the individual members’ fitness values. Typically the group under selection is a small cohesive social unit, and members’ interactions are of an altruistic nature. Examples of behaviours that appear to influence group selection include cooperative hunting, such as among lions and other social carnivores cooperative raising of young, such as in elephants and systems of predatory warning, such as those used by prairie dogs and ground squirrels.

The study of group selection has played an important role in informing other theories of selection and has shed light on the significance of altruistic behaviours observed in animals, including humans. However, it has been controversial since its introduction in the 19th century by British naturalist Charles Darwin. Often, selfless behaviours jeopardize the acting individuals’ fitness, possibly lowering their chances for leaving behind offspring. Darwin realized that this presented a problem for his theory of natural selection, for which the bearing and survival of offspring was a vital determinant of evolutionary success.

In the early 20th century, Darwin’s observations of group behaviour were explored by others in studies that focused on the evolution of certain physical traits and behaviours that appear to benefit social groups. But toward the middle of that century, following the rise of neo-Darwinism, in which Darwin’s theory of natural selection was synthesized with genetics (the modern evolutionary synthesis), the idea that selection acted on groups was largely dismissed. Many evolutionary biologists agreed that adaptation through selection at the level of the individual and the gene was of greater consequence than selection at the group level.

Natural Selection Examples

Example of Stabilizing Selection

For stabilizing selection, imagine a population of mice that lives in the woods. Some of the mice are black, some are white, and some are grey. If the mice had no predators, and no other forces acting on the color of their coat, it would have no reason to change and would only change randomly in response to certain mutations in the DNA. However, that is not the case with these mice. They have lots of predators.

Foxes and house cats prey on the mice during the day. In the nighttime, the owls and other predators scour the dark for dinner. Either way, the mice are in a tough position. But not all the mice face the same risk at all times. In the daytime, the black mice are much easier to spot, and predators eat more black mice. The white mice stand out at night. This means owls eat more white mice at night. The grey mice are the only ones who survive more both during the day and at night. By the next generation, there will be many less black and white mice to reproduce.

Example of Directional Selection

It is important to consider different traits in the same population of animals. Imagine again the population of mice living in the woods. Instead of their color, consider a trait that runs on a continuous scale. Imagine that the mice vary in size from a normal sized mouse to something much larger than a rat. Although the mice are the same species, they grow into many different sizes. The predators, however, have a terrible time trying to catch and eat the largest of the mice. Not only do the large mice weigh more, but they can fight back. The smaller mice are mostly defenseless and provide the perfect sized snack.

If this were the case, and nothing was holding them back, the mice would get much larger. This is directional selection. This is probably what happened in the case of the capybara, a giant South-American rodent. Like our fictional rodents, the pressures of their environment have caused them to be much larger than any other rodent known to man. Many rodents find different advantages in being small, which is why most rodents have remained a certain size. These advantages could be as simple as the ability to hide or the availability of food, but animals of certain sizes do better for different reasons, and populations can change size over time.

Example of Diversifying Selection

Okay, last time with the mice. But this time, consider a new trait in the population. Let’s imagine that some of the mice begin to grow skin flaps between their front and back legs. Effectively, it makes a parachute that allows them to glide away from predators. Mice that fully have the skin flaps do really well and are almost always able to escape predators. Likewise, mice without the flaps avoid the trees and open spaces that mice with flaps venture into, and are much better at hiding from predators. The largest selective force is against the mice in the middle.

Without the ability to glide away, mice somewhere in the middle cannot escape predators at a rate that can help them reap the benefits of the trees. At the same time, the half-flaps make it harder for them to run and hide from predators. Because of this weakness, many more of these mice from the middle of the spectrum are eaten. This begins to divide the population into two distinct traits. Eventually, this can lead to the mice becoming an entirely different species.

It is possible that this is how bats became the only flying rodents. Much like the imaginary scenario described, there are real rodents that do not fly, some that can glide, and bats. While the common ancestor between all of these animals might not have been called a rodent, they are all mammals. Much like our imaginary scenario, diversifying selection could have caused the population of the common ancestor to change and separate. In the real world, selective pressures are much more complex, and we can only guess at the exact historical relationship between animals.

Example of Sexual Selection

Look at a peacock. Try to imagine a functional use of that ridiculous tail. Stumped? Scientists were, too, until the mechanism of sexual selection was explained. This form of natural selection can sometimes select for functional adaptations, but often produces bizarre adaptations that only serve in attracting mates. In the case of the peacock, the colorful tail is used in a display meant to attract females. Males with larger tails and more dazzling colors are preferred to males with small tails. This peculiar preference seems to have no actual bearing on how successful the males could be in collecting food and reproducing, but because of the preference of the females, all male peacocks have large, colorful tails.

Interestingly, this pattern of males becoming the more decorated of the genders holds true for many bird species. Male ducks, many male tropical birds, and even the male common house sparrow are all much more decorated than their female counterparts. This is also seen in some reptiles. In fact, many animals have adapted strange displays or methods of decorating their nest to attract mates. The selection can work in both ways and depends mostly on which sex can be choosier in selecting a mate.

Example of Predator-Prey Selection

The fastest land predator is the cheetah. Cheetahs did not get extremely fast for no reason. The cheetah’s main prey item, the antelope, is fast as well. Which one got fast first will forever remain a mystery, but the fact is, these two species drive each other to be faster.

Faster cheetahs experience an advantage over other cheetahs in that they catch more antelope, and can support a much larger family. Eventually, slow cheetahs will die off, and the fast cheetah population will explode catching antelope. The antelope population, responding to the new selection, are also more successful when they are fast enough to avoid the cheetahs. Thus the antelope population is also being directionally selected for faster animals.

Scientists theorize that this give-and-take between the predator and prey populations is responsible for shaping many of their defining traits. In fact, scientists were baffled why the American Pronghorn, a species that resembles antelope in size and speed, would exist considering the lack of cheetahs in North America. Without a predator fast enough to catch you, at a certain point the extra speed is of no great advantage. Scientists remained baffled until the fossils of a cheetah-like predator were found in North America. Unlike the cheetahs of Africa, the cheetahs of North America did not survive human expansion, and the pronghorn is left without a predator.

Natural selection is a simple mechanism that causes populations of living things to change over time. In fact, it is so simple that it can be broken down into five basic steps, abbreviated here as VISTA: Variation, Inheritance, Selection, Time and Adaptation.

Variation and Inheritance

Members of any given species are seldom exactly the same, either inside or outside. Organisms can vary in size, coloration, ability to fight off diseases, and countless other traits. Such variation is often the result of random mutations, or "copying errors," that arise when cells divide as new organisms develop.

When organisms reproduce, they pass on their DNA--the set of instructions encoded in living cells for building bodies--to their offspring. And since many traits are encoded in DNA, offspring often inherit the variations of their parents. Tall people, for example, tend to have tall children.

Selection: Survival and Reproduction

Environments cannot support unlimited populations. Because resources are limited, more organisms are born than can survive: some individuals will be more successful at finding food, mating or avoiding predators and will have a better chance to thrive, reproduce, and pass on, their DNA. Small variations can influence whether or not an individual lives and reproduces. Differences in color, for instance, aid some individuals in camouflaging themselves from predators. Sharper eyes and claws help an eagle catch its dinner. And brighter coloration improves a male peacock's chances of attracting a mate.

Time and Adaptation

In generation after generation, advantageous traits help some individuals survive and reproduce. And these traits are passed on to greater and greater numbers of offspring. After just a few generations or after thousands, depending on the circumstances, such traits become common in the population. The result is a population that is better suited--better adapted--to some aspect of the environment than it was before. Legs once used for walking are modified for use as wings or flippers. Scales used for protection change colors to serve as camouflage.

The 3 Types of Natural Selection

Natural selection is defined as a process or a “force” that allows for organisms better adapted to their environment to better survive and produce more offspring. The theory of natural selection was first founded by Charles Darwin. The process of natural selection is important and is a driving force for evolution. For organisms to evolve, there needs to be differences in traits between organisms that provide certain advantages or disadvantages, and it is these traits that natural selection acts upon.

When it comes to natural selection, there are three different types of selection that can occur. These types include the following:

Stabilizing Selection

This type of natural selection occurs when there are selective pressures working against two extremes of a trait and therefore the intermediate or “middle” trait is selected for. If we look at a distribution of traits in the population, it is noticeable that a standard distribution is followed:

Example: For a plant, the plants that are very tall are exposed to more wind and are at risk of being blown over. The plants that are very short fail to get enough sunlight to prosper. Therefore, the plants that are a middle height between the two get both enough sunlight and protection from the wind.

Directional Selection

This type of natural selection occurs when selective pressures are working in favour of one extreme of a trait. Therefore when looking at a distribution of traits in a population, a graph tends to lean more to one side:


Example: Giraffes with the longest necks are able to reach more leaves to each. Selective pressures will work in the advantage of the longer neck giraffes and therefore the distribution of the trait within the population will shift towards the longer neck trait.

Disruptive Selection

This type of natural selection occurs when selective pressures are working in favour of the two extremes and against the intermediate trait. This type of selection is not as common. When looking at a trait distribution, there are two higher peaks on both ends with a minimum in the middle as such:

Example: An area that has black, white and grey bunnies contains both black and white rocks. Both the traits for white and black will be favored by natural selection since they both prove useful for camouflage. The intermediate trait of grey does not prove as useful and therefore selective pressures act against the trait.

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Natural Selection Is the Only Mechanism for Evolution

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While natural selection is the largest driving force behind evolution, it is not the only mechanism for how evolution occurs. Humans are impatient and evolution through natural selection takes an extremely long time to work. Also, humans seem to not like to rely on letting nature take its course, in some cases.

This is where artificial selection comes in. Artificial selection is a human activity designed to choose the traits that are desirable for species whether it be color of flowers or breed of dogs. Nature is not the only thing that can decide what is a favorable trait and what is not. Most of the time, human involvement and artificial selection are for aesthetics, but they can be used for agriculture and other important means.

How can we decide at which level does natural selection apply? - Biology

Natural selection at work

Scientists have worked out many examples of natural selection, one of the basic mechanisms of evolution.

Any coffee table book about natural history will overwhelm you with full-page glossies depicting amazing adaptations produced by natural selection, such as the examples below.

Orchids fool wasps into "mating" with them. Katydids have camouflage to look like leaves. Non-venomous king snakes mimic venomous coral snakes.

Behavior can also be shaped by natural selection. Behaviors such as birds' mating rituals, bees' wiggle dance, and humans' capacity to learn language also have genetic components and are subject to natural selection. The male blue-footed booby, shown to the right, exaggerates his foot movements to attract a mate.

In some cases, we can directly observe natural selection. Very convincing data show that the shape of finches' beaks on the Galapagos Islands has tracked weather patterns: after droughts, the finch population has deeper, stronger beaks that let them eat tougher seeds.

In other cases, human activity has led to environmental changes that have caused populations to evolve through natural selection. A striking example is that of the population of dark moths in the 19th century in England, which rose and fell in parallel to industrial pollution. These changes can often be observed and documented.


Selection experiments can be done on natural populations. They reveal that adaptive evolution can be much more rapid than previously thought and open up the possibility of complimenting laboratory selection experiments with studies of natural populations. This conclusion is reinforced by a much larger number of non-experimental studies of adaptation that are linked to a time reference and hence allow us to make inferences about the rate of evolution and show that this inference of a high potential rate of evolution is obtainable in a diversity of organisms. A combination of evaluations of selection in the field and laboratory also reveals that population size and structure can affect the outcome and genetic basis of selection. Because the laboratory imposes a specific population size and structure that most often does not relate well to natural populations, the kind of response seen in the laboratory may also fail to represent how organisms are likely to evolve in the field. The repeatability of adaptive evolution at the phenotypic level in response to specific selection pressures has already been demonstrated under both field (e.g., Reznick et al. 1996a Losos et al., 1997) and laboratory (e.g., Rainey and Travisano, 1998 Travisano and Rainey, 2000) conditions. However, much remains to be learned about the repeatability of the underlying genetic architecture of these events. Future studies that specifically evaluate factors such as population size and structure under laboratory and field conditions could provide insight into this largely unexplored area of research. Finally, a combination of laboratory and field work reveals that studying organisms in the laboratory alone often means studying evolution in the absence of trade-offs that are normally present in nature. These trade-offs arise because organisms in nature typically occur within a mosaic of heterogeneous environments and under a diversity of selection pressures. Thus, pleiotropic effects and other fitness costs associated with alleles that would otherwise be favored in response to a given form of selection suggest that the absence of any fitness tradeoffs can yield a diversity of laboratory artifacts. In summary, we argue that our understanding of how phenotypes and genotypes respond to selection will be better informed by studies of adaptation in both laboratory and natural conditions.

From the Symposium Selection Experiments as a Tool in Evolutionary and Comparative Physiology: Insights into Complex Traits presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 5–9 January 2004, at New Orleans, Louisiana.

Disruptive selection Disruptive Selection:
A type of selection that removes individuals from the center of a phenotypic distribution and thus causes the distribution to become bimodal. occurs when natural selection favors both extremes of continuous variation. Over time, the two extreme variations will become more common and the intermediate states will be less common or lost. Disruptive selection can lead to two new species.

This might happen in shallow water among rocks. Light-colored oysters are more cryptic Cryptic Coloration:
Coloration that allows an organism to match its background and hence become less vulnerable to predation or recognition by prey. (less easy for a predator to see) because they match the rock color. Dark-colored oysters blend into the shadows cast by the rocks. In this case, intermediate-colored oysters would be most heavily preyed upon by the crabs, and very light and very dark oysters would survive to reproduce.

Getting fatter

The realisation that people in the developed world are in effect choosing to prevent their genes from surviving beyond them has led evolutionary biologist Stephen Stearns to look at evolution in the current generations in a radical way.

A long-term medical study of a small town in Massachusetts, called Framingham, allowed him to look at the medical history of thousands of women going back to the middle of the 20th Century, and to calculate how the people that are having children differ from the population as a whole.

It has left him in no doubt that people - at least in Framingham - are still evolving and in a surprising direction.

"What we have found with height and weight basically is that natural selection appears to be operating to reduce the height and to slightly increase their weight."

This was not just a case of people eating more and there was no evidence to suggest the trend of people putting on weight and losing height would continue indefinitely.

In any case, the changes were very small and very slow, similar to those at work in Darwin's evolutionary studies.

Interestingly, Stearns believes that rather than sheltering us from natural selection, the changes that we've made to the world may actually be driving our evolution.

"We see rapid evolution when there's rapid environmental change and the biggest part of our environment is culture, and culture is exploding," says Prof Stearns.

"That's I really think the take-home message of the Framingham study, that we are continuing to evolve, that biology is going to change with the culture and it's just a matter of not being able to see it because we're stuck right in the middle of the process right now."

Technology may have limited the impact of evolutionary forces such as predation and disease, but that does not mean humans have stopped evolving.

Far from it, in a world of globalisation, rapidly advancing medical and genetic science and the increasing power of individuals to determine their own life choices, more powerful forces may come into play.

The direction of our future evolution is likely to be driven as much by us as by nature. It may be less dependent on how the world changes us, but ever more so on our growing ability to change the world.

Horizon: Are We Still Evolving? will be on BBC Two at 2100 on Tuesday 1 March 2011 or watch online afterwards via BBC iPlayer