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5.12: Rates of Speciation - Biology

5.12: Rates of Speciation - Biology


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Learning Outcomes

  • Explain the two major theories on rates of speciation

Scientists around the world study speciation, documenting observations both of living organisms and those found in the fossil record. In terms of how quickly speciation occurs, two patterns are currently observed: gradual speciation model and punctuated equilibrium model.

In the gradual speciation model, species diverge gradually over time in small steps. In the punctuated equilibrium model, a new species undergoes changes quickly from the parent species, and then remains largely unchanged for long periods of time afterward (Figure 1). This early change model is called punctuated equilibrium, because it begins with a punctuated or periodic change and then remains in balance afterward. While punctuated equilibrium suggests a faster tempo, it does not necessarily exclude gradualism.

Practice Question

Which of the following statements is false?

  1. Punctuated equilibrium is most likely to occur in a small population that experiences a rapid change in its environment.
  2. Punctuated equilibrium is most likely to occur in a large population that lives in a stable climate.
  3. Gradual speciation is most likely to occur in species that live in a stable climate.
  4. Gradual speciation and punctuated equilibrium both result in the divergence of species.

[reveal-answer q=”912830″]Show Answer[/reveal-answer]
[hidden-answer a=”912830″]Answer b is false.[/hidden-answer]

The primary influencing factor on changes in speciation rate is environmental conditions. Under some conditions, selection occurs quickly or radically. Consider a species of snails that had been living with the same basic form for many thousands of years. Layers of their fossils would appear similar for a long time. When a change in the environment takes place—such as a drop in the water level—a small number of organisms are separated from the rest in a brief period of time, essentially forming one large and one tiny population. The tiny population faces new environmental conditions. Because its gene pool quickly became so small, any variation that surfaces and that aids in surviving the new conditions becomes the predominant form.

Visit this website to continue the speciation story of the snails.

Speciation

Speciation is a process within evolution that leads to the formation of new, distinct species that are reproductively isolated from one another.

Anagenesis, or ‘phyletic evolution’, occurs when evolution acts to create new species, which are distinct from their ancestors, along a single lineage, through gradual changes in physical or genetic traits. In this instance, there is no split in the phylogenetic tree. Conversely, ‘speciation’ or cladogenesis arises from a splitting event, where a parent species is split into two distinct species, often as the result of geographic isolation or another driving force involving the separation of populations.

The reproductive isolation that is integral to the process of speciation occurs due to reproductive barriers, which are formed as a consequence of genetic, behavioral or physical differences arising between the new species. These are either pre-zygotic (pre-mating) mechanisms, for example, differences in courtship rituals, non-compatible genitalia, or gametes, which are unable to fertilize between species. Alternatively, they are post-zygotic (post-mating), for example zygote mortality or the production of sterile offspring. Reproductive isolation leads to reinforcement of the distinction between species through natural selection and sexual selection.


Varying Rates of Speciation

Scientists around the world study speciation, documenting observations both of living organisms and those found in the fossil record. As their ideas take shape and as research reveals new details about how life evolves, they develop models to help explain rates of speciation. In terms of how quickly speciation occurs, two patterns are currently observed: gradual speciation model and punctuated equilibrium model.

In the gradual speciation model , species diverge gradually over time in small steps. In the punctuated equilibrium model, a new species undergoes changes quickly from the parent species, and then remains largely unchanged for long periods of time afterward (Figure). This early change model is called punctuated equilibrium, because it begins with a punctuated or periodic change and then remains in balance afterward. While punctuated equilibrium suggests a faster tempo, it does not necessarily exclude gradualism.


Speciation: Definition, Classification and Modes | Zoology

Species has been considered to be the product of the opportunistic process of evo­lution. Taxonomist encounters difficulties in trying to delimit species mainly due to two aspects of evolution: variability within populations and the existence of incipient species.

The formation of new species is referred to as speciation. There are different attitudes towards speciation. Some authors even deny the existence of speciation as a distinct natural phenomenon. For many biologists, every speciation event has its individuality, being the outcome of a unique mixture of different ingredients, such as geo­graphical context, and the number and type of genetic structures involved.

For others, there is a very limited range of fundamentally different modes of speciation. While, a few researchers believe that speciation involving sexually reprodu­cing organisms are fundamentally the same. These different attitudes reflect the uneven attention paid to recognise several different and partly independent aspects of speciation.

Is speciation an adaptive process? This is perhaps one of the most basic of the many questions regarding the causes of speciation. The most prevalent view is, perhaps, expressed by Herman Muller (1940), Ernst Mayr (1963) and many later authors.

According to them “reproductive isolation evolves as a by-product of genetic changes that occur for other reasons, so that specia­tion is an incidental, non-adaptive consequence of divergence of populations”.

Rate of the Speciation Process:

When considering the rate of the process, two modes of speciation can be distin­guished:

(i) Conventional, gradual specia­tion occurring in the course of a long series of generations, and

(ii) Instantaneous specia­tion.

In population genetic terms, Templeton (1982) similarly distinguished between the slow speciation by divergence and the quick speciation by transilience (Table 3.3B).

Mechanism of Speciation:

To understand the mechanism of specia­tion, one must have a clear knowledge of bio­logical species concept. A species is considered to be a group of actually or poten­tially interbreed natural population that is reproductively isolated from other such groups.

Thus, the individuals of a species interbreeds with one another, but no inter­breeding occurs between two different species. As a result of interbreeding, gene flow occurs either within the population or between populations (Fig. 3.2).

Gene flow occurs between members of the same species but no gene flow occurs between two diffe­rent species. Each species maintains its integrity as long as its gene pool does not mix with that of other species. The mixing of gene pool will not be possible without breeding of interspecies. Such interbreeding is not possi­ble when there are genetic differences between the two groups. Species, thus, evolve owing to genetic divergence and isolating mechanisms.

If a single randomly mating population gives rise to two reproductively isolate popu­lations, then it is generally not sufficient for a single mutation to confer reproductive iso­lation on its bearers. Such type of mutation (say, X2) may arise in a single or a few indi­viduals and in heterozygous condition (X1X2).

Its reproductive fitness will be low­ered, irrespective of the fact whether it takes place in post-zygotic isolation (example, steri­lity) or pre-zygotic isolation (example, failure to mate with the ‘normal’ type). Therefore, it will generally be eliminated by natural selec­tion.

In the case of polygenic trait as a cause of reproductive isolation, the problem is that recombination generates intermediates. Thus, the problem of speciation is how two different populations can be formed without intermediates.

There are various controversies concer­ning speciation. These are due to the fact that the opponents did not see clearly that specia­tion has two aspects — genetic and populational. These two factors are not naturally exclusive alternatives rather, both are always involved simultaneously.

Classification of Speciation:

The modes of speciation have been classi­fied by two authors separately (Table 3.3). The one classified by Mayr (1963) is generally followed.

The evolution of reproductive barrier based on several or many allele substitutions has been referred by Mayr as gradual speciation. Such gradual speciation may occur under three geographic settings— allopatric speciation, parapatric speciation, and sympatric speciation.

Other than the above-mentioned ones, some instances are also known where species have arisen from hybrids. Here speciation occurs chiefly by polyploidy or even by chro­mosomal rearrangement.

Modes of Speciation:

A. Allopatric Speciation:

When two related populations are geo­graphically separated by an extrinsic barrier, physical barrier (such as topography, water or land) or un-favourable habitat, it results in the evolution of gradual genetic reproductive barriers between them. This results in the for­mation of separate species and the process is called allopatric speciation.

At the beginning the gene pool of the two related populations (allopatric popu­lation) is similar. Due to the formation of the geographic barrier, there is no possibility for the mixing up of genes between the populations. The environmental factors act upon the two populations differently. The two populations are under the operation of natural selection and genetic drift — which leads to genetic divergence.

When genetic divergence has developed, the populations, which once made effective interbreeding, fail to interbreed when they are brought together by migration or by the disappea­rance of the barrier. Thus, reproductive isolation has come into existence and the two populations are now considered to be separate species.

In species that disperse little or are strongly tied to a particular habitat, extrinsic barrier may isolate populations on a “micro- geographic” scale, such as segregated habi­tats within a lake. Such extrinsic barriers need not reduce gene exchange to zero, but may reduce to a very low level. All evolu­tionary biologists are of the opinion that allopatric speciation does occur in nature and is the prevalent mode of speciation in ani­mals.

Kinds of Allopatric Speciation:

Two major models of allopatric speciation have been postulated, differing in population structure and genetic dynamics:

(a) Vicariant Speciation:

Vicariance occurs when two rather widespread popula­tions are divided either by the emergence of an extrinsic barrier, or the extinction of inter­vening populations, or migration into a separate region (Fig. 3.3A). One such exam­ple is the emergence of the Isthmus of Panama during the Pliocene.

It divided many marine organisms into Pacific and Caribbean populations, some of which have diverged into distinct species. Vicariant speciation is supposed to occur by the operation of natural selection and per­haps by genetic drift, in both the populations separately and, perhaps, to a greater extent in one than the other.

(b) Peripatric Speciation:

Peripatric speciation is another major mode of allo­patric speciation. It occurs when a colony that is derived from a much widespread parent population diverges and acquires reproductive isolation (Fig. 3.3B). This model, sometimes called founder-effect speciation, is a very controversial one, as a great deal of controversy surrounds the genetic changes postulated for such popula­tions. Many population genetists believe that the genetic changes underlying peripatric speciation are the same as those in vicariant speciation.

According to Mayr (1954, 1963), Templeton (1980) and others, this process is thought of as speciation due to a shift, initia­ted by genetic drift and followed by natural selection between adaptive peaks. It has been seen in many birds and animals that isolated populations with restricted distributions often are highly divergent, even though their ecological environment appears similar to that of the parent.

Mayr explained such divergence, propo­sing that:

(1) Genetic change could be very rapid in localised populations founded by a few indi­viduals that are cut off from gene exchange with the main body of the species. Differences in ecological selection plays a vital role because the environment of a small area (where the founder species are) is often more homo­geneous than that of a large area. Therefore, the conflicting pressures that act on a widespread population is less numerous here.

(2) Allele frequencies at some loci differ from those in the parent population because of accidents of sampling (that is genetic drift), as a few colonists (the founders) will carry only some of the alleles from the source population, and at altered frequencies (such initial alteration of allel frequencies led Mayr to coin the term founder effect).

As epistatic interactions among genes affect fitness, this initial change in allele frequencies at some loci alters the selective value of genotypes at other interacting loci. Thus, the “genetic environment” of the other loci is altered.

(3) As the colony is cut off from the gene flow among populations the genetic environ­ment of the colony, that imposes selection on alleles (including new mutations) is less variable, more homozygous and may favour alleles that confer high fitness in specific combinations with certain alleles at other loci.

According to Mayr (1963) the epistatic interactions are so strong that each genetic change might trigger others, so that massive genetic change might take place, yielding reproductive isolation as a by-product. Such substantial evolution might occur so rapidly and/or so localised a geographic scale, that it will seldom be documented in the fossil record. This hypothesis may thus help to explain the rarity of fossilized transitional forms among species.

An example of founder-effect speciation is the robin, Petrocia multi-colour. The bright plumage of the male is quite contrasting with the duller plumage of the female throughout eastern Australia. But among the various Melanesian islands present nearby, the male robins have “female” colouration on some, while the females have “male” colouration on others.

Another example of peripatric speciation is the kingfisher, Tanysiptera galeata. It shows only slight variation in plumage throughout climatically very different regions of New Guinea. However, it has given rise to several markedly different forms on the various small islands present nearby.

B. Parapatric Speciation:

Parapatric speciation is the evolution of reproductive isolation between populations that are continuously distributed in space, so that there is substantial movement of indivi­duals and, hence, gene flow between them.

Here the populations of a widespread species diverge by adaptation to different environ­ments. Divergent selection—even at a narrow environmental discontinuity—may oppose gene flow and result in reproductive isolation.

The importance of parapatric speciation is a matter of dispute mainly because the empirical evidence is usually ambiguous. It is a matter of question as to how reproduc­tive isolation develops in the face of gene exchange. A probable answer is that diver­gent selection for the different gene combina­tions may be stronger than the rate of gene flow.

If gene flow is low, as probability exists over a very wide area, then relatively weak selection suffices to establish clines at the relevant loci. If gene flow is appreciable, as happens between neighbou­ring populations, then, due to strong diver­gent selection, a steep cline is formed. To say that divergent selection is strong then it has to be presumed that there exists a barrier to gene exchange.

This barrier is caused due to reproductive failure of individuals with the “wrong” genotype or phenotype that migrate across the border. Therefore, in principle, strong divergent selection at certain loci, causing a steep cline, acts like a physical barrier, resulting in genetic diversity at other more weakly selected loci. Subsequently, clines at various loci may tend to develop at the same location, resulting in a hybrid zone.

The best possible example of parapatric speciation is Authoxanthum adoratum, one of several grasses that evolved tolerance to heavy metals in the vicinity of mines. Several populations, under very strong selection for heavy metal tolerance, have diverged from neighbouring non-tolerant populations, not only in tolerance but also in flowering time.

Moreover, they self-pollinate more frequently, as they have become more self-compatible. Both these characteristics provide sufficient reproductive isolation from adja­cent non-tolerant genotypes.

C. Sympatric Speciation:

Sympatric speciation is the evolution of reproductive isolation within a randomly mating population (Fig. 3.5). As the two populations inhabit the same place and in the same habitat, they have the same needs, with regard to food, space and other requirements of life. This competition brings about the operation of Gause’s law or competitive exclusion principle, which states that two populations cannot continue to occupy the same habitat indefinitely.

Sympatric speciation is one of the most controversial subjects in evolutionary biology. Sympatric speciation would arise if a bio­logical barrier to gene exchange arose within the confines of panmictic (randomly mating) population or if speciation occurs despite high initial gene flow.

Many authors put for­ward that spatial segregation was necessary for speciation. Mayr (1982) demonstrated that many supposed cases are unconvincing and that the sympatric speciation hypothesis must overcome severe theoretical difficulties.

However, the one mode of sympatric speciation that is above any controversy is speciation by polyploidy in plants. Here, a single instantaneous change reproductively isolates a new polyploid from its ancestor. Any model of sympatric speciation must overcome the difficulty of how to reduce the frequency of the intermediate genotypes that would act as a channel for gene exchange.

Possible examples of sympatric specia­tion:

Some authors are of the opinion that sympatric speciation are quite frequent in certain groups of organisms.

1. Treehoppers, Enchenopa binotata, consist of six sympatric sibling species, each restricted to a different genus of host plant. As their eggs hatch at different times, they reach reproductive maturity and mate at somewhat different times during the sum­mer.

Wood and Keese (1990) suggested that gene flow among the species was initially reduced by differences in the timing of their life history caused directly by the host plants and that this reduction allowed subsequent genetic divergence in host preference and mate preference.

2. The apple maggot fly, Rhagoletis pomonella, shows genetic divergence due to habitat isolation and is undergoing the pro­cess of sympatric speciation. Subsequent studies by Feder (1993) have shown this to be one of the few convincing cases of incipient sympatric speciation.

Grant (1971) defined quantum speciation as the budding off of a new and very different daughter species from a semi-isolated peri­pheral population of the ancestral species. Quantum speciation (Fig. 3.6) is a more rapid and abrupt mode of species formation.

This type of speciation is based on the observation of Drosophila inhabiting Hawaii islands. These islands are not older than 7 million years. Nevertheless, they contain about 700 species of Drosophila, and each of these species have no sub-species.

Hawaii islands are said to be characte­rised by volcanic eruptions. These volcanic eruptions kill the pre-existing flora and fauna. Occasionally gravid female flies are carried to this area affected by volcanoes. In such a habitat, the fly population increases rapidly. Since this area is free from predators, natural selection is relaxed temporarily and this leads to the development of new genetic variation.

As the area becomes thickly popu­lated, this expansion may be followed by a population crash, which will eliminate almost all the individuals. A few flies that recover from the crash are acted upon by natural selection and genetic drift. This leads to the formation of genetic divergence and the failure of interbreeding with the original population by the development of reproduc­tive isolation.

E. Polyploidy and Hybrid Speciation:

These are special consi­derations that play a greater role in diversi­fication and speciation in plants than in animals. Speciation by polyploidy has two distinctions:

1. It is the only known mode of instan­taneous speciation by a single gene­tic event.

2. It is the only mode of sympatric spe­ciation.

Among animals, polyploidy is rare. Sexually reproducing polyploids are known in a few groups of fishes (particularly in trout and sucker fishes) and frogs. Polyploid ani­mals are often parthenogenetic, such as wee­vils, grasshoppers, salamanders and lizards. Polyploidy, however, is very common in plants.

Polyploidy generally occurs due to failure of the reduction division in meiosis yielding 2n gametes. Union of such a gamete with haploid (normal) or diploid gametes result in triploid (3n) and tetraploid (4n) zygotes, respectively.

Hybridization may sometimes give rise to distinct species. In plants, some genotypes of diploid hybrids are fertile and are reproductively isolated from the parent species, and so give rise to new species. It may increase in frequency, forming a distinct population. Verne Grant (1971) named such speciation recombinational speciation.

Several researchers have produced “hybrid species” of plants experimentally through independent translocations, such that parts of chromosomes A and B of species 1 are reciprocally trans-located in species 2. Similar reciprocal translocations are done for other chromosomes.

Although, in most cases, crosses of such types result in largely sterile progeny, nevertheless, an F2 that is homozy­gous for one of these combinations will form fertile hybrids. One such example is the experiment of Grant (1966) who crossed Gilia malior with G. mudocensis — resulting in 99% sterile F1. However, he was able to obtain several F2 plants and reared inbred lines by self-fertilization.

Although such hybridization may be a common mode of speciation in plants, it is rare in animals. One such possible example, described by Bullini (1994), is the minnow, Gila seminuda, of Western North America.

Rates of Speciation:

The rate of speciation is related to two concepts:

(a) The rapidity with which populations of a species evolve reproductive isolation.

(b) The numbers of species that arise per unit of time. The rate of increase in the number of species depends on both the rate of origin and the rate of extinction of species.

The time required for speciation or in other words for the evolution of reproductive isolation is highly variable. However, geo­logical evidence, combined with estimates of divergence time based on molecular data, suggests that generally the time required for full reproductive isolation is about 3 million years.

Factors affecting speciation rate:

The factors affecting speciation rate are gene­rally:

1. Abundant topographic barriers that provide opportunities for allopatric divergence

2. Low dispersal rates that reduce gene flow among populations

3. Strong sexual selection, which may cause divergence in mate recognition systems

4. Probably ecological specialisation, which may limit species to isolated patches of habitat and afford the opportunity for divergence

5. Perhaps bottlenecks in population size, which may facilitate genetic peak shifts or substitution of chromo­some rearrangements, and

6. Genetic properties (example, degree of epistasis) that may affect the rate of evolution of reproductive incom­patibility (these are, however, poorly understood).


18.3 Reconnection and Rates of Speciation

In this section, you will explore the following questions:

  • What are the pathways of species evolution in hybrid zones?
  • What are the two major theories on rates of speciation?

Connection for AP ® Courses

Speciation can both occur gradually over time in small steps or in bursts of change known as punctuated equilibrium. With punctuated equilibrium, a species may remain unchanged for long periods of time. The primary influencing factor on changes in speciation rate is environmental change.

Information presented and the examples highlighted in the section support concepts outlined in Big Idea 1 of the AP ® Biology Curriculum Framework. The AP ® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® exam questions. A learning objective merges required content with one or more of the seven science practices.

Big Idea 1 The process of evolution drives the diversity and unity of life.
Enduring Understanding 1.C Life continues to evolve within a changing environment.
Essential Knowledge 1.C.1 Speciation and extinction have occurred throughout Earth’s history.
Science Practice 5.1 The student can analyze data to identify patterns or relationships.
Learning Objective 1.20 The student is able to analyze data related to questions of speciation and extinction throughout the Earth’s history.

Teacher Support

Hybrid zones provide scientists with spaces from which to research the factors that cause reproductive isolation, and thus, speciation. The spatial patterns of hybrid zones reveal much about these factors and allow for inferences to be made about the number and degree of obstacles to gene flow as well as the number and types of contacts between species.

You may wish to identify for students examples of hybrid zones, or encourage them to research zones for themselves. Pose questions to students about why hybrid zones might be viewed a “natural experiments” in which to study the process of speciation. Encourage them to compare and contrast the information they find about different hybrid zones. Have them consider what questions they might ask about the species in the zones and what information they might expect to gain from asking them.

Speciation occurs over a span of evolutionary time, so when a new species arises, there is a transition period during which the closely related species continue to interact.

Reconnection

After speciation, two species may continue interacting indefinitely or even recombine. Individual organisms will mate with any nearby individual who they are capable of breeding with. An area where two closely related species continue to interact and reproduce, forming hybrids, is called a hybrid zone. Over time, the hybrid zone may change depending on the fitness of the hybrids and the reproductive barriers (Figure 18.24). If the hybrids are less fit than the parents, reinforcement of speciation occurs, and the species continue to diverge until they can no longer mate and produce viable offspring. If reproductive barriers weaken, fusion occurs and the two species become one. Barriers remain the same if hybrids are fit and reproductive: stability may occur and hybridization continues.

Visual Connection

  1. stability, fusion, reinforcement
  2. allopatric speciation, sympatric speciation, fusion
  3. convergent evolution, divergent evolution, no evolution
  4. natural selection, genetic drift, gene flow

Hybrids can be either less fit than the parents, more fit, or about the same. Usually hybrids tend to be less fit therefore, such reproduction diminishes over time, nudging the two species to diverge further in a process called reinforcement. This term is used because the low success of the hybrids reinforces the original speciation. If the hybrids are as fit or more fit than the parents, the two species may fuse back into one species (Figure 18.25). Scientists have also observed that sometimes two species will remain separate but also continue to interact to produce some hybrid individuals this is classified as stability because no real net change is taking place.

Varying Rates of Speciation

Scientists around the world study speciation, documenting observations both of living organisms and those found in the fossil record. As their ideas take shape and as research reveals new details about how life evolves, they develop models to help explain rates of speciation. In terms of how quickly speciation occurs, two patterns are currently observed: gradual speciation model and punctuated equilibrium model.

In the gradual speciation model, species diverge gradually over time in small steps. In the punctuated equilibrium model, a new species undergoes changes quickly from the parent species, and then remains largely unchanged for long periods of time afterward (Figure 18.25). This early change model is called punctuated equilibrium, because it begins with a punctuated or periodic change and then remains in balance afterward. While punctuated equilibrium suggests a faster tempo, it does not necessarily exclude gradualism.

Visual Connection

  1. There is a significant change in the environment over time, such as the breakup of a supercontinent due to tectonic activity.
  2. A species that has a competitor outcompetes it and drives it to extinction, freeing up more resources.
  3. There is a sudden and significant change in the environment, such as a volcanic eruption that divides a population that once shared a habitat.
  4. There is a stable and unchanging environment in which a species can flourish.

The primary influencing factor on changes in speciation rate is environmental conditions. Under some conditions, selection occurs quickly or radically. Consider a species of snails that had been living with the same basic form for many thousands of years. Layers of their fossils would appear similar for a long time. When a change in the environment takes place—such as a drop in the water level—a small number of organisms are separated from the rest in a brief period of time, essentially forming one large and one tiny population. The tiny population faces new environmental conditions. Because its gene pool quickly became so small, any variation that surfaces and that aids in surviving the new conditions becomes the predominant form.

Link to Learning

Visit this website to continue the speciation story of the snails.

Which term is used to describe the continued divergence of species based on the low fitness of hybrid offspring?

Which components of speciation would be least likely to be a part of punctuated equilibrium?

What do both rate of speciation models have in common?

Both models continue to conform to the rules of natural selection, and the influences of gene flow, genetic drift, and mutation.

Describe a situation where hybrid reproduction would cause two species to fuse into one.

If the hybrid offspring are as fit or more fit than the parents, reproduction would likely continue between both species and the hybrids, eventually bringing all organisms under the umbrella of one species.

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    Art Connection

    In (a) gradual speciation, species diverge at a slow, steady pace as traits change incrementally. In (b) punctuated equilibrium, species diverge quickly and then remain unchanged for long periods of time.

    Which of the following statements is false?

    1. Punctuated equilibrium is most likely to occur in a small population that experiences a rapid change in its environment.
    2. Punctuated equilibrium is most likely to occur in a large population that lives in a stable climate.
    3. Gradual speciation is most likely to occur in species that live in a stable climate.
    4. Gradual speciation and punctuated equilibrium both result in the divergence of species.

    The primary influencing factor on changes in speciation rate is environmental conditions. Under some conditions, selection occurs quickly or radically. Consider a species of snails that had been living with the same basic form for many thousands of years. Layers of their fossils would appear similar for a long time. When a change in the environment takes place—such as a drop in the water level—a small number of organisms are separated from the rest in a brief period of time, essentially forming one large and one tiny population. The tiny population faces new environmental conditions. Because its gene pool quickly became so small, any variation that surfaces and that aids in surviving the new conditions becomes the predominant form.


    Art Connection

    In (a) gradual speciation, species diverge at a slow, steady pace as traits change incrementally. In (b) punctuated equilibrium, species diverge quickly and then remain unchanged for long periods of time.

    Which of the following statements is false?

    1. Punctuated equilibrium is most likely to occur in a small population that experiences a rapid change in its environment.
    2. Punctuated equilibrium is most likely to occur in a large population that lives in a stable climate.
    3. Gradual speciation is most likely to occur in species that live in a stable climate.
    4. Gradual speciation and punctuated equilibrium both result in the divergence of species.

    The primary influencing factor on changes in speciation rate is environmental conditions. Under some conditions, selection occurs quickly or radically. Consider a species of snails that had been living with the same basic form for many thousands of years. Layers of their fossils would appear similar for a long time. When a change in the environment takes place—such as a drop in the water level—a small number of organisms are separated from the rest in a brief period of time, essentially forming one large and one tiny population. The tiny population faces new environmental conditions. Because its gene pool quickly became so small, any variation that surfaces and that aids in surviving the new conditions becomes the predominant form.


    5.12: Rates of Speciation - Biology

    Speciation is a fundamental issue in evolutionary biology, but it is both fascinating and frustrating: we know it does happen but it its an historical phenomenon so it is difficult to observe. The two camps of evolutionary biologists best equipped to deal with speciation (in terms of mechanism, population geneticists in terms of time-frames, paleontologists) are both incapable of "seeing" speciation except in very special situations. We must rely on strong inference to properly understand speciation. This inference is in many cases very rigorous and scientific although it is historical, i.e., requires an interpretation of what has gone on in the past.

    Defining speciation depends on one's species concept . (Recall species concepts: typological, evolutionary, biological, recognition). In its simplest form speciation is lineage splitting the resulting lineages are genetically isolated and ecologically distinct. This implies that something intrinsic about the new lineages (an aspect of its biology, e.g., genetics) makes/keeps them distinct. Speciation then must involve the evolution of intrinsic barriers to gene exchange . Intrinsic barriers can related in many ways to extrinsic barriers to gene exchange (abiotic factors limiting gene flow: rivers, isolated islands, glaciers). A variant of a species could be adapted to live in a particular environment that is spatially distinct from other types of environmental conditions here an intrinsic component contributes to an extrinsic barrier. The notion of the evolution of barriers to gene exchange applies to virtually all species concepts since unlimited gene exchange between two populations/species would prevent the evolution of the defining principles of a given species concept: 1) true typological differences must have a genetic basis, 2) evolutionary lineages would not have their own "evolutionary tendencies" with homogenization due to gene exchange, 3) reproductive isolation (either pre- or postmating) would not be maintained with unlimited gene flow, and 4) mate recognition systems could not be maintained as distinct with unlimited gene exchange.

    Without the evolution of some intrinsic barrier to gene exchange, fusion of the two incipient species would be one likely outcome (populations would blend back into one), or extinction of one or the other lineages (one population out competed [at the individual level!] the diverged sister population leaving only one population).

    There are may models which have been proposed that enable barriers to gene exchange to evolve as argued by Ernst Mayr, geographic isolation provides the most effective barrier. We thus consider the allopatric model :

    1. continuous distribution split into two (or more) sub populations

    2. differentiation in allopatry (different selection regimes not necessarily selection for speciation)

    3. if populations come into secondary contact, no gene flow (= speciation complete)

    If no gene flow after secondary contact, speciation was completed in allopatry . Speciation would then be viewed as a byproduct of divergence in allopatry . What happens after secondary contact is a matter of great debate: If the two differentiated forms mix or hybridize this may provide the context for selection for assortative mating also called reinforcement of premating isolation (reinforcement hypothesis). In this case speciation was not completed in allopatry and fate of the two populations depends on the outcome of the interaction upon secondary contact.

    Patterns predicted from the action of reinforcement:

    Selection in zone of overlap for increased premating isolation. See artificial demonstration with a selection experiment (fig. 16.4, pg. 432).

    Reinforcement model assumes that hybrids are less fit (=means by which selection for further isolation can operate). This assumes that post mating barriers arise first and that premating barriers arise as a result of selection in sympatry these assumptions may not hold in all cases. However, if premating barriers evolved first, there might be little hybridization (speciation complete?) if there was no postmating barrier, even with small amounts of hybridization the two forms would fuse back together because there would be no selection against hybrids!

    Reinforcement is actually a special case of character displacement which is the accentuation of differences between species (or forms) by selection against the individuals of similar phenotype (reinforcement = reproductive character displacement and is achieved by selection against hybrids). If reinforcement is true, we should expect to see displacement of characters associated with pre-mating barriers to gene exchange in areas of secondary contact. Some cases we do: calling songs of anurans Frequently such reproductive character displacement is not observed . When "reinforcement-like" patterns are observed, one has to be sure that the phenotypic shift is actually an evolutionary response to the presence of the other incipient species and not to some other clinal variation (e.g., ecological factors that generate parallel clines).

    Problems with reinforcement: other possible outcomes: fusion of the two populations because differentiation was sufficiently slight that selection against hybrids is weak relative to the gene flow between forms. extinction of one or the other of the two forms. Quite likely when there is selection against heterozygotes. In population genetic terms, equivalent to heterozygote disadvantage AA, Aa, aa with fitnesses 1,1-s,1, a metastable equilibrium

    Selection against hybrids within the zone of secondary contact only favors displacement in sympatry gene flow in from allopatry will swamp the effect. One could view such hybrid zones as genetic canyons of lost alleles. Another important question: if selection against hybrids is the driving force for reproductive character displacement, how will the genes for the different components of isolation/recognition sweep through the allopatric regions of the two species ranges where there is no hybridization, hence no selection?

    Another allopatric model is the Peripatric model referring to populations surrounding the main part of the current species range. See fig. 16.5, pg 434.

    1. Small isolated populations

    2. Genetic drift via population bottleneck or founder event => new allele frequencies

    3. new "genetic environment" => different response to selection than in main population

    4. effect is a major genetic change = " genetic revolution "

    One consequence is that speciation may not be dichotomous . Important consequence: rapid divergence, unlikely to leave fossil intermediates (these possibilities will come in to play when we discuss "punctuated equilibrium" later).

    A variant on this theme proposed by Hampton Carson an influential evolutionary biologist from the University of Hawaii is Founder-Flush speciation :

    1. population initiated with small number of individuals (founders)

    2. flush in population size relaxed selection during this phase low fitness recombinants survive

    3. crash in population size selection and drift determine which genotypes survive.

    Carson's view: two "parts" to the genome: the " open variability system" and the " closed variability system" Open system has much variability , responds rapidly to selection (loci encoding allozyme polymorphisms such as enzymes in glycolysis and Krebs cycle, etc.) closed system is resistant to selection less variable ) loci encoding courtship song, developmental patterns, etc . ) In Carson's view the closed system is reorganized during the flush-crash cycles, leads to a genetic change that contributes to reproductive isolation/mate recognition.

    Questions about the founder flush speciation: how small is population after crash?, how long does population stay at reduced population size? Could retain a large portion of the genetic variation after one crash extended bottle necks will be more effective in reducing variation.

    These questions also could apply to Mayr's peripatric speciation model

    Parapatric Model of speciation. Ranges of two differentiated forms are contiguous and non-overlapping. Patterns of discontinuities between differentiated forms/populations may be due to secondary contact after a period in allopatry, or the discontinuity could be due to primary differentiation in situ . One cause of this might be a steep environmental gradient or habitat boundary (see fig. 16.3 pg. 428). With selection on loci that affect reproductive isolation/mate recognition, populations can become differentiated. Will be apparent in the formation of a cline . Can lead to sufficient divergence of reproductive/mating characteristics that barrier to gene flow is established (e.g., plants growing on mine tailings have diverged in flowering time ).

    Studies of parapatric distributions are frequently concerned with the concordance of clines . Selection acting on one locus/trait can impose a cline on another character if the two characters/loci are linked. Are clines superimposed, shifted, different slopes. Slatkin (1973) has shown that the width of a cline is: See fig. 16.9, page 439 text uses different letters for equation). Different loci may have different cline shapes due to different strengths of selection acting on them.

    The text is a bit misleading about Parapatric speciation. It might lead one to believe that when a hybrid zone is observed, parapatric speciation is involved. This is not true since the hybrid zone may be the result of secondary contact after allopatry, rather than primary differentiation at the hybrid zone interface. Here again we need to determine the relative importance of the allopatric phase and the parapatric interaction in determining the outcome of speciation (or fusion). The cricket hybrid zone (fig. 16.8, page 438) is in fact the result of allopatry followed by secondary contact (my personal knowledge), but Ridley does not let you know this.

    Non-allopatric models of speciation are controversial but not impossible. Sympatric speciation can be modeled with a two locus polymorphism, one locus (A) affecting fitness (in this case by affecting fitness in terms of survival on one of two alternative hosts/patches), and another locus (B) affecting mate choice which is crucial in the evolution of assortative mating , a barrier to gene exchange (proposed by John Maynard-Smith in 1966)

    AA Aa aa BB Bb bb
    host 1 1+s 1 1 mate w/ AA no preference mate w/ aa
    host 2 1 1 1+t

    These selective regimes maintain polymorphism at the A locus as in a multiple niche polymorphism considered in the population genetics section. These sets of fitness/mating values will result in the evolution of associations (e.g., linkage disequilibria) between the A and the B locus (e.g., AABB individuals and aabb individuals will be found in the populations with few intermediates.

    these have high fitness these have low fitness

    The green lacewings (Genus Chrysoperla formerly Chrysopa ) seem to exhibit patterns of host preference and mate choice similar to that presented above (studied by the Taubers, Cornell University). One form is adapted to one host/habitat and a second to another this habitat preference appears to be controlled by a single locus with other modifying loci (some evolutionists have not accepted the lacewing data as conclusive).

    See the other sympatric speciation model that involves variation in a resource base (fig. 16.10, pg. 443). This model still requires the evolution of associations (e.g., linkage disequilibrium) between fitness genes and behavior genes.

    But, if sympatric speciation is, if not common, at least possible, is the model really sym patric?: is it just microallopatric speciation (some argue NO if adults come up off their hosts into a mating swarm, but then proceed to mate). Another crucial issue is: what is the rate of recombination between these two types of loci since crossing over will break up favorable associations. A model of host preference and assortative mating invoking many genes (polygenic model) make it more difficult to maintain nonrandom associations. A general issue with all of these models is how much gene flow is tolerated. Evolution of barriers to gene exchange is the issue, gene flow = gene exchange how much gene flow can take place and still evolve barriers to the gene flow?? The answer depends on the genetic architecture of speciation (how many genes, how much divergence, etc. next lecture on genetics of speciation).

    Saltatory speciation: Richard Goldschmitt in the Material Basis of Evolution proposed the idea that Macromutations (mutations with big effects) would result in major developmental and phenotypic changes in their carriers producing the so-called hopeful monster . Ridiculed at the time recently gained a new readership due to the molecular characterization of genes that cause major phenotypic effects (more later on evolution of development). Big problem remains: who is the hopeful monster going to mate with?

    Chromosomal speciation: Consider a diploid with 2N = 4 chromosomes. If two such individuals failed to undergo the reduction division of meiosis their gametes would be 2N=4. If these gametes were used in fertilization of one another, a new chromosomal number would be established: 4N = 8. If this became stabilized as a new chromosomal type (and this is common in plants), this new type can be reproductively isolated from the original 2N = 4 species. The reproductive isolation would be due to an imbalance of chromosome sets in the new zygote: N = 2 gamete crossed to an N = 4 gamete results chromosomal type of 3N = 6. There can be two consequences with this imbalance: i) inviability due to failure during development or ii) instability during chromosome segregation could result in gametes with an incomplete set of chromosomes (aneuploidy). These consequences could have the effect of a reproductive barrier between the original 2N = 4 and the polyploid 4N = 8 type. Speciation can be nearly instantaneous when such chromosomal events are involved (multiples of even numbered ploidy levels: can produce gametes with some exceptions multiples of odd numbered ploidy levels: usually cannot produce gametes due to imbalance of haploid complements) => speciation. Thus polyploid hybrids are frequency genetically isolated from their progenitors .

    The simple inversion model (figure 16.17, pg. 457) illustrates another way that chromosomal factors might play a role in speciation.

    How should we think about speciation events? What are the models of divergence: is speciation like a peak shift in an adaptive landscape, or is speciation a gradual divergence process on a flat adaptive landscape? Main issue is whether the peak itself shifts and hence the population shifts with it, or whether the two alternative peaks already exist and the problem is shifting between the two alternatives.

    Some fundamental issues in thinking about speciation: 1) does speciation require allopatry or can speciation occur in non-allopatric contexts (sympatric, parapatric)? 2) does speciation require changes in many genes or can changes in a few specific genes lead to speciation? 3) is speciation itself adaptive or does speciation occur as a byproduct of adaptive responses to other pressures? 4) what determines the rates of speciation ? (some lineages speciate at very different rates).


    New Speciation Model Challenges Evolution, Supports Creation

    Which model, naturalistic evolution or supernatural creation, best explains the pattern of life’s history on Earth? If a test produces “strikingly divergent results” for the expectations of a model, what does that tell us? A new study on speciation and extinction rates provides persuasive evidence.

    Model Tenets
    A fundamental tenet of all naturalistic models for the history of Earth’s life is that natural changes in the genomes of life will be responsible for the observed changes in the physical body structures (morphology) of life. Consequently, evolutionary trees (phylogenies, see figure 1) developed from the observed patterns in present-day genomes and the presumed natural rates of change of those genomes (molecular clocks). Assuming that strictly natural processes are responsible for the changes occurring throughout the history of life, the phylogenetic trees should match the morphological changes and the timing of those changes observed in the fossil record (or paleontological trees—see figure 2).

    The same kind of match between the paleontology and phylogenetics can be realized if God intervened throughout life’s history. However, apparently only supernatural interventions can explain significant mismatches between phylogenetic and paleontological trees.

    Figure 1: Phylogenetic Tree of Life Derived from Completely Sequenced Genomes.The center represents the presumed first life-form on Earth. The genomes denoted on the outer circle are based on actual genetic data. The branching patterns in the inner circle presume that all species are entirely related to one another through strictly natural processes. Image credit: Ivica Letunic

    Figure 2: Spindle Diagram of the Presumed Evolution of Vertebrates.Width of the spindles indicates the number of extant families or the number of families represented in the fossil record. The curved (presumed) connecting lines are not supported by any physical remains. Image credit: Peter Bockman

    In an open-access paper published in Nature Communications 1 , four computational biologists and biochemists led equally by Daniele Silvestro and Rachel Warnock concede:

    “The fossil record and molecular phylogenies of living species can provide independent estimates of speciation and extinction rates, but often produce strikingly divergent results.” 2

    Silvestro, Warnock, and their two colleagues do not concede, however, that supernatural interventions explain the “strikingly divergent results.” They attempt to offer a possible naturalistic explanation.

    Divergence Is Real and Striking
    Biologists use over a dozen different definitions of a species. In their paper, the Silvestro-Warnock team defines a species as “ an identifiable taxonomic unit (a lineage) that can persist through time, give rise to other species, and become extinct.” 3

    The team first recognizes that since “extant and fossil species are samples of the same underlying diversification process,” 4 if the diversification process is by strictly natural means, researchers expect that in all cases the phylogenetic (presumed evolutionary) trees will match the paleontological (fossil record) trees. To put it another way, a match is expected since “methods used to estimate rates [of change] from fossils and phylogenies are based on the same underlying mathematical birth-death theory.” 5 The team then documents that evolutionary biologists can no longer deny the frequent and striking divergences between phylogenetic and paleontological trees.

    The Silvestro-Warnock team cited a recent study of extant terrestrial Carnivora. 6 There, the estimated mean species longevity based on fossil evidence was 2.0 million years, contrasted with 9.8 million years derived from phylogenetics. They also cited a study demonstrating incongruence between phylogenies and fossils for primates. 7 They noted that, at least for mammals, the occurrences of congruence are few. 8

    Speciation rates derived from phylogenetics consistently supersede those derived from the fossil record, while derived extinction rates are consistently lower than speciation rates. Perhaps the best studied example (see featured image) is for cetaceans (whales, dolphins, and porpoises). The Silvestro-Warnock team cited research showing:

    “Phylogenetic estimates of diversification rates among cetaceans suggest speciation has exceeded extinction over the past 12 Myr 9 implying diversity has increased towards the recent. In contrast, analyses of the cetacean fossil record indicate extinction has exceeded speciation over this same interval, and that the diversity of cetaceans was in fact much higher than it is today .” 10

    In other words, the naturalistic biological evolution model based on phylogenetics predicts that introduction of new species has exceeded extinctions, but the fossil record shows that the reverse is true. The research team did not address the fact that the discrepancies between phylogenetics and the fossil record appear to increase with the complexity and the adult body size of the genus. By contrast, such a correlation is predicted from a biblical creation model perspective for life. 11

    Silvestro, Warnock and collaborators do point out that several other researchers have attempted to explain the discrepancies by underestimates of the statistical and systematic errors in the two methods. However, the discrepancies in fact are much too large to be attributed to these errors. 12

    Attempted Reconciliation
    The Silvestro-Warnock team suggests that many of the discrepancies between phylogenetics and the fossil record are due to sensitivities to different speciation modes. They identify three distinct modes of speciation that can leave behind fossil evidence without impacting the calculated phylogenetic trees:

    1. Cladogenesis via budding: a speciation event that gives rise to one new species. The ancestral species persists and no extinction occurs.
    2. Cladogenesis via bifurcation: a speciation event that gives rise to two new species, replacing the ancestral species, which becomes extinct.
    3. Anagenetic speciation: evolutionary changes along a lineage that result in the origination of one new species and the extinction of the ancestral species.

    They also point out that extinction without replacement is a frequent occurrence, where a species becomes extinct without leaving any descendants. More simply put, the fossil record includes extinct and extant (living) species whereas phylogenetic data typically include extant species only.

    Silvestro, Warnock, and their colleagues developed a model in which they unify budding, bifurcation, anagenesis, and extinction in a single “ birth−death chronospecies ” (BDC) process. Their BDC model shows that phylogenetic and paleontological speciation and extinction rate estimates will only be equal if all speciation has occurred through budding. Furthermore, they demonstrate that “even in an ideal scenario with fully sampled and errorless data sets, speciation and extinction rates can only be equal across phylogenetic and stratigraphic inferences if all speciation events have occurred through budding and no speciation has occurred through bifurcation or anagenesis” 13 (emphasis added). Their BDC model also reveals that phylogenetic analysis indicating extinction equal to zero does not imply that no extinction occurred.

    Actual Reconciliation
    The team ’ s BDC model establishes that relative to the fossil record, phylogenetics always underestimates extinction rates. The fossil record, which is largely incomplete, underestimates the true extinction rates. Much higher extinction rates pose a serious challenge to all strictly naturalistic models for Earth’s life because higher extinction rates require higher speciation rates to explain the increasing diversity of life observed in the fossil record throughout life’s history.

    This requirement of higher speciation rates is all the more problematic for Earth’s most advanced species. For mammals, birds, and advanced plants, the observed extinction rates far exceed the observed speciation rates during the era of human existence (God’s seventh day when, according to Genesis 2, God ceased from his creation work and allowed natural processes operate).

    The Silvestro-Warnock BDC model also exposes a fundamental limitation in naturalistic explanations for the history of Earth’s life. Since all naturalistic models require more than one speciation mode, and since the only way to reconcile phylogenetics and paleontology is to posit just one speciation mode, something other than strictly natural processes must operate.

    Some evolutionary biologists will insist on the caveat that perhaps some unknown natural process might salvage a reconciliation between phylogenetics and paleontology. However, it is difficult to conceive how a natural process of sufficient magnitude to reconcile phylogenetics and paleontology could remain undiscovered. It appears to me that a creation model positing that the supernatural Creator intervened at several times throughout life’s history to replace life-forms driven to extinction fully reconciles this “discrepancy.” I am reminded of a verse (Psalm 104:24) from the longest of the creation psalms:

    How many are your works, Lord! In wisdom you made them all the earth is full of your creatures.

    Featured image: Nine Different Cetacean Species. Featured image credit: Little Jerry, Creative Commons Attribution


    Watch the video: Speciation (October 2022).