Speciation

For the electrochemical phenomenon, see Ion speciation.

Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook was the first to coin the term 'speciation' for the splitting of lineages or "cladogenesis," as opposed to "anagenesis" or "phyletic evolution" occurring within lineages.[1][2][3] Whether genetic drift is a minor or major contributor to speciation is the subject matter of much ongoing discussion.

There are four geographic modes of speciation in nature, based on the extent to which speciating populations are isolated from one another: allopatric, peripatric, parapatric, and sympatric. Speciation may also be induced artificially, through animal husbandry, agriculture, or laboratory experiments.

Natural speciation

Phyletic gradualism, above, consists of relatively slow change over geological time. Punctuated equilibrium, bottom, consists of morphological stability and rare, relatively rapid bursts of evolutionary change (see the illustration at the bottom of the page).
Comparison of allopatric, peripatric, parapatric and sympatric speciation

All forms of natural speciation have taken place over the course of evolution; however, debate persists as to the relative importance of each mechanism in driving biodiversity.[4]

One example of natural speciation is the diversity of the three-spined stickleback, a marine fish that, after the last ice age, has undergone speciation into new freshwater colonies in isolated lakes and streams. Over an estimated 10,000 generations, the sticklebacks show structural differences that are greater than those seen between different genera of fish including variations in fins, changes in the number or size of their bony plates, variable jaw structure, and color differences.[5]

Speciation rate

There is debate as to the rate at which speciation events occur over geologic time. While some evolutionary biologists claim that speciation events have remained relatively constant over time, some palaeontologists such as Niles Eldredge and Stephen Jay Gould have argued that species usually remain unchanged over long stretches of time, and that speciation occurs only over relatively brief intervals, a view known as punctuated equilibrium. (See last subheading under Darwin's Dilemma below.)

Allopatric

Main article: Allopatric speciation

During allopatric (from the ancient Greek allos, "other" + Greek patrā, "fatherland") speciation, a population splits into two geographically isolated populations (for example, by habitat fragmentation due to geographical change such as mountain building). The isolated populations then undergo genotypic and/or phenotypic divergence as: (a) they become subjected to dissimilar selective pressures; (b) they independently undergo genetic drift; (c) different mutations arise in the two populations. When the populations come back into contact, they have evolved such that they are reproductively isolated and are no longer capable of exchanging genes. Island genetics is the term associated with the tendency of small, isolated genetic pools to produce unusual traits. Examples include insular dwarfism and the radical changes among certain famous island chains, for example on Komodo. The Galápagos islands are particularly famous for their influence on Charles Darwin. During his five weeks there he heard that Galápagos tortoises could be identified by island, and noticed that finches differed from one island to another, but it was only nine months later that he reflected that such facts could show that species were changeable. When he returned to England, his speculation on evolution deepened after experts informed him that these were separate species, not just varieties, and famously that other differing Galápagos birds were all species of finches. Though the finches were less important for Darwin, more recent research has shown the birds now known as Darwin's finches to be a classic case of adaptive evolutionary radiation.[6]

Peripatric

Main article: Peripatric speciation

In peripatric speciation, a subform of allopatric speciation, new species are formed in isolated, smaller peripheral populations that are prevented from exchanging genes with the main population. It is related to the concept of a founder effect, since small populations often undergo bottlenecks. Genetic drift is often proposed to play a significant role in peripatric speciation.

Case Studies

Parapatric

Main article: Parapatric speciation

In parapatric speciation, there is only partial separation of the zones of two diverging populations afforded by geography; individuals of each species may come in contact or cross habitats from time to time, but reduced fitness of the heterozygote leads to selection for behaviours or mechanisms that prevent their inter-breeding. Parapatric speciation is modelled on continuous variation within a "single", connected habitat acting as a source of natural selection rather than the effects of isolation of habitats produced in peripatric and allopatric speciation.

Parapatric speciation may be associated with differential landscape-dependent selection. Even if there is a gene flow between two populations, strong differential selection may impede assimilation and different species may eventually develop.[8] Habitat differences may be more important in the development of reproductive isolation than the isolation time. Caucasian rock lizards Darevskia rudis, D. valentini and D. portschinskii all hybridize with each other in their hybrid zone; however, hybridization is stronger between D. portschinskii and D. rudis, which separated earlier but live in similar habitats than between D. valentini and two other species, which separated later but live in climatically different habitats.[9]

Ecologists refer to parapatric and peripatric speciation in terms of ecological niches. A niche must be available in order for a new species to be successful.

Case Studies

Sympatric

Main article: Sympatric speciation

Sympatric speciation refers to the formation of two or more descendant species from a single ancestral species all occupying the same geographic location.

Often-cited examples of sympatric speciation are found in insects that become dependent on different host plants in the same area.[10][11] However, the existence of sympatric speciation as a mechanism of speciation is still hotly contested. Scientists have argued that the evidences of sympatric speciation are in fact examples of micro-allopatric, or heteropatric speciation.

The best illustrated example of sympatric speciation is that of the cichlids of East Africa inhabiting the Rift Valley lakes, particularly Lake Victoria, Lake Malawi and Lake Tanganyika. There are over 800 described species, and according to estimate, there could be well over 1,600 species in the region. All the species have diversified from a common ancestral fish (Oryzias latipes) about 113 million years ago. Their evolution is cited as an example of both natural and sexual selection.[12][13] A 2008 study suggests that sympatric speciation has occurred in Tennessee cave salamanders.[14] Sympatric speciation driven by ecological factors may also account for the extraordinary diversity of crustaceans living in the depths of Siberia's Lake Baikal.

Example of three-spined sticklebacks

The three-spined stickleback (Gasterosteus aculeatus)

Freshwater three-spined sticklebacks, which have been studied by Dolph Schluter, were once thought to provide an intriguing example best explained by sympatric speciation. Schluter and colleagues found two different species of three-spined sticklebacks in each of five different lakes. Each lake contained a large benthic species with a large mouth that feeds on large prey in the littoral zone, as well as a smaller limnetic species with a smaller mouth that feeds on the small plankton in open water. DNA analysis indicated that each lake was colonized independently, presumably by a marine ancestor, after the last ice age. It also showed that the two species in each lake are more closely related to each other than they are to any of the species in the other lakes.The two species in each lake are reproductively isolated; neither mates with the other. However, aquarium tests showed that the benthic species from one lake is able to mate with the benthic species from the other lakes. Likewise the limnetic species from the different lakes are able to mate with each other. These benthic and limnetic species even display their mating preferences when presented with sticklebacks from Japanese lakes. A Canadian benthic prefers a Japanese benthic over its close limnetic relative from its own lake.

The researchers concluded that in each lake there had been great competition within a single original species for limited resources. This led to disruptive selection — competition favoring fishes at either extreme of body size and mouth size over those nearer the mean, as well as assortative mating — each size preferred mates like it. The result was a divergence into two subpopulations exploiting different food in different parts of the lake.The fact that this pattern of speciation occurred the same way on three separate occasions suggests strongly that ecological factors in a sympatric population can cause speciation.

However, the DNA evidence cited above is from mitochondrial DNA (mtDNA), which can often move easily between closely related species ("introgression") when they hybridize or engage in despeciation. A more recent study,[15] using genetic markers from the nuclear genome, shows that limnetic forms in different lakes are more closely related to each other (and to marine lineages) than to benthic forms in the same lake. The three-spine stickleback is now usually considered an example of "double invasion" (a form of allopatric speciation) in which repeated invasions of marine forms have subsequently differentiated into benthic and limnetic forms. The three-spine stickleback provides an example of how molecular biogeographic studies that rely solely on mtDNA can be misleading, and that consideration of the genealogical history of alleles from multiple unlinked markers (i.e. nuclear genes) is necessary to infer speciation histories.

Speciation via polyploidization

Polyploidy is a mechanism that has caused many rapid speciation events in sympatry because offspring of, for example, tetraploid x diploid matings often result in triploid sterile progeny.[16] However, not all polyploids are reproductively isolated from their parental plants, and gene flow may still occur for example through triploid hybrid x diploid matings that produce tetraploids, or matings between meiotically unreduced gametes from diploids and gametes from tetraploids (see also hybrid speciation).

It has been suggested that many of the existing plant and most animal species have undergone an event of polyploidization in their evolutionary history.[17][18] Reproduction of successful polyploid species is sometimes asexual, by parthenogenesis or apomixis, as for unknown reasons many asexual organisms are polyploid. Rare instances of polyploid mammals are known, but most often result in prenatal death.

Hawthorn fly

One example of evolution at work is the case of the hawthorn fly, Rhagoletis pomonella, also known as the apple maggot fly, which appears to be undergoing sympatric speciation.[19] Different populations of hawthorn fly feed on different fruits. A distinct population emerged in North America in the 19th century some time after apples, a non-native species, were introduced. This apple-feeding population normally feeds only on apples and not on the historically preferred fruit of hawthorns. The current hawthorn feeding population does not normally feed on apples. Some evidence, such as the fact that six out of thirteen allozyme loci are different, that hawthorn flies mature later in the season and take longer to mature than apple flies; and that there is little evidence of interbreeding (researchers have documented a 4-6% hybridization rate) suggests that sympatric speciation is occurring. The emergence of the new hawthorn fly is an example of evolution in progress.[20]

Speciation via hybrid formation

Main article: Hybrid speciation

Reinforcement

Reinforcement, also called the Wallace effect, is the process by which natural selection increases reproductive isolation.[21] It may occur after two populations of the same species are separated and then come back into contact. If their reproductive isolation was complete, then they will have already developed into two separate incompatible species. If their reproductive isolation is incomplete, then further mating between the populations will produce hybrids, which may or may not be fertile. If the hybrids are infertile, or fertile but less fit than their ancestors, then there will be further reproductive isolation and speciation has essentially occurred (e.g., as in horses and donkeys.)

The reasoning behind this is that if the parents of the hybrid offspring each have naturally selected traits for their own certain environments, the hybrid offspring will bear traits from both, therefore would not fit either ecological niche as well as either parent. The low fitness of the hybrids would cause selection to favor assortative mating, which would control hybridization. This is sometimes called the Wallace effect after the evolutionary biologist Alfred Russel Wallace who suggested in the late 19th century that it might be an important factor in speciation.[22]
Conversely, if the hybrid offspring are more fit than their ancestors, then the populations will merge back into the same species within the area they are in contact.

Reinforcement favoring reproductive isolation is required for both parapatric and sympatric speciation. Without reinforcement, the geographic area of contact between different forms of the same species, called their "hybrid zone," will not develop into a boundary between the different species. Hybrid zones are regions where diverged populations meet and interbreed. Hybrid offspring are very common in these regions, which are usually created by diverged species coming into secondary contact. Without reinforcement, the two species would have uncontrollable inbreeding. Reinforcement may be induced in artificial selection experiments as described below.

Artificial speciation

New species have been created by domesticated animal husbandry, but the initial dates and methods of the initiation of such species are not clear. For example, domestic sheep were created by hybridisation, and no longer produce viable offspring with Ovis orientalis, one species from which they are descended.[23] Domestic cattle, on the other hand, can be considered the same species as several varieties of wild ox, gaur, yak, etc., as they readily produce fertile offspring with them.[24]

The best-documented creations of new species in the laboratory were performed in the late 1980s. William Rice and G.W. Salt bred fruit flies, Drosophila melanogaster, using a maze with three different choices of habitat such as light/dark and wet/dry. Each generation was placed into the maze, and the groups of flies that came out of two of the eight exits were set apart to breed with each other in their respective groups. After thirty-five generations, the two groups and their offspring were isolated reproductively because of their strong habitat preferences: they mated only within the areas they preferred, and so did not mate with flies that preferred the other areas.[25] The history of such attempts is described in Rice and Hostert (1993).[26][27]

Diane Dodd used a laboratory experiment to show how reproductive isolation can evolve in Drosophila pseudoobscura fruit flies after several generations by placing them in different media, starch- and maltose-based media.[28]

Dodd's experiment has been easy for many others to replicate, including with other kinds of fruit flies and foods.[29] Research in 2005 has shown that this rapid evolution of reproductive isolation may in fact be a relic of infection by Wolbachia bacteria.[30]

Alternatively, these observations are consistent with the notion that sexual creatures are inherently reluctant to mate with individuals whose appearance or behavior is different from the norm. The risk that such deviations are due to heritable maladaptations is very high. Thus, if a sexual creature, unable to predict natural selection's future direction, is conditioned to produce the fittest offspring possible, it will avoid mates with unusual habits or features.[31][32][33][34][35] Sexual creatures will then inevitably tend to group themselves into reproductively isolated species.[34]

Genetics

Few speciation genes have been found. They usually involve the reinforcement process of late stages of speciation. In 2008 a speciation gene causing reproductive isolation was reported.[36] It causes hybrid sterility between related subspecies.

Hybrid speciation

Main article: Hybrid speciation

Hybridization between two different species sometimes leads to a distinct phenotype. This phenotype can also be fitter than the parental lineage and as such natural selection may then favor these individuals. Eventually, if reproductive isolation is achieved, it may lead to a separate species. However, reproductive isolation between hybrids and their parents is particularly difficult to achieve and thus hybrid speciation is considered an extremely rare event. The Mariana Mallard is thought to have arisen from hybrid speciation.

Hybridisation is an important means of speciation in plants, since polyploidy (having more than two copies of each chromosome) is tolerated in plants more readily than in animals.[37][38] Polyploidy is important in hybrids as it allows reproduction, with the two different sets of chromosomes each being able to pair with an identical partner during meiosis.[39] Polyploids also have more genetic diversity, which allows them to avoid inbreeding depression in small populations.[40]

Hybridization without change in chromosome number is called homoploid hybrid speciation. It is considered very rare but has been shown in Heliconius butterflies [41] and sunflowers. Polyploid speciation, which involves changes in chromosome number, is a more common phenomenon, especially in plant species.

Gene transposition as a cause

Theodosius Dobzhansky, who studied fruit flies in the early days of genetic research in 1930s, speculated that parts of chromosomes that switch from one location to another might cause a species to split into two different species. He mapped out how it might be possible for sections of chromosomes to relocate themselves in a genome. Those mobile sections can cause sterility in inter-species hybrids, which can act as a speciation pressure. In theory, his idea was sound, but scientists long debated whether it actually happened in nature. Eventually a competing theory involving the gradual accumulation of mutations was shown to occur in nature so often that geneticists largely dismissed the moving gene hypothesis.[42]

However, 2006 research shows that jumping of a gene from one chromosome to another can contribute to the birth of new species.[43] This validates the reproductive isolation mechanism, a key component of speciation.[44]

Human speciation

Humans have genetic similarities with chimpanzees and bonobos, their closest relatives, suggesting common ancestors. The central idea of biological evolution is that all life on Earth shares a common ancestor. Evolution means that we are all distant cousins: humans and oak trees, hummingbirds and whales. Human beings are all descendants of a LUCA, the last universal common ancestor. The LUCA splits into branches of different species. This split is called a speciation event. Over a large number of years, evolution produces diversity in forms of life due to evolutionary changes, such as: gene flow, mutations, migration, genetic drift, and natural selection.[45]

The variants in shared ancestral species is said to be due to multiple genetic lineages. Roughly one-quarter of our genome shares no immediate ancestry with chimpanzees. It was determined that the human genetic lineage must have started evolving before the differentiation of humans, chimps, and gorillas.[46]

Analysis of genetic drift and recombination using a Markov model suggests humans and chimpanzees speciated apart 4.1 million years ago.[47] Even though there are similarities an article demonstrates that the human genome is a mosaic with respect to evolutionary history. The variants in shared ancestral species is said to be due to multiple genetic lineages.[46]

Darwin's dilemma

In addressing the question of the origin of species, there are two key issues: (1) what are the evolutionary mechanisms of speciation, and (2) what accounts for the separateness and individuality of species in the biota? Since Darwin's time, efforts to understand the nature of species have primarily focused on the first aspect, and it is now widely agreed that the critical factor behind the origin of new species is reproductive isolation.[48] Next we focus on the second aspect of the origin of species.

Darwin's dilemma: Why do species exist?

In The Origin of Species, Charles Darwin interpreted biological evolution in terms of natural selection, but was perplexed by the clustering of organisms into species.[49] Chapter 6 of Darwin's book is entitled "Difficulties of the Theory". In discussing these "difficulties" he noted "First, why, if species have descended from other species by fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion, instead of the species being, as we see them, well defined?" This dilemma can be referred to as the absence or rarity of transitional varieties in habitat space.

Another dilemma, related to the first one, is the absence or rarity of transitional varieties in time (see diagram at the bottom of the page). Darwin pointed out that by the theory of natural selection "innumerable transitional forms must have existed", and wondered "why do we not find them embedded in countless numbers in the crust of the earth." That clearly defined species actually do exist in nature in both space and time implies that some fundamental feature of natural selection operates to generate and maintain species.

The effect of sexual reproduction on species formation

It has been argued that the resolution of Darwin's dilemmas lies in the fact that out-crossing sexual reproduction has an intrinsic cost of rarity.[50][51][52][53][54] The cost of rarity arises as follows. If, on a resource gradient, a large number of separate species evolve, each exquisitely adapted to a very narrow band on that gradient, each species will, of necessity, consist of very few members. Finding a mate under these circumstances may present difficulties when many of the individuals in the neighborhood belong to other species. Under these circumstances, any species that happens, by chance, to increase in population size (at the expense of one or other of its neighboring species, if the environment is saturated), this will immediately make it easier for its members to find sexual partners. The members of the neighboring species, whose population sizes have decreased, will experience greater difficulty in finding mates, and therefore form pairs less frequently than in the larger species. This has a snowball effect, with large species growing at the expense of the smaller, rarer species, eventually driving them to extinction. Eventually, only a few species remain, each distinctly different from the other.[50][51][53] The cost of rarity not only involves the costs of failure to find a mate, but also indirect costs such as the cost of communication in seeking out a partner at low population densities.

A photograph of an African pygmy kingfisher, showing details of appearance and coloration that are shared by all African pygmy kingfishers to a high degree of fidelity.[55] A single illustration such as this is generally all that is required in an encyclopedia or field guide to describe, in intricate detail, the appearance of all the members of any given sexual species.

Bernstein et al.[51] argue furthermore that if an environmental gradient is populated by a single species which is perfectly adapted to only a small portion of that environment, it will be difficult for better-adapted individuals to pass their adaptation on to others in region. Such advantageous characteristics are unlikely to be due to a single altered gene, but rather to a combination of several altered genes, each of which, on its own, imparts little or no benefit to its carrier. If such an individual mates with a randomly selected mate, the advantageous combination of genes will be broken up, and the advantage lost, unless it happens to mate with another individual with the same advantageous combination of altered genes. This will be an exceptionally rare event, the consequence of which is that the species will be resistant to change over time or to the budding off of new species.

Rarity brings with it other costs. A rare or unusual feature is very seldom advantageous. In most instances, it will be indicative of a (non-silent) mutation, which is almost certain to be deleterious. It therefore behooves sexual creatures to avoid mates sporting rare or unusual features.[33][34] Should this be the case, then sexual populations will rapidly shed rare or peripheral phenotypic features, thus canalizing the entire external appearance, as illustrated in the accompanying illustration of the African pygmy kingfisher, Ispidina picta. This remarkable uniformity of all the members of a sexual species has stimulated the proliferation of field guides on birds, mammals, reptiles, insects, and many other taxons, in which each species can be described in full by means of a single illustration (or a pair of illustrations if there is sufficient sexual dimorphism). Once a population has become as homogeneous in appearance as is typical of most species (and is illustrated in the photograph of the African pygmy kingfisher), its members will avoid mating with members of other populations that look different from themselves.[35] Thus, the avoidance of mates displaying rare and unusual phenotypic features inevitably leads to reproductive isolation, one of the hallmarks of speciation.[21][56][57][58]

In the contrasting case of organisms that reproduce asexually, there is no cost of rarity; consequently, there are only benefits to fine-scale adaptation. Thus, asexual organisms very frequently show the continuous variation in form (often in many different directions) that Darwin expected evolution to produce, making their classification into "species" (more correctly, "morphospecies") very difficult.[33][34][51][59][60][61]

Punctuated evolution

Evolution is imposed on species or groups. It is not planned or striven for in some Lamarckist way.[62] The mutations on which the process depends are random events, and, except for the "silent mutations" which do not affect the functionality or appearance of the carrier, are thus usually disadvantageous, and their chance of proving to be useful in the future is vanishingly small. Therefore, while a species or group might benefit from being able to adapt to a new environment by accumulating a wide range of genetic variation, this is to the detriment of the individuals who have to carry these mutations until a small, unpredictable minority of them ultimately contributes to such an adaptation. Thus, the capability to evolve is a group adaptation, a concept discredited by (for example) George C. Williams,[63] John Maynard Smith[64] and Richard Dawkins[65][66][67][68] as selectively disadvantageous to the individual. .

If sexual creatures avoid mutant mates with strange or unusual characteristics,[33][34][69][32] then mutations that affect the external appearance of their carriers will seldom be passed on to the next and subsequent generations. They will therefore seldom be tested by natural selection. Evolution is, therefore, effectively halted or slowed down considerably. The only mutations that can accumulate in a population are ones that have no noticeable effect on the outward appearance and functionality of their bearers (i.e., they are "silent" or "neutral mutations", which can be, and are, used to trace the relatedness and age of populations and species.[33][70])

This implies that evolution can only occur if mutant mates cannot be avoided, as a result of a severe scarcity of potential mates. This is most likely to occur in small, isolated communities. These occur most commonly on small islands, in remote valleys, lakes, river systems, or caves,[71] or during the aftermath of a mass extinction.[70] Under these circumstances, not only is the choice of mates severely restricted but population bottlenecks, founder effects, genetic drift and inbreeding cause rapid, random changes in the isolated population's genetic composition.[71] Furthermore, hybridization with a related species trapped in the same isolate might introduce additional genetic changes. If an isolated population such as this survives its genetic upheavals, and subsequently expands into an unoccupied niche, or into a niche in which it has an advantage over its competitors, a new species, or subspecies, will have come in being. In geological terms this will be an abrupt event. A resumption of avoiding mutant mates will, thereafter, result, once again, in evolutionary stagnation.

Thus the fossil record of an evolutionary progression typically consists of species that suddenly appear, and ultimately disappear, in many cases close to a million years later, without any change in external appearance.[70][72][73] Graphically, these fossil species are represented by horizontal lines, whose lengths depict how long each of them existed. The horizontality of the lines illustrates the unchanging appearance of each of the fossil species depicted on the graph. During each species' existence new species appear at random intervals, each also lasting many hundreds of thousands of years before disappearing without a change in appearance. The exact relatedness of these concurrent species is generally impossible to determine. This is illustrated in the following diagram depicting the evolution of modern humans from the time that the Hominins separated from the line that led to the evolution of our closest living primate relatives, the chimpanzees.[73]


Hominin species distributed through time
Homo Australopithecus Ardipithecus Paranthropus Homo sapiens Homo heidelbergensis Homo erectus Paranthropus robustus Paranthropus boisei Paranthropus aethiopicus Homo ergaster Homo habilis Australopithecus sediba Australopithecus garhi Australopithecus africanus Australopithecus bahrelghazali Australopithecus afarensis Australopithecus anamensis Sahelanthropus Pleistocene Pliocene Miocene

For similar evolutionary time lines see, for instance, the paleontological list of African dinosaurs, Asian dinosaurs, the Lampriformes and Amiiformes.

See also

References

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