Speciation

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Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been 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] 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. Observed examples of each kind of speciation are provided throughout.[3]

Natural speciation[edit]

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.

All forms of natural speciation have taken place over the course of evolution; however it still remains a subject of debate 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[edit]

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.

Allopatric[edit]

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.

Observed instances

Island genetics, the tendency of small, isolated genetic pools to produce unusual traits, has been observed in many circumstances, including 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[edit]

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.

Observed instances

Parapatric[edit]

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.

Observed instances

Sympatric[edit]

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.[citation needed]

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]

Until recently,[when?] there has been a dearth of strong evidence that supports this form of speciation, with a general feeling that interbreeding would soon eliminate any genetic differences that might appear. But there has been at least one study, in 2008, that 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[edit]

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[edit]

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[edit]

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[edit]

Reinforcement[edit]

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[edit]

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]

Drosophila speciation experiment.svg

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]

Genetics[edit]

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.[31] It causes hybrid sterility between related subspecies.

Hybrid speciation[edit]

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.[32][33] 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.[34] Polyploids also have more genetic diversity, which allows them to avoid inbreeding depression in small populations.[35]

Hybridization without change in chromosome number is called homoploid hybrid speciation. It is considered very rare but has been shown in Heliconius butterflies [36] and sunflowers. Polyploid speciation, which involves changes in chromosome number, is a more common phenomenon, especially in plant species.

Gene transposition as a cause[edit]

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.[37]

However, 2006 research shows that jumping of a gene from one chromosome to another can contribute to the birth of new species.[38] This validates the reproductive isolation mechanism, a key component of speciation.[39]

Human speciation[edit]

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.[40][better source needed]

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.[41]

Analysis of genetic drift and recombination using a Markov model suggests humans and chimpanzees speciated apart 4.1 million years ago.[42] 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.[41]

Darwin's Dilemma[edit]

In addressing the question "what is 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 concept needed to understand the origin of new species is reproductive isolation.[43] Next we focus on the second aspect of the origin of species.

Darwin's dilemma: Why do species exist?[edit]

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.[44] 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. 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[edit]

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.[45][46][47][48][49] The cost of rarity arises from the simple requirement of out-crossing sex that it takes two individuals to mate. A cost-benefit analysis of sexual species splitting to form more species each with fewer individual organisms was performed.[45][46] The benefit of splitting is the finer adaptation of the smaller species resulting from the splits. By this approach, it was found that when a cost of rarity is introduced, a point of diminishing returns is reached with increasing splitting. When this point is reached, sexual species are stable and will not continue to split into smaller species. The analysis also indicated that new small, more finely adapted species cannot compete with larger species, even if moderately better adapted, because of the cost of rarity. 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 density. (See references[46][48] for numerous examples in nature.)

In the contrasting case of organisms that reproduce asexually, there is no cost of rarity; consequently there are only benefits to fine scale adaptation.

Punctuated evolution[edit]

Moderately adapted species that are already established are intrinsically stable because of the cost of rarity. Such species can only be replaced by new, substantially better adapted species. Potential emerging new species, that are only marginally better adapted than their established competitor(s), would have a high cost of rarity when they first arise. Thus, the few individuals in the better-adapted new species would be unable to compete unless their new adaptive features could compensate for the high initial cost of rarity. As a result, displacement of one species by another would have a quantum character. This view of species evolution is consistent with that of Eldridge and Gould,[50] that evolution is characterized by long periods of stasis punctuated by relatively rapid changes in species composition (see first diagram, above).

See also[edit]

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  50. ^ Eldridge N, Gould SJ. (1972) Punctuated equilibria: An alternative to phyletic gradualism. In Schopf, Thomas J. M. (1972). Models in paleobiology. San Francisco: Freeman, Cooper. ISBN 0-87735-325-5.  pp. 82–115.

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