Speciation

Speciation is the evolutionary process by which new biological species arise. There are four modes of natural speciation, based on the extent to which speciating populations are geographically isolated from one another: allopatric, peripatric, parapatric, and sympatric. Speciation may also be induced artificially, through animal husbandry or laboratory experiments. Observed examples of each kind of speciation are provided throughout.^[1]

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Natural speciation

All forms of natural speciation have taken place over the course of evolution, though it still remains a subject of debate as to the relative importance of each mechanism in driving biodiversity. ^[2]

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

Main article: allopatric speciation

During allopatric speciation, a population splits into two geographically isolated allopatric populations (for example, by habitat fragmentation due to geographical change such as mountain building or social change such as emigration). The isolated populations then undergo genotypic and/or phenotypic divergence as they (a) become subjected to dissimilar selective pressures or (b) they independently undergo genetic drift. When the populations come back into contact, they have evolved such that they are reproductively isolated and are no longer capable of exchanging genes.

Speciation occurs when a population is isolated from another population. The way is transformation when mutation and adaptation are a result from changing environmental conditions. The old species is then gradually replaced. The second way is divergence when the parent species arises one or more species that continues to exist.

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, like Komodo and Galápagos, the latter having given rise to the modern expression of evolutionary theory, after being observed by Charles Darwin. Perhaps the most famous example of allopatric speciation is Darwin's Galápagos Finches.

Peripatric

Main article: peripatric speciation

In peripatric speciation, new species are formed in isolated, small peripheral populations which 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

• Mayr bird fauna
• The Australian bird Petroica multicolor
• Reproductive isolation occurs in populations of Drosophila subject to population bottlenecking

Parapatric

Main article: parapatric speciation

In parapatric speciation, the zones of two diverging populations are separate but do overlap. There is only partial separation afforded by geography, so individuals of each species may come in contact or cross the barrier from time to time, but reduced fitness of the heterozygote leads to selection for behaviours or mechanisms which prevent breeding between the two species.

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

• Ring species
  • The Larus gulls form a ring species around the North Pole.
  • The Ensatina salamanders, which form a ring round the Central Valley in California.
  • The Greenish Warbler (Phylloscopus trochiloides), around the Himalayas.
• the grass Anthoxanthum has been known to undergo parapatric speciation in such cases as mine contamination of an area.

Sympatric

Main article: sympatric speciation

In sympatric speciation, species diverge while inhabiting the same place. Examples of sympatric speciation are found in insects which become dependent on different host plants in the same area.

  Polyploidy is a mechanism often attributed to causing some speciation events in sympatry. Not all polyploids are reproductively isolated from their parental plants, so an increase in chromosome number may not result in the complete cessation of gene flow between the incipient polyploids and their parental diploids (see also hybrid speciation).

Observed instances

Polyploidy is observed in many species of both plant and animal:

• wheat
• Salsify or goatsbeard
• Cichlids of Lake Victoria, Lake Tanganyika and Lake Malawi
• Xenopus laevis, an African frog

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.

Reinforcement

Reinforcement is the process by which natural selection increases reproductive isolation.^[3] 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 no further reproductive isolation and speciation has essentially occurred (e.g., as in horses and donkeys.) 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 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. And also without reinforcement they will have uncontrollable interbreeding. 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.^[4] 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.^[5]

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 such as light/dark and wet/dry. Each generation was placed into the maze, and the groups of flies which 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 would not breed with each other even when doing so was their only opportunity to reproduce.^[6]

Diane Dodd was also able to show allopatric speciation by reproductive isolation in Drosophila pseudoobscura fruit flies after only eight generations using different food types, starch and maltose.^[7] Dodd's experiment has been easy for many others to replicate, including with other kinds of fruit flies and foods.^[8]

The history of such attempts is described in Rice and Hostert (1993).^[9]

Genetics

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 arose from hybrid speciation.

Hybridization without change in chromosome number is called homoploid hybrid speciation. It is considered very rare but has been shown in Heliconius butterflies ^[10] and sunflowers. Polyploid speciation, which involves changes in chromosome number, is a more common phenomena, 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.^[11]

However, recent research shows that jumping of a gene from one chromosome to another can contribute to the birth of new species.^[12] This validates the reproductive isolation mechanism, a key component of speciation.^[13]

Interspersed repeats

Main article: Interspersed repeat

Interspersed repetitive DNA sequences function as isolating mechanisms. These repeats protect newly evolving gene sequences from being overwritten by gene conversion, due to the creation of non-homologies between otherwise homologous DNA sequences. The non-homologies create barriers to gene conversion. This barrier allows nascent novel genes to evolve without being overwritten by the progenitors of these genes. This uncoupling allows the evolution of new genes, both within gene families and also allelic forms of a gene. The importance is that this allows the splitting of a gene pool without requiring physical isolation of the organisms harboring those gene sequences.

Human speciation

Humans have genetic similarities with chimpanzees and gorillas, suggesting common ancestors. Analysis of genetic drift and recombination using a Markov model suggests humans and chimpanzees speciated apart 4.1 million years ago.^[14]

See also

• Species problem
• Heteropatry
• Chronospecies
• Frozen plasticity
• Koinophilia

References

Further reading

• Coyne, J. A. & Orr, H. A. (2004). Speciation. Sunderlands, Massachusetts: Sinauer Associates, Inc. ISBN 0-87893-089-2. 
• Grant, V. (1981). Plant Speciation, 2nd Edit., New York: Columbia University Press. ISBN 0-231-05113-1. 
• Mayr, E. (1963). Animal Species and Evolution. Harvard University Press. ISBN 0-674-03750-2
• White, M. J. D. (1978). Modes of Speciation. San Francisco, California: W. H. Freeman and Company. ISBN 0-716-70284-3.