Parapatric speciation

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Parapatric speciation , or parapatric speciation is a scientific model having a speciation (, also called species speciation of Latin type) of two new species from a species of origin in biogeographically adjacent partial regions of the spatially coherent distribution range of species of origin postulated. It practically forms the middle range between the allopatric speciation , in which the new species arise in spatially completely separate (called disjoint ) regions, and the sympatric speciation , in which they arise in the same area.

The term parapatric speciation was introduced in 1955 by the biologist Hobart Muir Smith in addition to the previously defined terms allopatric and sympatric speciation. Whether parapatric speciation occurs at all has long been controversial in evolutionary biology. The important evolutionary biologist Ernst Mayr considered them to be completely, or almost, impossible and not occurring in nature, only the allopatric speciation is important. Allopatric speciation is still considered to be the standard hypothesis of speciation, but most evolutionary biologists now also consider sympatric and parapatric speciation to be real. In a certain way, parapatric speciation is now often considered to be the normal case, since the prerequisites for perfect and strict allopatry or sympatry are possibly only rarely given in nature, so that the models originally thought of as strictly separate opposites are no longer regarded as equally useful .

The model

The parapatric speciation is based on an original species that inhabits a spatially extensive distribution area (area). Under these conditions, no permanent genetic differences develop within an ideal population , even in very large areas, as possible differences would be leveled out again due to the spatial mix due to migrating ( migrating ) individuals and their subsequent mating with individuals from the immigration area ( panmixie ). This factor that counteracts the differences is known as gene flow .

In fact, within spatially extended populations one sometimes observes hereditary, temporally permanent differences that can apparently sometimes persist despite the homogenizing influence of the gene flow. For example, a certain feature can be very gradual in different regions, in which, for example, individuals in the northern part of the area are slightly larger than in the southern, without a sharp boundary anywhere: this is called a kline . This is possible if either the individuals are not very mobile and prefer to mate only with close neighbors, or if a strong selection in different areas favors different characteristics, which means that migrating individuals (and their offspring) have less fitness in the "wrong" area . If there are further favorable circumstances, for example that individuals in regions for which they are particularly well adapted ecologically, also prefer each other as mating partners (this is called assortative mating ), the characteristics of populations in neighboring regions can possibly develop apart without this There should be a spatial separation between them, which prevents mating due to lack of contact. There are therefore feature differences ( divergence ) without spatial separation ( separation ). In the course of time, isolation mechanisms can develop between the genetically but not spatially separated populations , which then make mating not only less likely, but ultimately completely impossible, which would genetically isolate the two earlier subpopulations . This would have created two new species from one original species.

evidence

Parapatric speciation therefore presupposes that the splitting of a species is also possible if there is gene flow between the emerging species, and that the differences between the newly formed species later also compared to occasional mating due to imperfect isolation mechanisms that lead to introgression due to hybridization , can persist. There are now a number of convincing case studies for corresponding processes in nature, and they could also be made plausible on the basis of theoretical model assumptions. On the one hand, populations can be prevented from direct mating contact for reasons other than geographical, for example if plant clusters bloom on different soil types at different times. On the other hand, differences that ultimately lead to speciation can arise and persist under certain conditions even under the influence of moderate gene flow.

Individual evidence

  1. ^ Ernst Mayr (1982): Speciation and Macroevolution. Evolution 36 (6): 1119-1132.
  2. ↑ Jerry A. Coyne & H. Allen Orr: Speciation. Sinauer Publishers, Sunderland, Mass., USA, 2004. ISBN 0-87893-091-4
  3. ^ Roger K. Butlin, Juan Galindo, John W. Grahame (2008): Sympatric, parapatric or allopatric: the most important way to classify speciation? Philosophical Transactions of the Royal Society B 363: 2997-3007. doi: 10.1098 / rstb.2008.0076
  4. ↑ John A. Endler: Geographic Variation, Speciation, and Clines. Princeton University Press, 1977. ISBN 978-0-691-08192-2
  5. ^ BM Fitzpatrick, JA Fordyce, S. Gavrilets (2009): Pattern, process and geographic modes of speciation. Journal of Evolutionary Biology 22: 2342-2347. doi: 10.1111 / j.1420-9101.2009.01833.x
  6. ↑ Jody Hey (2006): Recent advances in assessing gene flow between diverging populations and species. Current Opinion in Genetics & Development 16: 592-596. doi: 10.1016 / j.gde.2006.10.005
  7. ↑ Patrik Nosil (2008): Speciation with gene flow could be common. Molecular Ecology 17: 2103-2106. doi: 10.1111 / j.1365-294X.2008.03715.x (open access).
  8. ↑ Chung-I Wu & Chau-Ti Ting (2004): Genes and Speciation. Natur Reviews Genetics 5: 114-122 doi: 10.1038 / nrg1269