Sympatric speciation
As sympatric speciation (also: sympatric speciation or sympatric species emergence ) one describes the emergence of new species in the area of the original species (s). The term was by the evolutionary biologist Ernst Mayr coined.
Sympatric speciation and gene flow
In sympatric speciation, similar to allopatric speciation , genetic isolation can play an important role (example: in speciation through polyploidization , the gene flow is interrupted). It has long been controversial whether speciation can take place at all without interrupting the gene flow (i.e. without isolation). Some (especially older) theoretical models predict that speciation cannot take place without prior isolation. But now there are more advanced models that predict sympatric speciation without interrupting the gene flow if two conditions are met:
- strong selection,
- the choice of sexual partner must be correlated with the factor that determines the selection .
In addition to the theoretical predictions of these models, there is a growing number of publications that indicate that sympatric speciation with gene flow is not only theoretical, but also real:
A first type of evidence of sympatric speciation comes from studies examining differentiations within species. Such studies could show that a genetic differentiation into different ecotypes or morphs within the same species, i.e. a preliminary stage for speciation, can arise even without geographical isolation. Such studies may reveal species as they evolve.
A second type of evidence of sympatric speciation comes from studies that investigate already differentiated sister species . In these species the species splitting is already in the past. In order to rule out allopatric speciation to a large extent, sister species are investigated which are restricted to a small common area of distribution and whose species splitting up in the past did not present any geographical barriers.
Examples of sympatric diversification within a species (species in the making)
Birds
In the middle-ground finch , two morphs with beaks of different sizes were observed, the females of which preferably mate with males of the same morph. Singing differences due to the different beak sizes were given as the reason for this “preference” when choosing a partner .
A new population of blackcap has developed over the last few decades, which overwinters in the British Isles instead of visiting the traditional winter quarters (North Africa, South Spain). One possible reason for this behavior is seen in winter feeding of songbirds, which is particularly popular in the UK. With isotope analyzes it could be shown that both populations are reproductively isolated from one another, although they differ neither morphologically nor in behavior otherwise. But it is clear (from other migratory bird research) that hybrids would take an intermediate migratory route between them, which would lead them from Central Europe out to the open Atlantic. The slightly shifted breeding period could represent a possible isolation mechanism between the populations.
Examples of completed sympatric speciation
Kentia palms
During the sympatric speciation on the isolated Lord Howe Island , two palm species emerged from one parent species ( Kentia palm ). Adaptation to different substrates and shifting the flowering period played an important role in this case.
fishes
Sympatric speciation can be found in cichlids in isolated lakes, e.g. B. in the Apoyo crater lake (Nicaragua) as well as in the Barombi Mbo and in the Bermin crater lake (both in Cameroon). The cichlid species of these lakes each descend from an immigrant species, but today they differ significantly in their morphology and ecological niche . A allopatric speciation can be ruled out in these small crater lakes.
Sympatric speciation through homoploid hybridization
Phytophagous bored flies meet their mating partners on their host plant. An immigrant plant species offers hybrids a new food source and is at the same time a separate meeting point for the new hybridogenic drill fly.
In contrast to the examples in the following section (polyploidization), the number of chromosomes in the boredfly hybrids is not increased ( homoploid ). The sympatric speciation with a change to a new host can also be understood as a borderline case to a very small-scale allopatric speciation .
Sympatric speciation through polyploidization
A distinction is made between two forms of speciation through polyploidization :
- Allopolyploidisierung ( allo = different): After an interspecific hybridisation (or genus junction), which normally (to infertile sterile leads) hybrids, produced by (random) doubling of chromosomes equivalents (genomic mutation) a polyploid organisms normal fertility shows. It can be tetraploid (four sets of chromosomes ) or amphidiploid (two different double sets of chromosomes). Examples: Culture wheat , Jostabeere .
- Autopolyploidization: ( auto = self): Here the duplication of the chromosome set takes place without a previous crossing of species. The offspring are strictly tetraploid (four identical sets of chromosomes). Example: Tetra rye .
Species formation through polyploidization is a sudden isolation process that starts with individuals and leads to new species that cannot be fertile crossed with the original forms. This species formation only plays a greater role in the plant world , as polyploid animals almost always have disorders in the formation of their sexes.
A common case in the speciation of plants is based on species that normally only reproduce asexually, but which rarely and in exceptional cases can lead to fertile crosses. Each species here consists of the offspring of such a cross (so it is a clone). This speciation mechanism very quickly leads to a split into numerous species, which are mostly more or less distributed locally and can be very similar to one another. In botany these are often called "small species". Species formation according to this pattern has produced a considerable percentage of the plant species in Central Europe, whereby these small species can often only be identified by a few specialists. The genera blackberry ( Rubus ), dandelion ( Taraxacum ) and hawkweed ( Hieracium ), which are very species-rich and notoriously critical to their identification, owe their abundance of species to this mechanism.
See also
swell
- Species and their formation . In: WK Purves, D. Sadava, GH Orians, HC Heller (eds.): Biology . 7th edition. Spectrum academic publisher, ISBN 978-3-8274-2007-7 , pp. 585-588 .
- ^ GF Turner, MT Burrows: A Model of Sympatric Speciation by Sexual Selection Proceedings of the Royal Society B, 260 (1359), pp. 287-292 doi : 10.1098 / rspb.1995.0093
- ^ Sarah K. Huber: Reproductive isolation of sympatric morphs in a population of Darwin's finches. Proceedings of the Royal Society B, published online on May 15, 2007, doi : 10.1098 / rspb.2007.0224
- ↑ Bearhop, S. et al. (2005): Assortative Mating as a Mechanism for Rapid Evolution of a Migratory Divide. Science 310: 502-504. online at www.sciencemag.org (registration required)
- ^ V. Savolainen, MC Anstett, C. Lexer, I. Hutton, JJ Clarkson, MV Norup, MP Powell, D. Springate, N. Salamin, WJ Baker: Sympatric speciation in palms on an oceanic island. Nature 441 (7090): 210-213 doi : 10.1038 / nature04566
- ↑ Marta Barluenga, Kai N. Stölting, Walter Salzburger, Moritz Muschick, Axel Meyer: Sympatric speciation in Nicaraguan crater lake cichlid fish. Nature 439: 719-723 doi : 10.1038 / nature04325 , February 9, 2006
- ^ Ulrich K. Schliewen, Barbara Klee: Reticulate sympatric speciation in Cameroonian crater lake cichlids. Frontiers in Zoology 2004, 1: 5 doi : 10.1186 / 1742-9994-1-5
- ↑ Ulrich K. Schliewen, Diethard Tautz, Svante Pääbo: Sympatric speciation suggested by monophyly of crater lake cichlids. Nature 368, 629-632 doi : 10.1038 / 368629a0
- ↑ Dietmar Schwarz, Benjamin M. Matta, Nicole L. Shakir-Botteri, Bruce A. McPheron: Host shift to an invasive plant triggers rapid animal hybrid speciation Nature 436, 546-549 doi : 10.1038 / nature03800