Sympatric speciation
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Sympatry is one of four theoretical models for the phenomenon of speciation. In complete contrast to allopatry, species undergoing sympatric speciation are not geographically isolated by, for example, a mountain or a river. The speciating populations generally share the same territory.
A debate almost since the beginning of popular evolutionary thought, sympatric speciation is still a highly contentious issue. By 1980 the theory was largely unfavourable given the void of empirical evidence available, and more critically the required conditions which scientists expect. Ernst Mayr, one of the foremost thinkers on evolution, completely rejected sympatry outright, ushering in a climate of hostility towards the theory. Since the 1980s, a more progressive ideology has been adopted. While still debatable, well documented empirical evidence now exists, and the development of sophisticated theories incorporating multilocus genetics have followed.
A number of models have been proposed to account for this mode of speciation. The most popular, disruptive speciation, was first put forward by John Maynard Smith in 1962. Maynard Smith suggested that homozygous individuals may, under particular environmental conditions, have a greater fitness than those with alleles heterozygous for a certain trait. Under the mechanism of natural selection, therefore, homozygosity would be favoured over heterozygosity, eventually leading to speciation.
There is also at least one well-known ecological phenomenon, allochrony, which offers some empirical evidence that sympatric speciation has taken place, as many examples exist of allochronic species which are each others' nearest relatives ("sister taxa").
Rhagoletis pomonella, the apple maggot, may be currently undergoing sympatric or, more precisely, heteropatric (see heteropatry) speciation. The apple feeding race of this species appears to have spontaneously emerged from the hawthorn feeding race in the 1800 - 1850 AD time frame, after apples were first introduced into North America. The apple feeding race does not now normally feed on hawthorns, and the hawthorn feeding race does not now normally feed on apples. This may be an early step towards the emergence of a new species.
Sympatric speciation events are vastly most common in plants when they double or triple their number of chromosomes, resulting in a condition called polyploidy.
A rare example of sympatric speciation in animals is the divergence of "resident" and "transient" Orca forms in the northeast Pacific. Resident and transient orcas inhabit the same waters, but avoid each other and do not interbreed. The two forms hunt different prey species and have different diets, vocal behaviour, and social structures.
[edit] See also
[edit] Reference
- ^ McPheron et al. 1988. Nature 336:64-66
- ^ Smith, D.C. 1988. Nature 336:66-67
- ^ Feder et al. 1988. Nature 336:61-64
- ^ Sympatric speciation in Nicaraguan crater lake cichlid fish. By: Barluenga, Marta; Stölting, Kai N.; Salzburger, Walter; Muschick, Moritz; Meyer, Axel. Nature, 2/9/2006, Vol. 439 Issue 7077, p719-723.
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Speciation guide
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| Basic concepts: species | chronospecies | speciation | cline |
| Modes of speciation: allopatric | peripatric | parapatric | sympatric | polyploidy | paleopolyploidy |
| Auxiliary mechanisms: sexual selection | assortative mating | punctuated equilibrium |
| Intermediate stages: hybrid | Haldane's rule | ring species |
