Yet another installment of dry biology …
Hybridization occurs when a species introduces its alleles to another species. Many hybrids are sterile and do not survive to form their own species. This can be overcome by polyploidy, the doubling of chromosomes in alleles. Polyploids are interfertile among themselves, but not with the parent species. Hence, a new species is born!
Selection for hybrids is typically not in their favor. Often they are unable to overcome ecological competition or find a mate (two species with the same ecological need cannot coexist!).
Areas in which hybridization is common is called a “hybrid zone”. These are examples of “stepped clines”. A cline is a geographic gradient in the frequency of genes; in a stepped cline, the gradient is more dramatic.
In a primary hybrid zone, two populations that have been kept apart come back together. Either reproductive isolation is complete or incomplete. If it is complete, then the two populations cannot mate and speciation has occurred. If it is incomplete, the two populations can mate and produce hybrids. If these hybrids are unfavorable, the two populations will continue to separate and form new species by parapatric or sympatric speciation.
In a secondary hybrid zone, two previously separate populations which speciated allopatrically come in contact and cross-fertilize. Isolation is increased if the hybrids are unfavorable.
A graduate slide back into the same species is possible by gene flow.
An area in which hybridization with selection against hybrids is called a “tension zone.”
In practice it is difficult to distinguish between secondary and primary hybrid zones. Secondary hybrid zones are believed to be more common. Certain landscapes are more apt to have hybrid zones than others.
In nature, certain hybrid zones demonstrate the existence of “novel alleles”, alleles that were not present in the parents. It is probably that natural selection favors these alleles.
But when do we define a new species?
Some examples of hybrid animals:
Mule - female horse / male donky
Hinny - female donkey / male horse
Wolfdog - domestic dog / wolf
Beefalo - American bison / domestic cow
Liger (or Tigon) - Tiger / Lion
Cama - camel / llama
Wolphin - false killer whale / bottlenose dolphin
Related posts:
0 Responses to “hybrids… a little late”