A major task for evolutionary biologists is to explain the origin of biodiversity.
Almost nothing is known from hybridization studies about the inheritance of courtship behavior of females, or of their responsiveness to particular male signals.
Although speciation is ecologically driven, reproductive (mating) interactions at secondary contact are far from irrelevant.
Closely related species of birds are also chromosomally similar.
Despite the evidence against reinforcement, the basis of premating isolation does not evolve entirely in allopatry.
Evidence of epistasis from hybridization studies is more scarce.
Exchange of breeding individuals between two populations tends to homogenize their gene pools.
For example, an ecologist's world can make perfect sense, in the short term at least, in the absence of evolutionary considerations.
Genes that underlie the capacity to receive, use and transmit information are the evolving properties.
If continental speciation is promoted by temporarily insular conditions the demography and genetics of founding populations may be of importance.
If evolutionary divergence in allopatry has been minor before the interchange, the members of the two populations are likely to treat each other as potential mates and interbreeding will ensue.
If postmating isolation evolves in allopatry, hybridization at the sympatric stage will be lacking, or at most extremely rare and of little consequence.
In broad outline the last stages of speciation are known, but the genetic basis to the physiological details of postmating isolation are not.
Islands are known to differ in the food supply available to ground finches, mainly seeds.
Males transmit signals in courtship through behavioral displays.
Plumage features constitute a major component of courtship signals.
Species can be recognized by their morphological characteristics and songs.
The divergence of songs in the new population away from those in the progenitor population would only be prevented if these processes were balanced by repeated immigration and subsequent breeding: song flow.
The evolution of reproductive isolation in Darwin's finches is therefore the evolution of differences in song and in morphology.
The independent role of morphology in mate choice is revealed by the rare instances where the usual association between song and morphology is disrupted.
The process of speciation is completed with the cessation of genetic exchange.
The rare interbreeding of species and the mating pattern of the hybrids provide further evidence of the importance of song in mate choice.
The theory of founder effects does not explain how novel features like plumage traits arise.
Thus mating of females was strictly along the lines of paternal song.
Thus the genetic basis to the origin of bird species is to be sought in the inheritance of adult traits that are subject to natural and sexual selection.
Thus the genetics of speciation of Darwin's finches through the evolution of reproductive isolation are very different from the genetics of Drosophila as described by Dobzhansky and elaborated by many others.
To summarize, the particular song a male sings, and the behavioral responses of females to song and morphological signals, are not genetically inherited in a fixed manner but are determined by learning early in life.
We observe closely related species in sympatry and infer how they evolved from a common ancestor.
With rare exceptions sympatric species pair and breed conspecifically, and as a result are reproductively isolated from each other.
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