The process of speciation is completed with the cessation of genetic exchange.
— Peter R. Grant
Almost nothing is known from hybridization studies about the inheritance of courtship behavior of females, or of their responsiveness to particular male signals.
Plumage features constitute a major component of courtship signals.
Exchange of breeding individuals between two populations tends to homogenize their gene pools.
The independent role of morphology in mate choice is revealed by the rare instances where the usual association between song and morphology is disrupted.
We observe closely related species in sympatry and infer how they evolved from a common ancestor.
The theory of founder effects does not explain how novel features like plumage traits arise.
Males transmit signals in courtship through behavioral displays.
Islands are known to differ in the food supply available to ground finches, mainly seeds.
Genes that underlie the capacity to receive, use and transmit information are the evolving properties.
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.
Closely related species of birds are also chromosomally similar.
Evidence of epistasis from hybridization studies is more scarce.
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.
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.
Species can be recognized by their morphological characteristics and songs.
