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.
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Almost nothing is known from hybridization studies about the inheritance of courtship behavior of females, or of their responsiveness to particular male signals.
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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.
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Closely related species of birds are also chromosomally similar.
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Exchange of breeding individuals between two populations tends to homogenize their gene pools.
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We observe closely related species in sympatry and infer how they evolved from a common ancestor.
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Evidence of epistasis from hybridization studies is more scarce.
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Plumage features constitute a major component of courtship signals.
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Males transmit signals in courtship through behavioral displays.
