Sympatry affects the evolution of genetic versus cultural determination of song

One hypothesis explaining the species richness of the songbirds is based on the fact that these species generally acquire their songs socially rather than genetically. Here we consider the outcome of secondary contact between birds that have developed distinct song dialects in allopatry. We ask 2 questions: 1) given a fixed probability of learning, how do the songs evolve in sympatry and 2) would evolution tend to increase or decrease the genetic, versus cultural, determination of song in this situation? We assume that the 2 local dialects can be both genetically determined and learned via song copying, whereas a third category of song, mixed song, is only learned. We evaluate both questions assuming 3 different ways that mixed song can be generated in addition to learning via copying. We find that when the probability of learning is fixed, local dialects can generally be maintained even when a substantial majority of males produce learned songs. Song differentiation will, however, be lost if song learning itself can evolve. This is partly a result of a positive feedback mechanism; mixed songs are readily accepted by all females, leading to selection favoring an allele which increases the probability that males will produce those songs. This in turn increases the frequency of mixed songs, making it harder for dialects to be maintained. As human disturbance alters species ranges, partially differentiated populations may increasingly come into contact; this work predicts that song differentiation may indeed tend to be lost under a significant proportion of these conditions. Copyright 2008, Oxford University Press.