Flycatcher song in allopatry and sympatry – convergence, divergence and reinforcement

Abstract

The theory of reinforcement predicts that natural selection against the production of unfit hybrids favours traits that increase assortative mating. Whether culturally inherited traits, such as bird song, can increase assortative mating by reinforcement is largely unknown. We compared songs of pied (Ficedula hypoleuca) and collared flycatchers (F. albicollis) from two hybrid zones of different ages with songs from allopatric populations. Previously, a character divergence in male plumage traits has been shown to reinforce premating isolation in sympatric flycatchers. In contrast, we find that the song of the pied flycatcher has converged towards that of the collared flycatcher (mixed singing). However, a corresponding divergence in the collared flycatcher shows that the species differences in song characters are maintained in sympatry. Genetic analyses suggest that mixed song is not caused by introgression from the collared flycatcher, but rather due to heterospecific copying. Circumstantial evidence suggests that mixed song may increase the rate of maladaptive hybridization. In the oldest hybrid zone where reinforcement on plumage traits is most pronounced, the frequency of mixed singing and hybridization is also lowest. Thus, we suggest that reinforcement has reduced the frequency of mixed singing in the pied flycatcher and caused a divergence in the song of the collared flycatcher. Whether a culturally inherited trait promotes or opposes speciation in sympatry may depend on its plasticity. The degree of plasticity may be genetically determined and accordingly under selection by reinforcement.

Songs of Darwin's finches diverge when a new species enters the community

“…When introgression occurs, the species exchange one component (genes) but not the other (song). In this respect they differ from some other species where hybrids sing elements of the songs of both of the parental species (43,55). Thus in contrast to many other species of birds the defining feature of a population of Darwin's finches is a cultural learned signal.…”

Early song discrimination by nestling sparrows in the wild

“…When closely related species come into contact, courtship signals could diverge as a consequence of reproductive character displacement (Grant and Grant, 2010;Kirschel et al, 2009;Seddon, 2005), or converge when there is no selection against heterospecific copying (Haavie et al, 2004;Laiolo, 2012;Secondi et al, 2003;Sorjonen, 1986;Tobias and Seddon, 2009;Vokurková et al, 2013). We currently lack a mechanistic explanation for these contrasting patterns of song divergence versus convergence; the presence or absence of early song discrimination could be one such mechanism that determines whether cultural traits diverge or converge.…”

Natural and sexual selection against hybrid flycatchers

“…Both species are sexually dimorphic in plumage and males of the two species differ in plumage characters (Svensson 1992) and song (e.g. Haavie et al 2004), both of which are important in species recognition (Saether et al 1997;Wiley et al 2005;Qvarnström et al 2006). Males generally arrive at the breeding grounds a week earlier than females (Pärt & Gustafsson 1989) and start to defend a territory.…”