Abstract
Hummingbirds have developed a remarkable diversity of learned vocalizations, from single-note songs to phonologically and syntactically complex songs. In this study we evaluated if geographic song variation of wedge-tailed sabrewings (Campylopterus curvipennis) is correlated with genetic divergence, and examined processes that explain best the origin of intraspecific song variation. We contrasted estimates of genetic differentiation, genetic structure, and gene flow across leks from microsatellite loci of wedge-tailed sabrewings with measures for acoustic signals involved in mating derived from recordings of males singing at leks throughout eastern Mexico. We found a strong acoustic structure across leks and geography, where lek members had an exclusive assemblage of syllable types, differed in spectral and temporal measurements of song, and song sharing decreased with geographic distance. However, neutral genetic and song divergence were not correlated, and measures of genetic differentiation and migration estimates indicated gene flow across leks. The persistence of acoustic structuring in wedge-tailed sabrewings may thus best be explained by stochastic processes across leks, in which intraspecific vocal variation is maintained in the absence of genetic differentiation by postdispersal learning and social conditions, and by geographical isolation due to the accumulation of small differences, producing most dramatic changes between populations further apart.
Highlights
Understanding the processes and mechanisms by which phenotypic diversity arises is fundamental in evolutionary biology
Most of research has focused in the role that natural selection can play in the processes of population divergence, an interest to study the role of stochastic processes involved in phenotypic and population divergence has raised lately [1,2,3,4,5]
Divergence in acoustic signals is interesting because population differences can accrue over short evolutionary timescales through learning and cultural evolution [3], and because the rapid change of a signal-receiver communication system could reduce gene flow between populations, promoting neutral genetic differentiation [6,7,8,9,10]
Summary
Understanding the processes and mechanisms by which phenotypic diversity arises is fundamental in evolutionary biology. Song divergence among populations can be driven by three non-mutually exclusive processes: (1) selection of particular acoustic traits due to environmental or habitat characteristics, where different frequencies of sound travel best in different environments or because ambient noise differences between environments [9,13,14,15,16,17,18,19]; (2) sexual, social and cultural selection, where this kind of selection might cause rapid evolution of acoustic signals involved in mating because the attractiveness of novelty, the potential for runaway change and the absence of well-defined optima [20,21]; and (3) genetic and cultural drift, where song divergence occurs in the absence of any kind of selection [22,23,24]. Hummingbirds have developed the trait of vocal learning [27], and they sing apparently single-note songs [29,30,31], there are species intermediate in vocal complexity [30,32,33], and species rivaling passerines with intricate, phonologically and syntactically complex songs [14,27,30,34,35,36,37,38]
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