Punctuated evolution in the learned songs of African sunbirds

Learned traits are thought to be subject to different evolutionary dynamics than other phenotypes, but their evolutionary tempo and mode has received little attention. Learned bird song has been thought to be subject to rapid and constant evolution. However, we know little about the evolutionary modes of learned song divergence over long timescales. Here, we provide evidence that aspects of the territorial songs of Eastern Afromontane sky island sunbirds Cinnyris evolve in a punctuated fashion, with periods of stasis of the order of hundreds of thousands of years or more, broken up by evolutionary pulses. Stasis in learned songs is inconsistent with learned traits being subject to constant or frequent change, as would be expected if selection does not constrain song phenotypes over evolutionary timescales. Learned song may instead follow a process resembling peak shifts on adaptive landscapes. While much research has focused on the potential for rapid evolution in bird song, our results suggest that selection can tightly constrain the evolution of learned songs over long timescales. More broadly, these results demonstrate that some aspects of highly variable, plastic traits can exhibit punctuated evolution, with stasis over long time periods.

Seasonality and social factors, but not noise pollution, influence the song characteristics of two leaf warbler species

Changes in the acoustic signalling of animals occupying urban ecosystems is often associated with the masking effects of noise pollution, but the way in which they respond to noise pollution is not straightforward. An increasing number of studies indicate that responses can be case specific, and some species have been found to respond differently to high levels of natural versus anthropogenic noise, as well as different levels of the latter. While the perception of noise between species may vary with its source, amplitude and temporal features, some species may possess broader environmental tolerance to noise pollution, as they use higher frequency vocalizations that are less masked by low-frequency urban noise. In this study, we explored the song variation of two closely related leaf warblers, the Common Chiffchaff Phylloscopus collybita and the Willow Warbler Phylloscopus trochilus, inhabiting urban green spaces and nonurban forests. The main goal of our study was to evaluate the impact of moderate levels of noise pollution on the songs of species which use higher frequency vocalizations and large frequency bandwidth. Previous studies found that the Common Chiffchaff modified their song in response to intense noise pollution, while no such data is available for the Willow Warbler. However, the majority of urban green spaces, which serve as wildlife hot spots in urban environments are usually polluted with moderate noise levels, which may not mask the acoustic signals of species that communicate with higher frequency. We analysed the spectral and temporal song parameters of both warblers and described the ambient noise present in males’ territories. Additionally, we looked at the social and seasonal aspects of bird song, since there is more than just noise in urban ecosystems which may affect acoustic communication. We found no evidence for noise-related bird song divergence in either species, however, we showed that social factors, time of day and season influence certain Common Chiffchaff and Willow Warbler song characteristics. Lack of noise-related bird song divergence may be due to the relatively low variation in its amplitude or other noise features present within the song frequency range of the studied species. Similar results have previously been shown for a few songbird species inhabiting urban ecosystems. Although in many cases such results remain in the shadow of the positive ones, they all contribute to a better understanding of animal communication in urban ecosystems.

Song divergence indicates an unclear relationship between the Neotropical and Nearctic Horned Larks

Background Some studies have indicated that the Horned Larks (Eremophila alpestris) should be considered as a species complex. Recently it split into four species or clades and seven subclades based on genetic, morphological, and biogeographic data. However, other aspects like song divergence have not been studied and several subspecies have not been evaluated, leading to important information gaps in this group of birds. In this work, we aimed to assess the differences in song traits and playback response between the Nearctic subclade and the Neotropical or Colombian subspecies E. a. peregrina. Methods We compared six song traits between these groups and performed field playback experiments, to test the response of the Neotropical larks to both songs. We tested the difference in the variables for separate as well as by principal component analysis (PCA). Results We found significant differences (p < 0.05) in the individual song traits and the PCA analysis between the two groups. Further, the PCA analysis showed a clearer divergence of the Neotropical songs in comparison to the Nearctic songs of different locations within North America. Similarly, the playback analysis showed a significantly lower response of E. a. peregrina to the songs of the Nearctic larks. Conclusions Besides this song divergence, there are important ecological and biogeographic differences between the Neotropical and Nearctic Horned Larks, that indicate an unclear relationship between these two groups. Thus, further morphological and genetic studies are required to clarify the taxonomy of the Neotropical Horned Lark and define if they share the same evolutionary history as the other subspecies of the Nearctic subclade.

Sympatry drives colour and song evolution in wood-warblers (Parulidae)

Closely related species often exhibit similarities in appearance and behaviour, yet when related species exist in sympatry, signals may diverge to enhance species recognition. Prior comparative studies provided mixed support for this hypothesis, but the relationship between sympatry and signal divergence is likely nonlinear. Constraints on signal diversity may limit signal divergence, especially when large numbers of species are sympatric. We tested the effect of sympatric overlap on plumage colour and song divergence in wood-warblers (Parulidae), a speciose group with diverse visual and vocal signals. We also tested how number of sympatric species influences signal divergence. Allopatric species pairs had overall greater plumage and song divergence compared to sympatric species pairs. However, among sympatric species pairs, plumage divergence positively related to the degree of sympatric overlap in males and females, while male song bandwidth and syllable rate divergence negatively related to sympatric overlap. In addition, as the number of species in sympatry increased, average signal divergence among sympatric species decreased, which is likely due to constraints on warbler perceptual space and signal diversity. Our findings reveal that sympatry influences signal evolution in warblers, though not always as predicted, and that number of sympatric species can limit sympatry's influence on signal evolution.

Inferring punctuated evolution in the learned songs of African sunbirds

Signals used in animal communication, especially those that are learned, are thought to be prone to rapid and/or regular evolution. It has been hypothesized that the evolution of song learning in birds has resulted in elevated diversification rates, as learned song may be subject to especially rapid evolution, and song is involved in mate choice. However, we know little about the evolutionary modes of learned song divergence over timescales relevant to speciation. Here we provide evidence that aspects of the territorial songs of Eastern Afromontane sky island sunbirds Cinnyris evolve in a punctuated fashion, with periods of stasis, on the order of hundreds of thousands of years or more, broken up by strong evolutionary pulses. Stasis in learned songs is inconsistent with learned traits being subject to constant or frequent change, as would be expected if selection does not constrain song phenotypes, or if novel phenotypes are frequently advantageous. Learned song may instead follow a process resembling peak shifts on adaptive landscapes. While much research has focused on the potential for rapid evolution in bird song, our results suggest that selection can tightly constrain the evolution of learned songs over fairly long timescales. More broadly, these results demonstrate that some aspects of highly variable, plastic traits can exhibit punctuated evolution, with stasis over fairly long time periods.

Parallel genomic architecture underlies repeated sexual signal divergence in Hawaiian Laupala crickets

When the same phenotype evolves repeatedly, we can explore the predictability of genetic changes underlying phenotypic evolution. Theory suggests that genetic parallelism is less likely when phenotypic changes are governed by many small-effect loci compared to few of major effect, because different combinations of genetic changes can result in the same quantitative outcome. However, some genetic trajectories might be favoured over others, making a shared genetic basis to repeated polygenic evolution more likely. To examine this, we studied the genetics of parallel male mating song evolution in the Hawaiian cricket Laupala . We compared quantitative trait loci (QTL) underlying song divergence in three species pairs varying in phenotypic distance. We tested whether replicated song divergence between species involves the same QTL and whether the likelihood of QTL sharing is related to QTL effect size. Contrary to theoretical predictions, we find substantial parallelism in polygenic genetic architectures underlying repeated song divergence. QTL overlapped more frequently than expected based on simulated QTL analyses. Interestingly, QTL effect size did not predict QTL sharing, but did correlate with magnitude of phenotypic divergence. We highlight potential mechanisms driving these constraints on cricket song evolution and discuss a scenario that consolidates empirical quantitative genetic observations with micro-mutational theory.

Parallel genomic architecture underlies repeated sexual signal divergence in Hawaiian Laupala crickets

ABSTRACTWhen the same phenotype evolves repeatedly, we can explore the predictability of genetic changes underlying phenotypic evolution. Theory suggests that genetic parallelism is less likely when phenotypic changes are governed by many small-effect loci compared to few of major effect, because different combinations of genetic changes can result in the same quantitative outcome. However, some genetic trajectories might be favored over others, making a shared genetic basis to repeated polygenic evolution more likely. To examine this, we studied the genetics of parallel male mating song evolution in the Hawaiian cricket Laupala. We compared quantitative trait loci (QTL) underlying song divergence in three species pairs varying in phenotypic distance. We tested whether replicated song divergence between species involves the same QTL and the likelihood that sharing QTL is related to phenotypic effect sizes. Contrary to theoretical predictions, we find substantial parallelism in polygenic genetic architectures underlying repeated song divergence. QTL overlapped more than expected based on simulated QTL analyses. Interestingly, QTL effect size did not predict QTL sharing, but did correlate with magnitude of phenotypic divergence. We highlight potential mechanisms driving these constraints on cricket song evolution and discuss a scenario that consolidates empirical quantitative genetic observations with micro-mutational theory.

Complex spatiotemporal variation in processes shaping song variation

Understanding factors that contribute to song divergence bolsters our understanding of signal evolution and reproductive isolation. Hybrid zones often occur across environmental gradients; as such, they are excellent places to examine how signals diverge and how differentiation is maintained. We studied song variation across two hybrid zones, one old and one recent, of Tufted Titmouse (Baeolophus bicolor) and Black-crested Titmouse (B. atricristatus), across an environmental gradient where the two titmouse populations meet. In the recent zone, noise and vegetation structure were correlated with several song characteristics, but in the older zone, these features did not correlate despite similar gradients in song features. Our data, combined with previous studies, suggest that despite overall similarities in characteristics, songs in the older zone may be more shaped by sexual selection, whereas songs in the young zone are shaped by environment. Thus, even within the same species, processes shaping signal structure can vary spatially and temporally.