Global song divergence in barn swallows (Hirundo rustica): exploring the roles of genetic, geographical and climatic distance in sympatry and allopatry

Abstract

Divergence in acoustic signals plays an important role in the production and maintenance of biodiversity in numerous taxa. In this study, we assess patterns of acoustic divergence in geographically isolated and sympatric subspecies of barn swallows (Hirundo rustica), including analyses of whether song differentiation varies with geographical isolation, genetic distance and climatic distance. We provide the first description of geographical variation in song among five of six currently recognized barn swallow subspecies. Temporal traits describing terminal trills were the most distinct song traits among subspecies, adding to growing evidence that trills are important in speciation among many birds, insects and fish. Across a ~6000 km transect of Russia, acoustic distance was predicted by genetic and geographical distance, but not climatic distance. We also found no reproductive character displacement of song traits in a contact zone between H. r. rustica and H. r. tytleri. Based on patterns discovered in this study, we infer an important role of sexual selection, genetic and/or cultural drift in the gradual build-up of acoustic divergence, which is accelerated in small populations.

Analysis of female song provides insight into the evolution of sex differences in a widely studied songbird

“…This is consistent with the theory that elaborate female song may be selected against in certain species (Kleindorfer et al, 2016a; Wallace, 1868), and provides a mechanism supporting the loss of song in many North Temperate female birds (Odom et al, 2014). This is significant because, while the vast literature on male birdsong has informed many aspects of sexual selection theory (Gil & Gahr, 2002; Nowicki et al, 2002), and male song has been shown to play a key role in premating isolation and speciation (Alcaide et al, 2014; Irwin et al, 2001; Sosa-López et al, 2016; Wilkins et al, 2013, 2018), we know relatively little about female vocal signaling, even in classic study systems like barn swallows.…”

Evolution of communication signals and information during species radiation

“…whether species-specific information is selected for or against -if at all-, remains an open question 4 , 5 . Previous studies on signal divergence have emphasized the direct role of natural and social/sexual selection (‘sensory drive model’) 6 – 14 , the indirect consequences of ecological selection on traits related to signal production (‘magic traits’) 15 – 17 , as well as the random effects of neutral mechanisms (genetic and cultural drifts) 4 , 18 . The relative weight of these processes is still debated 4 , 5 , 7 , 18 .…”

“…Previous studies on signal divergence have emphasized the direct role of natural and social/sexual selection (‘sensory drive model’) 6 – 14 , the indirect consequences of ecological selection on traits related to signal production (‘magic traits’) 15 – 17 , as well as the random effects of neutral mechanisms (genetic and cultural drifts) 4 , 18 . The relative weight of these processes is still debated 4 , 5 , 7 , 18 . For instance, while between-species competition may be a strong driver of signal divergence by promoting niche partitioning 19 – 21 , some studies advocate that competition has little effect 22 , 23 .…”

“…Similarly, while sexual selection is emphasized as an important driver of signal divergence 13 , 27 , 28 , it could be of secondary importance compared to neutral genetic drift 22 , 29 – 32 . Furthermore, morphological constraints on signal production as well as phylogenetic history may limit evolutionary outcomes and trait diversity in the evolution of acoustic signals 4 , 18 .…”

Sister species diverge in modality‐specific courtship signal form and function

“…Understanding the origin and maintenance of such signal diversity remains at the heart of many evolutionary studies. Research testing the relative roles and importance of different drivers of signal divergence—for example, ecological, genetic, and social/sexual selection—remains an active and stimulating field of study (Bailey et al., 2019; Derryberry et al., 2018; Garcia et al., 2020; Hebets et al., 2013; Wilkins et al., 2018).…”