| Abstract: | Divergent selection and local adaptation are responsible for many phenotypic differences between populations, potentially leading to speciation through the evolution of reproductive barriers. Acoustic divergence, in particular, is often the basis of assortative mating and behavioral isolation in birds. However, some species have different singing styles used in distinct social contexts, and songs of each style may change at different rates over time and space. Mate recognition largely depends on learning, generally resulting in discrimination against nonlocal stimuli. Nevertheless, there may also be geographical variation in discrimination, allowing inference on the mechanisms behind recognition. Morphologic divergence can cause reproductive isolation too, and a strong association between bill traits and foraging niche, at a given place and time, may indicate ecological speciation is at play and how.
This thesis focused on reed buntings (Emberiza schoeniclus), a small passerine that breeds in marshlands throughout the Palearctic. Populations of three subspecies were studied: migratory E. s. schoeniclus, breeding in northern and central Europe; and the residents E. s. lusitanica from northwest Iberia and E. s. witherbyi from southeast Iberia and southern France (hereafter schoeniclus, lusitanica and witherbyi, respectively).
Four main objectives were addressed using different approaches: (1) phenotypic divergence was evaluated to determine the extent of local adaptation to migration and diet, using linear measurements and geometric morphometrics; (2) song divergence between subspecies and between the three singing styles of each subspecies were tested searching for effects of sexual selection, using spectral traits and derived synthetic variables; (3) song discrimination level was determined to infer premating reproductive isolation, using playback of fast song to test territorial males; (4) association between bill morphology and foraging niche was evaluated searching for evidence of natural selection, by sampling blood in southwest Europe during the winter (when target subspecies co-occur) and performing stable isotope analyses (carbon and nitrogen).
As predicted, migratory subspecies were smaller and had longer and more pointed wings. Unexpectedly, their tail was longer, probably due to correlation with wing length. witherbyi, which feed on insects hiding inside reed stems during the winter, had a very thick bill; whereas schoeniclus, which feed on seeds, had thinner bills. Although smaller, lusitanica had thicker and longer bill than schoeniclus. Geometric morphometrics revealed that southern subspecies have a more convex culmen than schoeniclus, and that lusitanica and schoeniclus bills differ more in shape than in linear measurements. The two singing styles under sexual selection (dawn and fast songs, related to obtaining extrapair and social mates, respectively) diverged more than the slow songs (an all-clear signal to nest attending females). Multiple song traits differed between subspecies in all styles, with intermediate values for lusitanica. Cluster analyses of populations indicate that sexually selected styles better discriminate subspecies, describing a major split in song features between schoeniclus and the resident subspecies. It was found that witherbyi, and to some extent lusitanica, males largely ignored schoeniclus songs. However, witherbyi reacted less strongly to lusitanica than the converse. In contrast, schoeniclus males reacted equally strongly to all subspecies. schoeniclus showed a broader isotopic niche than southern subspecies, which seemed similar despite witherbyi more divergent bill. Stable isotope ratios were consistent with resident subspecies feeding on C3-plant-feeding insects, whereas schoeniclus diet also included C4 plant material.
Subspecies differing in several traits as theoretically predicted indicate local adaptation through natural selection. Greater song divergence in fast and dawn styles implies that sexual selection is important in this evolutionary process. Differential territorial defense provides evidence of premating reproductive isolation, and discrimination by southern subspecies seems associated with local adaptation. Recognition pattern agrees with ecological rather than neutral divergence, as predicted in ecological speciation processes. Isotopic niche divergence between northern and southern subspecies suggests that bill differentiated by natural selection through competition during winter.
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