The consequences of sexual selection in the common wall lizard : insights following secondary contact and non-native introductions

Reproductive characters often vary geographically within species. This has led to the suggestion that traits related to reproduction evolve rapidly and that sexual selection is an important cause of diversification. Using the common wall lizard, Podarcis muralis, this thesis explores the consequences of interactions between reproductive characters and sexual selection in two environmental contexts: (i) following secondary contact between lineages that differ in secondary sexual characters, and (ii) following introduction to a cooler and more seasonal nonnative environment. To address this, I adopted an integrated approach, combining information on associations between reproductive characters, social behaviour, and reproductive success in an experimental setting with extensive documentation of phenotypic variation across native and non-native populations. Common wall lizards occupy a wide geographic range, spanning from western Spain to Turkey. Their phylogeographic structure is complex and composed of several genetically and phenotypically distinct lineages. In the first part of this thesis I examined how divergence in male sexual characters between two lineages ‚Äö- the Italian lineage, where males have highly exaggerated sexual traits, and the Western European lineage, where sexual traits in males are less expressed ‚Äö- mediates sexual selection and ultimately patterns of hybridization and introgression following secondary contact. Specifically, I combined an investigation of behavioural interactions and patterns of paternity in experimentally replicated mixed-lineage populations with genetic and phenotypic data from three independent zones of secondary contact. Experimentally, I show that Italian males have a significant advantage over Western European males in competition for females, leading to overall greater courtship and mating success, and consequently, asymmetric hybridization. Patterns of genetic and phenotypic introgression following secondary contact mirrored this directionality. Nuclear microsatellite markers revealed a westwards shift in the position of the hybrid cline compared to mitochondrial markers. Furthermore, clines in male visual sexual characters were shifted even further westwards into the Western European lineage, indicative of the rapid and adaptive displacement of Western European male sexual phenotypes. Combined with a lack of evidence for negative effects on hybrid offspring survival and their reproductive characters, these results demonstrate an important role for pre-copulatory sexual selection through male-male competition in shaping the genetic and phenotypic consequences of secondary contact. I then examined the consistency of these effects across different communication channels, specifically comparing the above results for visual characters with chemical characteristics of male femoral secretions used as scent marks. Despite chemical communication being considered an important feature of lizard reproductive behaviour, I find little evidence for a role of divergence between the lineages in chemical characters in hybridization or sexually selected introgression. In contrast to the extensive introgression of the visual characteristics of the Italian lineage into the Western European lineage, patterns of introgression in chemical profiles resembled that of nuclear microsatellite markers, implying that genetic divergence in chemical characters is selectively neutral. These results highlight the potentially differing functions for visual and chemical communication channels in lizards. Chemical characters in wall lizards may function primarily as an individual-based recognition system. In the second part of this thesis I examined divergence in female and male reproductive characters in response to a different climatic selection regime. Wall lizards that have been recently introduced into England (outside of their native distribution) experience a cooler, more seasonal climate that effectively restricts offspring recruitment to the first clutch of the season. This should exert strong directional selection on the reproductive investment of both females and males. Consistent with an adaptive response to climate, I show that non-native females in England produce relatively larger and heavier first seasonal clutches and smaller and lighter second seasonal clutches compared to native females. Despite non-native male fitness also depending almost entirely on the first clutch of the season, examination of male behaviour, dominance hierarchies, and phenotypic associations with mating and fertilization success in experimental populations revealed that non-native males do not alter their sexual strategies and compete aggressively to fertilize the second clutches from non-native females. These results highlight the potential for sex-specific limitations on rapid adaptive shifts in reproductive characters when male behaviour has been shaped by sexual selection regimes in past environments. Combined, this thesis provides evidence of the potential ways in which sexual selection may shape the evolutionary and ecological trajectory of populations across different environmental contexts.