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Context dependent sexual selection and the dynamics of colonisation

Final Report Summary - ADMIXTURE (Context dependent sexual selection and the dynamics of colonisation)

Recent research suggests that multiple introductions of distinct lineages are common in invasive species. Despite this, the impact of admixture on the fitness of introduced populations, and consequently its contribution to their evolutionary potential, has received little attention. Sexual selection should be of particular importance within this context as it is fundamental to the maintenance of genetic variation, reproductive output, and hence population growth. Furthermore, sexual selection is predicted to be highly context-dependent. For example, variance in genetic diversity resulting from admixture should strongly influence the mating strategies employed by males and females, and thus, regulate both population dynamics and accelerate (or impede) the introgression of genotypes.

We have shown that human introductions of wall lizards (Podarcis muralis) into the United Kingdom have brought into secondary contact several genetically and phenotypically distinct lineages. The consequences of this genetic admixture, with respect to the ecological and evolutionary trajectory of introduced populations, will be dictated by the extent and outcome of interactions between the lineages. We found that females do not discriminate behaviourally between males of different origins despite large differences in male morphology and pheromone profiles as well as costs of hybridisation (including reduced embryo viability and fertility). Male competitive ability is also highly asymmetrical with Italian males dominating French males - a pattern consistent with differences in the degree of sexual dimorphism between individuals from the two lineages. The consequence of this greater asymmetry and lack of female recognition was a greater overall reproductive success of Italian males in mixed-origin semi-natural populations compared to French males. Despite this, paternity was still, on average, highly assortative. This could be due to male mate choice or male-male competition that results in assortative paternity in the absence of male choice per se.

These behavioural patterns allowed us to make a number of predictions regarding the evolutionary consequences of secondary contact, specifically, the extent to which it results in the formation of hybrid swarms versus reinforcement and completion of speciation. To test these predictions and examine the consequences of genetic admixture more widely we combined the above experimental work with detail data collected from introduced and native wall lizard populations. Preliminary data suggests that patterns of introgression in native and non-native hybrid zones are consistent with the predictions from our behaviour data. Specifically, our results suggest the introgression of Italians genes into French lineages. Furthermore, our data suggests that genetic admixture as a result of multiple introductions has resulted in a moderate loss of genetic diversity compared to native populations, but there is no evidence that this loss of genetic variation causes inbreeding depression. Combined these data will help us to assess the importance of genetic variation and mixing of animals from different origins for the evolutionary and ecological trajectories of introduced populations.