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In this thesis, I investigated different evolutionary forces in shaping genetic variation within a bottlenecked population of an island species, the Seychelles warbler (Acrocephalus sechellensis). I specifically explore pathogen-mediated selection within this system by using avian beta-defensins and toll-like receptor genes to examine functional variation. First, I characterise variation within both gene groups in this population and show that this species’ demographic history has had an overriding effect on selection and random drift is the predominant evolutionary force. I characterise variation within these gene groups across several other Acrocephalus species, in addition to looking at a specific locus in a prebottlenecked population in order to directly compare genetic variation pre- and postbottleneck. I use population genetic statistical methods to detect selection at several polymorphic genes and evaluate the robustness of these methods when applied to singlelocus sequence data, which may be lacking in power and not meet the demographic assumptions that come with these tests. To overcome this, I designed forward-in-time simulations based on microsatellite markers used in pre- and post-bottleneck populations of the Seychelles warbler. I am able to delineate the evolutionary effects of selection from drift and show that some toll-like receptor genes are indeed under positive balancing selection in spite of the recent bottleneck. I further explore how this variation is maintained by conducting association analyses investigating innate immune gene variation and its relationship with individual survival and malarial susceptibility / resistance. Environmental factors are also considered. By investigating the consequences of functional variation in a bottlenecked species we are able to assess its long-term viability and adaptive potential, whilst elucidating the evolutionary importance of maintaining genetic variation in natural populations. |
