The genomics of diversification in South Pacific Silvereyes (Zosterops lateralis)

Speciation is a dynamic process, shaped by interactions among different evolutionary mechanisms including natural selection, genetic drift, gene flow, and mutation. Over 150 years since the publication of ‘On the Origin of Species’ a major challenge in evolutionary biology is to determine the relative importance of these different mechanisms in the speciation process. In the era of genomics, a step towards addressing this challenge is to study the progression of speciation at the level of the genome. Using island colonisations by the silvereye (Zosterops lateralis), in this thesis I investigate the roles of gene flow, selection and drift in shaping patterns of divergence at both the genomic and phenotypic level. In Chapter 2, using populations where divergence timeframes and gene flow histories are known a priori, I documented how genomic patterns accumulate as populations progress along the speciation continuum. This chapter challenges the ubiquity of existing verbal models that explain the accumulation of genomic differences via the growth of genomic islands and instead supports the idea that divergence both within and outside of genomic islands is important during the speciation process. In Chapter 3, using the recent sequential colonization of south Pacific islands by the Tasmanian silvereye subspecies I provide the first assessment of how patterns of genomic divergence are shaped by the stochastic effects of drift following population founding. By tracking divergence across the genome, I demonstrate that population founding has an idiosyncratic effect on the buildup of divergence. In Chapter 4, using the human- mediated introduction of the silvereye to French Polynesia, I assess the role of drift and selection in driving rapid morphological change following the establishment of new populations. Despite only 80 years of separation from their New Zealand ancestors, French Polynesian silvereyes displayed significant changes in body and bill size and shape, but the magnitude of these changes are not large enough, considering effective population sizes, to require directional natural selection to explain them; instead drift alone is a sufficient explanation at the phenotypic level. However, identification of outlier loci at genes previously identified as candidates for bill size and body shape, also suggests selective processes are operating within this population. As a whole, this body of work furthers understanding of how different evolutionary mechanisms shape patterns of divergence at the level of the genome and how divergence unfolds following the establishment of new populations.