Evolutionary consequences of island colonisation in the silvereye: a genomic approach

Thesis

Deterministic evolutionary outcomes are widespread across the Tree of Life. For example, many insular organisms show repeated evolution of some traits, many of which are captured under the so‐called ’island syndrome’. A classic example is the repeated evolution of body size, but there are also less-explored consequences of island colonisation: a reduction in dispersal propensity, which is of particular interest because it can catalyse diversification. A great example of this concerns ’great speciators’, bird species present on multiple islands that, at the same time, show many subspecies. Within the taxon cycle framework, great speciators are species that, after an initial range expansion stage, have differentiated into subspecies. However, this pattern is not entirely intuitive, since great speciators are hypothesised to have high dispersal propensity in the colonisation stage but low in the differentiation stage. This evolutionary puzzle is known as the ”paradox of the great speciators”. Strong selection against dispersal ability after island colonisation is often invoked to resolve this apparent paradox, but the mechanisms underlying it are not well understood. In Chapter 2, I explored the great speciator continuum and the drivers of incipient speciation on islands and found that spatial effects are particularly important in determining the occurrence of great speciators. The silvereye (Zosterops lateralis), of the white‐eye bird family Zosteropidae, is a prolific natural coloniser of Southwest Pacific islands and has rapidly given rise to over 17 subspecies most of which are island-dwelling, making it a useful model to understand the evolutionary consequences of island colonisation. In Chapter 3, I used genome-wide data to build phylogenetic trees and networks to understand the evolutionary history of the silvereye. I found that continental subspecies are highly connected and can overcome major biogeographic barriers, while island populations are generally more differentiated. Island populations follow the island syndrome expectations: reduced dispersal propensity and increased body size. In Chapter 4, I took a candidate gene approach to explore whether the ”paradox of the great speciators” can be understood from a genetic perspective in this species. I found that some behavioural genes, like CREB1, are associated with a reduction in dispersal, and that, in a partial migrant population, migrants carry longer CLOCK alleles than sedentary individuals. Chapter 5 explored the mechanisms underlying the repeated evolution towards a larger body size in island silvereyes and found that standing genetic variation is the most common route towards gigantism but that de novo mutations can also play a role. As a whole, this body of work advances our understanding of the mechanisms leading to repeated evolution after island colonisation and the impact on diversification.

Authors: Estandía, A

• DOI: 10.5287/ora-ozyaeeao0
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English