Islands promote diversification of the silvereye species complex: a phylogenomic analysis of a great speciator
Geographic isolation plays a pivotal role in speciation by restricting gene flow between populations through distance or physical barriers. However, the speciation process is complex, influenced by the interplay between dispersal ability and geographic isolation, as seen in "great speciators" - bird species that simultaneously have broad island distributions but high levels of subspecific diversity. Comparing genomic population differentiation in species that occupy both continental and island settings can reveal the effects of different forms of geographic isolation and validate if the mechanism proposed to catalyse a great speciator pattern, that is dispersal reduction following island colonisation, has occurred. The silvereye (Zosterops lateralis) counts with 16 subspecies (11 occurring on islands), distributed on the Australian continent and numerous southwest Pacific islands. We compared continental and island patterns of divergence using whole genome and morphological data. Australian mainland populations showed low genetic population structure and morphological diagnosability, suggesting the species' dispersal propensity in a continental setting is sufficient to overcome multiple forms of geographic barriers and large geographic distances. In contrast, most island populations were highly genomically structured with clearer morphological diagnosability even if separated by relatively short geographic distances. The inferred reduction of dispersal propensity in island situations is consistent with the proposed model of great speciator formation. Our phylogenomic analyses revealed both concordance and discrepancies between morphological subspecies classifications and genomic data. Our findings illustrate how genomic and morphological patterns across different geographic contexts can help reveal when specific speciation stages, such as great speciators, are likely to emerge.