Résumé
The presence of congeneric taxa on the same island suggests the
possibility of in situ divergence, but can also result from multiple
colonizations of previously diverged lineages. Here, using genome-wide
data from a large population sample, we test the hypothesis that
intra-island divergence explains the occurrence of four geographic forms
meeting at hybrid zones in the Reunion grey white-eye (Zosterops
borbonicus), a species complex endemic to the small volcanic island of
Reunion. Using population genomic and phylogenetic analyses, we
reconstructed the population history of the different forms. We confirmed
the monophyly of the complex and found that one of the lowland forms is
paraphyletic and basal relative to others, a pattern highly consistent
with in situ divergence. Our results suggest initial colonization of the
island through the lowlands, followed by expansion into the highlands,
which led to the evolution of a distinct geographical form, genetically
and ecologically different from the lowland ones. Lowland forms seem to
have experienced periods of geographical isolation, but they diverged from
one another by sexual selection rather than niche change. Overall, low
dispersal capabilities in this island bird combined with both geographical
and ecological opportunities seem to explain how divergence occurred at
such a small spatial scale.