Résumé
On Lord Howe Island, speciation is thought to have taken place in situ in
a diverse array of distantly related plant taxa (Metrosideros, Howea and
Coprosma; Proc. Natl Acad. Sci. USA 108, 2011, 13188). We now investigate
whether the speciation processes were driven by divergent natural
selection in each genus by examining the extent of ecological and genetic
divergence. We present new and extensive, ecological and genetic data for
all three genera. Consistent with ecologically driven speciation, outlier
loci were detected using genome scan methods. This mechanism is supported
by individual-based analyses of genotype–environment correlations within
species, demonstrating that local adaptation is currently widespread on
the island. Genetic analyses show that prezygotic isolating barriers
within species are currently insufficiently strong to allow further
population differentiation. Interspecific hybridization was found in both
Howea and Coprosma, and species distribution modelling indicates that
competitive exclusion may result in selection against admixed individuals.
Colonization of new niches, partly fuelled by the rapid generation of new
adaptive genotypes via hybridization, appears to have resulted in the
adaptive radiation in Coprosma – supporting the ‘Syngameon hypothesis’.