Phylogeography of the highly dispersible landscape‐dominant woody species complex, Metrosideros, in Hawaii

AIM: Little is known about how diversification occurs within long‐lived, highly dispersible and continuously distributed groups. We examined the distribution of genetic variation within the woody genus Metrosideros across the Hawaiian Islands for insights into how diversification occurs within this animal‐pollinated, wind‐dispersed group. Among Hawaiian plants, Metrosideros is unique in its formation of continuous stands within islands that span a remarkable range of environments and comprise numerous predominantly single‐island taxa. LOCATION: Hawaiian Islands. TAXON: Metrosideros. METHODS: We performed population genetic analyses of variation at nine nuclear microsatellite loci from 1,486 adults of 23 Metrosideros morphotypes sampled from five main Hawaiian Islands plus additional Pacific Island populations. RESULTS: American Samoa and Tahiti populations clustered most closely with the older islands. Results also revealed isolation by distance across the archipelago, clustering of populations predominantly by island, and evidence of multiple colonizations or back‐colonizations of three islands. The number of genetic clusters peaked on islands of intermediate age, coincident with peak morphotype richness. All islands comprised a broad range of genetic distances among taxa with the greatest overall genetic distance observed on Oahu. The two taxa that are distributed broadly across the archipelago were weakly but significantly differentiated only on volcanically active Hawaii Island, where they partition early‐ and late‐successional environments. One of these taxa was positioned centrally both within individual‐island splitstree networks and across the archipelago‐wide network. MAIN CONCLUSIONS: Distance‐dependent gene flow contributes to isolation of Metrosideros across islands, especially on terminal islands. Morphological diversity likely accumulates rapidly within this group, likely associated with differential adaptation across heterogeneous environments, but isolation of gene pools through speciation within continuous Metrosideros stands likely requires persistent disruptive selection where environments are stable for long periods. The generalist, wet‐forest M. polymorpha var. glaberrima may play a central role in the generation of the group's many, largely island‐endemic, taxa.