Résumé
Although trees comprise a primary component of terrestrial species
richness, the drivers and temporal scale of divergence in trees remain
poorly understood. We examined the landscape-dominant tree, Metrosideros
polymorpha, for variation at nine microsatellite loci across 23
populations on young Hawai’i Island, sampling each of the island’s five
varieties throughout its full geographic range. For four varieties,
principal coordinate analysis revealed strong clustering of populations by
variety across the 10 430 km island, indicating partitioning of the
species into multiple evolutionarily significant units. The single
island-endemic form, riparian var. newellii, showed especially strong
differentiation from other varieties despite occurring in sympatry with
other varieties and likely evolved from a bog form on the oldest volcano,
Kohala, within the past 500 000 years. Along with comparable riparian
forms on other Pacific Islands, var. newellii appears to represent
parallel incipient ecological speciation within Metrosideros. Greater
genetic distance among the more common varieties on the oldest volcano and
an inverse relationship between allelic diversity and substrate age appear
consistent with colonization of Hawai’i Island by older, partially
diverged varieties followed by increased hybridization among varieties on
younger volcanoes. This study demonstrates that broad population-level
sampling is required to uncover patterns of diversification within a
ubiquitous and long-lived tree species. Hawaiian Metrosideros appears to
be a case of incipient radiation in trees and thus should be useful for
studies of divergence and the evolution of reproductive isolating barriers
at the early stages of speciation.