Evolution of 'ohi'a (Metrosideros polymorpha Gaud.) across Hawai'i's island-age gradient

Evolutionary divergence within a species is expected to involve accumulation of morphological differences and reproductive isolating barriers between populations. The tree species, Metrosideros polymorpha, dominates Hawaiian forests and comprises several varieties that apparently arose through adaptation to different habitats. Given the longer history of Metrosideros on older islands and the age gradient along the volcanic chain, we expect more distinct varieties in older regions, and more hybridization among varieties in younger regions. Morphological variation and timing of flowering (a mechanism of reproductive isolation) were examined at six mixed-variety populations at old (Ko'olau, O'ahu), intermediate (Kohala, Hawai'i), and young (East Hawai'i) substrates. Morphometric analyses supported the hypothesis that varieties are more distinct (indicating lower hybridization) at older sites; but differences in timing of flowering are not clearly greater. Other isolating mechanisms must be reducing hybridization among varieties at older sites. Future work should examine post-pollination reproductive isolating barriers, including hybrid fitness.