The genus Metrosideros Banks in Hawaii comprises five morphologically recognizable species. In the present study M. rugosa, M. tremuloides, and M. polymorpha were analysed for isozyme variation using starch gel electrophoresis. Fourteen populations from the island of Oahu were assessed for four enzymes encoded by six polymorphic loci. The populations exhibited high levels of variation with mean numbers of alleles per locus ranging from 2.30 to 3.30 with an overall mean of 2.97. The mean observed populational heterozygosity for different populations varied from 0.100 to 0.368 with a mean of 0.272. Gene diversity analysis indicated that 87.7 per cent of the total genetic variation resides within populations, while only 4.7 per cent was due to differentiation among taxa. The mean unbiased genetic identity for pairwise comparisons of species was 0.904, which is very high for congeneric flowering plant species. Therefore, in spite of distinct morphological divergence, the Metrosideros taxa studied did not show expected levels of divergence at genes coding for the enzymes studied. The data suggest that both M. rugosa and M. tremuloides have diverged recently from M. polymorpha. However, M. rugosa has diverged more (IBAR = 0.876) than M. tremuloides (IBAR = 0.917). This may therefore represent a case of recent and incipient speciation in the insular environment.
