POPULATION VARIATION AND HYBRIDIZATION - COMPARISON OF FINCHES FROM 2ARCHIPELAGOS

Some populations of Darwin's Finches (Emberizinae) are exceptionally v ariable in body size and beak traits as a result of introgressive hybr idization. A study of museum specimens of honeycreeper-finches (Cardue linae) from the Hawaiian islands was undertaken to see if the same phe nomenon was manifested by a different phyletic group of finches in a d ifferent archipelago. Five hundred and twenty-four specimens of the se ven species with finch-like bills were measured and their coefficients of variation were compared with those of the ground finch group (six species) of Darwin's Finches. Coefficients were smaller in the Hawaiia n finches. Sympatric and, hence, potentially hybridizing species on th e island of Hawaii were not consistently more variable than the allopa tric species on other islands in the archipelago. The one species with both sympatric and allopatric populations did not show greater variat ion in the sympatric population. There is little evidence from these c omparisons of hybridization occurring in the last 100 years. The diffe rence between the two finch faunas can be explained in terms of two fa ctors. Finches have been present for a longer time in the Hawaiian arc hipelago than in the Galapagos archipelago and have had more time to n ot only diversify but to evolve pre- and post-zygotic isolating mechan isms. In the generally less seasonal and floristically richer Hawaiian islands they have evolved greater dietary specializations. Beak trait s adapted to specialist feeding may have been under stronger stabilizi ng selection and hybrids (if formed) may have been at a strong disadva ntage in the absence of an ecological niche intermediate between the n iches of the two parental species. Results of published electrophoreti c studies of genetic variation suggest that the early phase of differe ntiation, involving occasional introgressive hybridization, may last f or up to 5 million years.