ADAPTIVE RADIATION AND GENETIC DIFFERENTIATION IN HAWAIIAN BIDENS

The genus Bidens (Asteraceae) has undergone extensive adaptive radiation on the Hawaiian Islands. The 19 species and eight subspecies endemic to Hawaii exhibit much more morphological and ecological differentiation than the continental members of the genus. However, the Hawaiian taxa have the same chromosome number and retain the capacity to interbreed in all possible combinations. Twenty-two populations of 15 Hawaiian taxa and four populations of American taxa were compared at 21 loci controlling eight enzyme systems. Populations of Hawaiian taxa are highly polymorphic. However, little genetic differentiation has occurred among taxa in spite of the high levels of genetic variability. Genetic identities calculated for pairs of populations show that populations of the same taxon are genetically more similar than are populations belonging to different taxa, but all values are high. The level of genetic differentiation that has occurred among the species of Hawaiian Bidens is comparable to the level of genetic differences found among populations within single continental plant species. Moreover, there is no correlation between the isozyme data and morphological data. No groups of taxa are evident in the genetic data, although morphological groups exist. Genetic differentiation at isozyme loci has not occurred at the same rate as the acquisition of presumably adaptive morphological and ecological characters in Hawaiian Bidens. Adaptive radiation may be limited to a few genes controlling morphological and ecological characters.