UBC Theses and Dissertations
Genetic differentiation of Hawaiian Bidens Helenurm, Kaius
Adaptive radiation is the evolutionary divergence of a group of organisms from a common ancestor to exploit different ecological niches. Bidens has undergone extensive adaptive radiation on the Hawaiian Islands. The 19 Hawaiian species exhibit much more morphological and ecological differentiation than the continental members of the genus. However, the Hawaiian taxa are chromosomally homogeneous and retain the capacity to interbreed in all possible combinations. Thus the morphological and ecological differentiation in Hawaiian Bidens has been attained without the existence of reproductive isolating mechanisms. Although some hybrid populations are known, hybridization usually does not occur in nature because the species are found in different habitats. Preliminary genetic studies have suggested that some of the morphological differences between taxa may be controlled by very few genes. Genetic differentiation may therefore not extend to other parts of the genome, such as to structural genes for enzymes. Most plant groups that have been studied electrophoretically show a correlation between morphological differentiation and genetic differentiation, but opposing predictions can be made about the extent of divergence at isozyme loci in Hawaiian Bidens. The morphological and ecological data suggest that the taxa are highly differentiated genetically, but the chromosomal similarity of species, the genetic studies of morphological characters and the absence of genetically controlled isolating mechanisms suggest that genetic differences among the taxa may be limited to only a small portion of the genome and may not include isozyme loci. Populations of the Hawaiian taxa of Bidens were compared at 23 loci controlling 9 enzyme systems. In general, populations are more polymorphic than populations of most other plant species that have been studied electrophoretically. Little genetic differentiation has occurred among taxa in spite of the high levels of genetic variability, however. Genetic identities calculated for pairs of populations show that populations of the same taxon are genetically more similar than populations belonging to different taxa, but all values are high. The genetic differentiation that has occurred among the taxa of Hawaiian Bidens is comparable to the genetic differences among populations of continental 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 adaptive morphological and ecological characters in Hawaiian Bidens. Adaptive radiation therefore does not require genetic change throughout the genome and may be limited to the genes controlling morphological and ecological characters.
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