My biosystematic and evolutionary studies have concentrated on Pacific coast annuals such as Amsinckia , Plectritis and Mimulus , and on endemic Hawaiian species of Bidens and their continental and Polynesian sister groups. Research on mating systems involves quantitative estimates of outcrossing rates and genetic variability in natural populations, including both isozyme and morphological polymorphisms. The systems of interest include heterostyly, gynodioecy, and closely related outcrossers and selfers. Heterostylous species are polymorphic for flowers with reciprocally different style and stamen lengths. Our studies of outcrossers and selfers have shown that the mating system is a major factor determining the genetic structure of natural plant populations.
Gynodioecious populations consist of separate hermaphrodite and female individuals.
Females are at a disadvan- tage compared to hermaphrodites because they do not contribute any genes to the next generation through pollen. To persist, they must have some compensating selective advantage. We hypothesize that the selective advantage of females is that they are obligately outcrossing, whereas hermaphrodites can self and their inbred progeny are less fit. I am also investigating adaptive radiation in Hawaiian Bidens . The 27 endemic taxa of Bidens on the Hawaiian Islands are extremely morphologically and ecologically diverse, yet all evolved from a single ancestral species which colonized from Central America.