Hybridization between Malacothrix polycephala and M . incana (Asteraceae) on San Nicolas Island, California

Natural hybridization between Malacothrix polycephala W . Davis (ined.), an annual, and M . incana (Nutt.) Torrey & A. Gray, a perennial, on San Nicolas Island, California was examined using herbarium studies, studies in the field, and studies of plants grown in growth chambers. Malacothrix incana has apparently been on San Nicolas Island for less than 25 yr, but M . polycephala is a long-time resident. The two species differ morphologically, physiologically, and in their banding patterns for the enzyme aspartatate aminotransferase. N o strong isolating mechanisms have been found between the two species and extensive hybridization is occurring on San Nicolas Island. Studies of growth chamber progenies indicate that natural hybridization has produced many combinations of the morphological and physiological features of the parental species. The evolutionary potential of hybridization between M . polycephala and M . incana appears to be considerable.