Beggar's ticks and tarweeds: Masters of adaptive radiation

The beggar's ticks (Bidens) and tarweeds (Madiinae) are very much more diversified morphologically and ecologically in the Hawaiian islands than in continental areas. Isozyme data and preliminary genetic analyses suggest that the adaptive features exhibited by these groups in Hawaii are based on relatively few gene loci. Each group has mechanisms promoting outcrossing: gynodioecy in the beggar's ticks, self-incompatibility in the tarweeds. While hybridization is not rare in the beggar's ticks, it is very common in the tarweeds. The tarweeds provide the only known instance of chromosomal evolution among studied examples of adaptive radiation. A number of other qualitative and quantitative differences exhibited by the tarweeds suggests the possibility of their greater antiquity in Hawaii compared to beggar's ticks and other groups.