FIGURE 1. Evolutionary diversification and adaptive radiation in the Galapagos Islands.

Large ground finch, Geospiza magnirostris (a), sitting on an Opuntia cactus. Both Geospiza and Opuntia have radiated in the Galapagos Islands, as has the snail genus Bulimulus (shown is Bulimulus reibischi) (b), producing phenotypically differentiated species that have adapted to different parts of the environment. Darwin noted the inter-island variation in Galapagos tortoises (Geochelone nigra) (c) and mockingbirds (Nesomimus parvulus) (d), but similar variation occurs in many other taxa on these islands, including marine iguanas (Amblyrhynchus cristatus, larger lizard) and lava lizards (genus Microlophus, smaller lizard on head of iguana) (e). (Panels a, d and e courtesy of H. Snell (Visual Escapes); panel b courtesy of C.Parent (University of Texas, Austin).)