Figure 6.1 Convergent evolution. Similar conditions in the deserts of the world have selected for similar water-conserving life forms in two groups of unrelated plants: (a) an organ pipe cactus (Stenocereus thurberi) in Organ Pipe Cactus National Monument, Arizona, and (b) a euphorb (Euphorbia virosa) in Namibia, Africa.

Photos by (a) All Canada Photos/Alamy; (b) imagebroker/Alessandra Sarti/Newscom.