Experimentally assessing the relative importance of predation and competition as agents of selection

Shape-shifting lizards Experimental tests of natural selection are rare, since few systems are understood well enough to allow predictions about the targets of selection. The Anolis lizards of the Caribbean are an exception. Decades of research have generated precise predictions about the targets (body size, limb length) and agents of selection (competition, predation) for this group. As a test of natural selection, Ryan Calsbeek and Robert Cox conducted large-scale population manipulations of anole lizards on six small Caribbean islands, with and without predatory birds and snakes. Although the presence of predators did influence lizard behaviour, it was increasing density of lizard populations that favoured larger size, longer legs and increased stamina. Intraspecific competition rather than predation seems to be more important in governing the evolution of these lizards.