Multiple observations of naturalists show that the shape of individuals of a species depends on the type of habitat. Indeed, there are natural forces that tend to select “ecomorphs” best able to live in a given habitat. But can we go beyond observation and reproduce experimentally this selective process? Ryan Calsbeek and Robert Cox of Dartmouth College, Hanover, New Hampshire, replied in the affirmative with their results on lizard populations in the Caribbean and America, the anoles, brown anole here (Anolis sagrei).

In theory, populations of anoles living on the American continent would be subject primarily to predation pressure and little competition between individuals. In fact, the lizards are relatively few in number per unit area, because the food is abundant and dispersed, while the populations of predators (snakes and birds) would be an easy life. Conversely, on islands, where space is restricted and limited food, predators are less numerous, but the competition between the anoles, more numerous per unit area, exerting a selective pressure.

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