Physical differences among anole species, for example in tails, limb proportions, and toepad size, evolved as a result of
natural selection in different environments. But how did different anole populations come to occupy different habitatsin the
first place, evolve these adaptations, and start on the path toward speciation?
Different populations compete for limited resources,such asfood. When competition for resourcesisintense, there is
positive selection for traits that favor the populations dividing up the environment. Different anole species, for example,
have moved into various habitatsranging from the grasses and bushesto the leaves in the forest canopy. Over many
generations, natural selection favors those traits among individuals that enable the populations to live more successfully in
their different environments. By specializing in a particular niche, each population can accrue a host of adaptations that
enable them to better avoid predation, more easily find mates, procure food not available to other populations, and even
find shelter.
The different niches may not be completely separated from each other. For example, the habitats of the twig anole and the
trunk-crown anole overlap high up in the trees, but when members of two different species encounter each other, they do
not mate. The traits that enable different species of anoles to thrive in different niches-----such as short legs or long tails-----
are notsufficient forspeciation to occur. In addition to those adaptations, traitsthat prevent mating and reproducing must
also change.
Dr. Jonathan Losos, Dr. Manuel Leal, and others have discovered that distinctive dewlap colors and shapes are traits that can
keep differentspecies of lizardsreproductively isolated. No two anole speciesliving in the same geographic location have
dewlaps of the same color orshape. (Figure 1 shows a variety of dewlap shapes and colors.) Male anoles display their
dewlaps as a social signal and in particular to attract females. The females of mostspecies also have dewlaps, which are
generally smaller than those of the males and are also used forsignaling. Females choose mating partners of the same
species, based on a variety of physical characteristicsincluding dewlap shape and colo