Adaptation and constraint in the evolution of specialization of Bahamian Anolis lizards

Abstract

Interspecific interactions affect habitat use and subsequent morphological adaptation in Anolis lizards. We examined populations of two species of Anolis lizards that evolved in the species-rich communities of Cuba and are now widespread in the Bahamas. Because the species occupy islands in the Bahamas that vary in the number of lizard species present and other characteristics, we predicted that directional selection should have led to morphological differentiation. In particular, we expected that populations on one-species islands should have evolved toward a generalist morphology because of the lack of competitors. Divergence in both species has been adaptive; populations that use wider perches have longer legs. Nonetheless, these differences are relatively minor, and none of the populations appears to have differentiated from its ancestral "ecomorph" type toward a more generalized morphology. This stasis mirrors a trend observed in the radiation of Caribbean anoles, which exhibits repeated instances of evolutionary specialization, but few or no cases of reversion to a more generalized condition. The explanation for this directionality of evolution is not obvious but probably involves the tendency of specialized species to continue using and further adapting the niches for which they are specialized even as conditions change.