A review of colour and pattern polymorphisms in anurans

Abstract

Species that exhibit polymorphism, the simultaneous occurrence of two or more discrete, genetically-based phenotypes in a population, are ideal for studying the microevolutionary forces that maintain genetic variation in nature. Many anuran species exhibit striking colour or dorsal pattern polymorphisms, and so provide an excellent system in which to study questions pertaining to the evolution and maintenance of polymorphisms. Despite the wide occurrence of pattern or colour polymorphisms in anurans (current records cite at least 225 species representing 35 genera and 11 families) surprisingly little conclusive work has been done on the inheritance and selective maintenance of this variation. The mode of inheritance has been investigated in 26 species, but conclusively demonstrated in only two. Forty-six species have been described as undergoing ontogenetic change, and 32 species have been described as sexually dimorphic. That anuran polymorphisms are under some sort of selection has been inferred from the large number of polymorphic species, from putative cases of apparent convergent evolution and the existence of identical polymorphisms in closely related species, from cyclical fluctuations in morph frequencies, and from a few observations of non-random survival during bouts of stress. The selective mechanisms maintaining these polymorphisms have been investigated in only 19 species. Most studies looked for physiological traits correlated with the polymorphism, rather than studying the most obvious mechanism, selective predation on colour/pattern itself. Thus, anuran polymorphisms remain a rich but largely unexploited system for studying the evolution of phenotypic variation in nature.