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A review of colour and pattern polymorphisms in anurans

2000; Oxford University Press; Volume: 70; Issue: 4 Linguagem: Inglês

10.1111/j.1095-8312.2000.tb00221.x

1095-8312

Eric A. Hoffman, Michael S. Blouin,

Plant and animal studies

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. There are at least 225 species representing 35 genera and 11 families of anurans that exhibit a colour or pattern polymorphism. These include several apparent cases of parallel polymorphism or convergent evolution. This diversity provides an excellent system in which to study polymorphism. The mode of inheritance has been investigated in 26 species. Although most studies indicate that CP is highly heritable, simple Mendelian inheritance has been conclusively demonstrated for CP polymorphisms in only two species (not counting albinos and other variants that are too rare to qualify as polymorphisms). There is an obvious need for more genetic studies. Forty-six species have been described as undergoing ontogenetic change, and 32 have been described as sexually dimorphic. An interesting area of research would be on the adaptive benefits of ontogenetic change, and on why anurans are sexually dimorphic. That anuran CP polymorphisms are under some sort of selection has been inferred by the large number of polymorphic species, the occurrence of apparent convergent evolution and the existence of identical polymorphisms in closely related species, the observation of consistent cyclical fluctuation in frequencies of morphs over short (seasonal) or long (annual) periods of time, and a few fortuitous cases of non-random survival (A. crepitans, and R. pipiens). The mechanisms behind the selective maintenance of colour and/or pattern polymorphism have been investigated in 19 species. Most of these studies attempted to determine if some fitness-related trait was correlated with morph type, and were mostly inconclusive. Although only four studies investigated direct selection by predators on CP polymorphism itself, two of these demonstrated differential selection (albeit in the laboratory). We believe that the simplest explanation for the maintenance of CP polymorphisms is indeed direct selection by visually-oriented predators. Well-designed studies are needed on the effects of predation on morph frequencies in the field. Although such studies are difficult to conduct, they are likely to be much more fruitful than yet more searching for correlated physiological traits. This remains a wide open and potentially rewarding area of study.