Colour polymorphism and correlated characters: genetic mechanisms and evolution

Abstract

Colour polymorphisms (CP's) continue to be of interest to evolutionary biologists because of their general tractability, importance in studies of selection and potential role in speciation. Since some of the earliest studies of CP, it has been evident that alternative colour morphs often differ in features other than colour. Here we review the rapidly accumulating evidence concerning the genetic mechanisms underlying correlations between CP and other traits in animals. We find that evidence for genetic correlations is now available for taxonomically diverse systems and that physical linkage and regulatory mechanisms including transcription factors, cis-regulatory elements, and hormone systems provide pathways for the ready accumulation or modification of these correlations. Moreover, physical linkage and regulatory mechanisms may both contribute to genetic correlation in some of the best-studied systems. These results raise the possibility that negative frequency-dependent selection and disruptive selection might often be acting on suites of traits and that the cumulative effects of such selection, as well as correlational selection, may be important to CP persistence and evolution. We consider additional evolutionary implications. We recommend continued efforts to elucidate the mechanisms underlying CP-correlated characters and the more frequent application of comparative approaches, looking at related species that vary in character correlations and patterns of selection. We also recommend efforts to elucidate how frequency-dependent selection may act on suites of characters.

Variation in founder groups promotes establishment success in the wild

“…We also found that variation in founder groups positively affected colour morph diversity in the established populations, indicating that founder diversity may also contribute to increased population persistence [5,7,28]. Association of colour pattern with other kinds of traits owing to genetic, developmental or functional associations are manifest in several species [29,30], and our results for pygmy grasshoppers may therefore be relevant also for other organisms.…”

Epistasis, phenotypic disequilibrium and contrasting associations with climate in the land snail Theba pisana

“…Visible polymorphisms, such as colour polymorphisms, have had a significant role in the study of adaptation, because they are readily accessible and often have clear adaptive significance. They are also valuable for examining the complexity of adaptation, because they are frequently correlated with other traits (McKinnon and Pierotti, 2010). Shell polymorphisms in snails are useful models for studying the complexity of adaptation, because likely mechanisms of selection are often clear in terms of selective predation or climatic selection (Jones et al, 1977;Clarke et al, 1978;Cain, 1983;Vermeij, 1993), allowing predictions of responses to changes in habitat or climate.…”

Heterospecific aggression bias towards a rarer colour morph

“…Indeed, if such biases in heterospecific aggression are also displayed by other species besides H. nicaraguensis, heterospecific aggression could help to explain the low frequencies of the gold morph more generally, especially within the Midas cichlid complex. Our results, therefore, suggest that heterospecific aggression should be taken into account when the complex interactions that are involved in maintenance of a stable (colour) polymorphism [2][3][4]48] are considered. Why should H. nicaraguensis territory holders be more aggressive towards gold than dark A. sagittae?…”

“…Indeed, studies of such species have provided evolutionary biologists with important insights into mechanisms underpinning individual variation and phenotypic diversity, as well as mechanisms of speciation [2][3][4][5][6]. Yet, despite these insights, the actual sources of selection that allow the persistence of different morphs within the same population remain poorly understood [3].…”