Polymorphic species, in which multiple variants coexist within a population, are often used as model systems in evolutionary biology. Recent research has been dominated by the hypothesis that polymorphism can be a precursor to speciation. To date, the majority of research regarding polymorphism and speciation has focused on whether polymorphism is maintained within a population or whether morphs within populations may diverge to form separate species (sympatric speciation); however, the geographical context of speciation in polymorphic systems is likely to be both diverse and complex. In this review, we draw attention to the geographic variation in morph composition and frequencies that characterises many, if not most polymorphic species. Recent theoretical and empirical developments suggest that such variation in the number, type and frequency of morphs present among populations can increase the probability of speciation. Thus, the geographical context of a polymorphism requires a greater research focus. Here, we review the prevalence, causes and evolutionary consequences of geographic variation in polymorphism in colour-polymorphic animal species. The prevalence and nature of geographic variation in polymorphism suggests that polymorphism may be a precursor to and facilitate speciation more commonly than appreciated previously. We argue that a better understanding of the processes generating geographic variation in polymorphism is vital to understanding how polymorphism can promote speciation.

Determining the mechanistic and genetic basis of animal coloration is essential to understand the costs and constraints on color production, and the evolution and maintenance of phenotypic variation. However, genes underlying structural color and widespread pigment classes apart from melanin remain largely uncharacterized, in part due to restricted taxonomic focus. We combined liquid chromatography-mass spectrometry and RNA-seq gene expression analyses to characterize the pigments and genes associated with skin color in the polymorphic lizard, Ctenophorus decresii. Throat coloration in male C. decresii may be a combination of orange, yellow, grey, or ultra-violet blue. We confirmed the presence of two biochemically different pigment classes, pteridines (self-synthesized) and carotenoids (acquired through the diet), in all skin colors. Orange skin had the highest levels of pteridine pigments while yellow skin tended to have higher levels of carotenoids, of which the vitamin A precursors β-carotene and β-cryptoxanthin have not been previously confirmed in reptiles. These results were confirmed by gene expression analyses, which detected 489 genes differentially expressed between the skin colors, including genes associated with pteridine production, provitamin A carotenoid metabolism, iridophore-specific synthesis, melanin synthesis, and steroid hormone pathways. For the majority of these 489 genes, however, our study reveals a new association with color production in vertebrates. These data represent a significant contribution to understanding the genetic basis of color variation in vertebrates and a rich resource for further studies.

BackgroundIdentifying the causes of intraspecific phenotypic variation is essential for understanding evolutionary processes that maintain diversity and promote speciation. In polymorphic species, the relative frequencies of discrete morphs often vary geographically; yet the drivers of spatial variation in morph frequencies are seldom known. Here, we test the relative importance of gene flow and natural selection to identify the causes of geographic variation in colour morph frequencies in the Australian tawny dragon lizard, Ctenophorus decresii.ResultsPopulations of C. decresii are polymorphic for male throat coloration and all populations surveyed shared the same four morphs but differed in the relative frequencies of morphs. Despite genetic structure among populations, there was no relationship between genetic similarity or geographic proximity and similarity in morph frequencies. However, we detected remarkably strong associations between morph frequencies and two environmental variables (mean annual aridity index and vegetation cover), which together explained approximately 45 % of the total variance in morph frequencies.ConclusionsSpatial variation in selection appears to play an important role in shaping morph frequency patterns in C. decresii. Selection associated with differences in local environmental conditions, combined with relatively low levels of gene flow, is expected to favour population divergence in morph composition, but may be counteracted by negative frequency-dependent selection favouring rare morphs.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0442-x) contains supplementary material, which is available to authorized users.