The function and evolution of colour polymorphism in the tawny dragon lizard

Abstract

© Dr Madeleine St Claire YewersColour polymorphic species are model systems to investigate the evolutionary processes that maintain intraspecific diversity within a population. Colour polymorphism occurs when two or more discrete, genetically inherited colour forms coexist within an interbreeding population. Almost ubiquitously, colour morphs differ in morphological, behavioural, ecological, life history and/or physiological traits in addition to colour that often form alternative strategies. Each strategy has optimal trait combinations that allow morphs to maximise fitness. Variation in the trait composition of morph-specific strategies has important implications for understanding life-history trade-offs and the maintenance of polymorphism within populations. Differences in morph composition and correlated traits between populations can also promote divergence and ultimately speciation. In this thesis, I investigate the evolutionary maintenance of colour polymorphism in the tawny dragon lizard, Ctenophorus decresii. I do so in two distinct ways; by assessing colour vision differences between genetically and geographically distinct monomorphic and polymorphic lineages, and by comparing a range of traits that could differentially affect the fitness of morphs within a colour polymorphic population. In the south of their range, male tawny dragons are monomorphic for throat coloration whereas in the north of their range, they are polymorphic. There are four discrete male colour morphs in polymorphic populations that vary in the presence/absence of yellow and orange coloration; the yellow morph, orange morph, grey morph and orange-yellow morph (a yellow background with a central orange patch). Throat colour is heritable, fixed for life, and is an important sexual signal. Males of the monomorphic southern lineage express ultraviolet (UV)-blue throat coloration, unlike males of the polymorphic northern lineage. Lineages meet at a narrow contact zone where genotypic admixture suggests potential barriers to gene flow and incipient speciation. I determined the cone photoreceptor spectral sensitivities using microspectrophotometry and opsin expression of the two lineages, to see if they differ, particularly in sensitivity to UV-blue wavelengths. I confirmed the presence of four single cone classes in both lineages and provide the first evidence of UV visual sensitivity in agamid lizards. However, whether the lineages differ in UV-blue sensitivity remains unresolved. Within a polymorphic northern population, I assessed a combination of traits associated with each colour morph. I found morph-specific alternative strategies differentiated by consistencies in behaviour and hormones. The orange morph has an aggressive strategy with high levels of androgens while the grey morph has a cautious strategy with low levels of baseline androgens. The yellow morph has a conditional strategy where its aggression depends on the colour of the intruder and exhibits a stress-induced androgen increase. The orange-yellow morph similarly shows aggression conditional on the colour of the intruder but it is the boldest with high levels of androgens. Morphs did not differ in performance (bite force) or space use, and there was no relationship between spatial arrangement and relatedness. However, genetic structure based on microsatellite markers indicated weak genetic differentiation between morphs. Therefore, there are minimal barriers to gene flow between morphs. Instead, frequency-dependent selection is likely to be maintaining polymorphism in the tawny dragon lizard