Geographic variation and speciation in the colour polymorphic tawny dragon lizard
AuthorMcLean, Claire Alice
AffiliationDepartment of Zoology, Faculty of Science
Document TypePhD thesis
CitationsMcLean, C. A. (2014). Geographic variation and speciation in the colour polymorphic tawny dragon lizard. PhD thesis, Department of Zoology, The University of Melbourne.
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© 2014 Dr. Claire Alice McLean
Polymorphism, the co-occurrence of multiple discrete variants (or morphs) within a single population, represents extreme intraspecific diversity and can provide critical insights into evolutionary processes. Many polymorphic species vary in the number, type, and frequency of morphs present in different populations, which is likely to be shaped by the interaction between spatially and temporally variable selection, genetic drift, and gene flow. This geographic variation can potentailly act as a precursor to speciation as assortative mating and/or genetic incompatabilities can arise between populations that differ in morph composition. Despite this, few studies have focused on geographic variation in polymorphism, its causes and evolutionary consequences. In this thesis, I studied geographic variation in the tawny dragon, Ctenophorus decresii, a small agamid lizard in which males are polymorphic for throat coloration. Using extensive field surveys and multi-locus phylogenetic data, I first investigated how biogeographic and demographic histories have shaped the current distribution of colour variation in C. decresii. I identified three geographically oriented lineages, each characterised by unique male throat coloration. Within South Australia there was a northern and a southern lineage, and the level of divergence between them indicated early quaternary separation. The northern lineage was polymorphic with four discrete throat colour morphs (orange, yellow, orange and yellow combined, and grey), and populations differed in the relative frequencies of morphs, while the southern lineage was monomorphic with blue-throated males. Populations from New South Wales were deeply genetically divergent, and subsequent morphological analyses confirmed that this lineage was a separate species, thus I described the Barrier Range dragon, Ctenophorus mirrityana. There was also notable morphological divergence between the South Australian lineages, and limited genetic introgression at a zone of secondary contact, suggesting potential barriers to gene flow. Throat coloration is likely to be a social signal in C. decresii, as males display their throats during courtship and territorial encounters. Using visual models, I assessed whether divergent coloration between the southern and northern lineage reflected local adaptation for increased detectability to conspecific lizards and/or increased crypsis to predators in different environments. I found that lineage specific throat colours (blue and orange) were more conspicuous to conspecifics against native lichen colours, therefore it appears that background colours have influenced the evolution of social signals in these lineages. Finally, I investigated the interaction between environmentally driven selection and gene flow in shaping geographic variation in morph frequencies in the northern lineage of C. decresii. I found that gene flow between populations was generally low and populations with similar morph frequencies were not necessarily genetically similar. Conversely, there were strong association between morph frequencies and measures of aridity and vegetation cover. This suggests an important role for selection in shaping population morph frequencies in C. decresii, and further research focusing on the northern lineage may help to answer questions about polymorphism maintenance and the link between geographic variation in polymorphism and speciation.
Keywordscolour; polymorphism; phylogenetics; population genetics; evolution; lizard
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