An investigation of the evolutionary diversification of a recent radiation of dwarf chameleons (Bradypodion) from KwaZulu-Natal Province, South Africa

[摘要] ENGLISH ABSTRACT: An important prerequisite for evolutionary change is variability in natural populations;however, when phenotypic and molecular rates of change differ, species delimitation isproblematic. Such discordance has been identified in a recent radiation of dwarfchameleons (Bradypodion) from KwaZulu-Natal Province, South Africa. This radiation iscomprised of several phenotypic forms, two of which have been classified taxonomically –Bradypodion melanocephalum and Bradypodion thamnobates. Early phylogenetic analysisdid not support the forms primarily because geographic sampling and the set of molecularmarkers used were appropriate for detecting deep divergences and, therefore, less effectivefor understanding species boundaries within a recent radiation. In this radiation, the formsare allopatric, occupy different habitats, and vary in size and colouration, suggesting localadaptation and ecological speciation. To test this hypothesis, morphometric and habitatdata were collected for each form to examine ecologically relevant morphologicaldifferences that reflect differential habitat use. Morphological differences were thenassociated with functional adaptations by testing locomotor performance and bite force.Next, fine-scale genetic sampling was used to examine lineage diversification using acombination of mitochondrial DNA and microsatellites. Spatial information wasincorporated into these analyses to quantify the genetic effects of landscape barriers ongenetic structure. Finally, ecological niche modelling was used to examine the abioticfactors involved in shaping the climatic niches of these chameleons, and to gain insightinto their biogeographic history. Results show morphological distinctions between phenotypic forms, with corresponding differences in performance, indicating functionaladaptations to habitats, which can be broadly classified as either open- or closed-canopyvegetation. Specifically, chameleons in open-canopy habitats have proportionally smaller forces and forefoot grip strengths. Varying degrees of sexual dimorphism were detected,with the closed-canopy forms being more dimorphic than the open-canopy forms. Thissuggests that sexual selection is the predominant force within the closed-canopy habitat,which are more protected from aerial predators, thereby enabling them to invest indimorphic traits for communication; while, in open-canopy habitats, natural selection is thepredominant force, ultimately enforcing their overall diminutive body size andconstraining performance. Genetic structure was observed, with the mitochondrial DNArevealing three genetic clusters and the microsatellites revealing seven. This likely reflectsthe different mutation rates and modes of inheritance between these two markers. Three ofthe microsatellite clusters were supported by morphological and ecological data andshould, therefore, be recognised as separate species. The remaining microsatellite clustersshowed discordance with the ecomorphological data; however, given their geneticdistinctiveness, they should be recognized as separate conservation units. The climaticniches of the three proposed species showed high to moderate levels of climatic stability,while the four proposed conservation units showed low climatic stability. These resultsindicate that this species complex is affected by both climatic niche conservatism andlability, which could explain the observed patterns of morphological and genetic diversity.In summary, these results support the hypothesis of ecological speciation within thisradiation.

[发布机构] Stellenbosch University