Inaugural Addresses (Botany and Zoology)

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Browsing Inaugural Addresses (Botany and Zoology) by Subject "Divergent selection"

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    Ecomorphological forms in Dwarf Chameleons (Bradypodion): assessment of functional morphology and gene flow across spatially adjacent habitat types
    (Stellenbosch : Stellenbosch University, 2013-03) Potgieter, Daniel Francois; Tolley, Krystal; Jansen van Vuuren, Bettine; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: Adaptive radiation is the process whereby clades, lineages, or species demonstrate rapid divergence into an array of phenotypic forms. Variation in ecological parameters, such as habitat use and morphology or behavioural traits related to communication; drive the evolution of ecologically relevant traits in specific habitat types. Nevertheless, such processes may be countered or enhanced by sexual selection pressures as selection acts on the phenotype to maximise reproductive output. Within the Cape Floristic region, species of dwarf chameleons (Bradypodion) are showing signs of such an adaptive radiation. Previous work on B. pumilum revealed intraspecific morphological differentiation, emphasised by functional differences in ecologically relevant traits, between those occupying the fynbos and forest/riverine thicket habitat types. Similar phenotypic divergences are hypothesised to have occurred in their allopatric, forest-dwelling neighbour, the Knysna Dwarf Chameleon (B. damaranum), given the presence of a closely related, morphologically divergent, undescribed species (B. sp. 1) found in the adjacent fynbos habitat type. With this in mind, functional morphological variation was examined between these two potential ecomorphological forms. A second unidentified fynbos species (B. sp. 2), which neighbours these species in its distribution, served to substantiate the proposed morphology~performance~habitat hypotheses. Given the chameleon’s strong reliance of vegetation type, habitat use was explored by examining the microhabitat relevant to chameleons and ascertaining whether this habitat is used randomly. To associate variation in morphology with differences in habitat use, differences in performance capabilities were quantified, particularly those associated with grip strength (hand and tail) and sprint speed. Furthermore, twelve microsatellite markers were used in combination with the ND4 mitochondrial marker to understand the fine scale patterns of gene flow both within and between habitat types. In response to the varied pressures experienced, differences in ecologically relevant traits are found between B. damaranum and the two fynbos species, particularly those related to locomotion (limb length) and bite force (head width). Furthermore, analysis of microhabitat features shows that the fynbos and forest habitat types are structurally different, facilitating differences in habitat use. Differences in performance also vary between vegetation types, with B. damaranum possessing stronger hand and tail grip forces as well as faster sprint speeds. Sexual dimorphism is also present; however it is more prominent in the forest-dwelling B. damaranum. Genetic analyses revealed high levels of geographical structure between B. sp. 1 and B. damaranum, suggesting the presence of a strong barrier to gene flow. Given the congruence between morphological divergence and genetic spatial patterns, it appears that this barrier is associated with habitat type. Within each habitat type, both mtDNA and microsatellite analyses reveal congruent patterns of structuring. These patterns are, however, not governed by barriers to gene flow, but rather via isolation by distance (based on microsatellite data). Furthermore, mtDNA analysis confirmed B. sp. 2 to be highly divergent, occupying a separate clade to B. sp. 1 and B. damaranum. The adaptive differences observed between B. damaranum and B. sp. 1, coupled with its overall resemblance to those observed in B. pumilum; suggest the presence of true chameleon ecomorphs in the genus Bradypodion. This coupled with the lack of gene flow between ecomorphs is indicative of a true allopatric diversification.