Zoology - Theses
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ItemPlumage pattern function and evolution: a phylogenetic and comparative approach( 2011)Visual patterns, such as bars and spots, are common in the animal kingdom. In no other group are patterns so exquisite in their arrangement and coloration than in birds. Although bird plumage patterns appear to be visually diverse there are only four types of patterns, which can be broadly categorized into irregular and regular patterning. That these types of irregular and regular patterning are recursive is intriguing and speaks of an underlying shared mechanism on which selection can act. The prevailing assumption is that patterns predominantly function in camouflage, however evidence suggests that they also function in communication in a small number of birds. In particular it has been suggested that barred plumage patterns could be a signal of individual quality. In visual ecology, communication and camouflage seem to be in conflict with one another – visual signals are often conspicuous whereas camouflage has evolved to provide concealment. These ideas of pattern function need not be incongruous if patterns evolved a) for camouflage first and were subsequently co-opted by sexual selection for communication, and/or b) some patterns, specifically barred plumage, evolved for both camouflage and communication to overcome this functional compromise. To test these alternative ideas of pattern evolution I test whether a) patterns were co-opted for signaling in the model group waterfowl and gamebirds, and b) if the evolution of sexual dimorphism in barred plumage indicate camouflage and ⁄ or signaling functions across the class Aves. Additionally, I investigated whether development poses a constraint on pattern evolution in waterfowl and gamebirds. Tracing the most probable evolutionary pathway of plumage pattern evolution revealed that the ancestral state of plumage was uniform coloration. From uniform coloration, patterns initially evolved to be predominantly monomorphic, and subsequently evolved to be sexually dimorphic. In sexually dimorphic patterns, barred plumage frequently evolved in females and males, suggesting a role for both camouflage and communication. However, dimorphic spotted plumage only evolved in males suggesting it predominantly evolved for communication. Overall, it is likely Plumage pattern function and evolution: a phylogenetic and comparative approach ii that patterns originally evolved for camouflage and were subsequently co-opted for signaling. Focusing on the evolution of barred patterns by comparing their prevalence between the sexes I found a higher frequency of female- rather than male-biased sexual dimorphism, indicating that camouflage is its most common function. But I also found that, compared with other pigmentation patterns, barred plumage is more frequently biased towards males and its expression more frequently restricted to adulthood, suggesting that barred plumage often evolves or is maintained as a sexual communication signal. This illustrates how visual traits can accommodate the apparently incompatible functions of camouflage and communication. Lastly, I studied the recurrence of irregular and regular plumage patterns to explore why there are different kinds of patterns that are broadly recursive. By modeling pattern evolution I derived directionality and show that where species exhibit a single pattern, selection need not be constrained by development. However, instances of irregular and regular patterns in the same species are a result of selection on existing patterns. Together this demonstrates that the evolution of patterns is not difficult and that states of multiple pattern types are a result of selection.
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ItemColour pattern evolution in Australian dragon lizards (Agamidae)( 2011)Many animals communicate using elaborate visual signals such as conspicuous colours and patterns, which are influenced by numerous selective forces. However, the relative influences of natural and sexual selection on types of colour and pattern exhibited and the number and complexity of diverse colour pattern elements remain unclear for most taxa. In this study, I applied phylogenetic comparative approaches to examine the evolution of colour patterns in 69 Australian agamid lizard species, a morphologically and ecologically diverse group that relies primarily on visual communication. I examined colour patterns on nine body regions, which include dorsal, lateral, and ventral areas. I tested for associations with indices reflecting different selective pressures from both visual predators and conspecifics. Indices of natural selection included habitat openness, life style, substrate types, and ecological generalism, and my indices of sexual selection were the degree of sexual dichromatism and body and head size dimorphism (SSD). I found that both carotenoid/pteridine-based yellow-red colours and melanin-based black colour are likely to be used as sexual signals in male agamid lizards, as indicated by their association with two indices of sexual selection, the degree of sexual dichromatism and sexual size dimorphism. The use of both yellow-red and black sexual signals suggests that the costs associated with different colour production mechanisms do not limit the expression of sexual signals. Furthermore, the prevalence of different types of colour on different body regions potentially reflects different signalling strategies in this group. I then examined the evolution of colour pattern complexity in Australian agamid lizards. I developed a novel approach to quantify colour pattern complexity based on the Shannon-Wiener species diversity index. Sex differences in colour pattern complexity and the degree of complexity in males were associated with SSD and sexual dichromatism, respectively. These results suggest that the evolution of colour pattern complexity is driven primarily by sexual selection. Greater colour pattern complexity of males than females suggests that sexual selection has led to signal innovation in males, involving the use of additional and/or novel signal types, rather than elaboration of existing signals, which does not influence complexity. By contrast, I found no strong associations between any ecological factors and colour patterns or with the degree of colour pattern complexity, which suggests that the influence of natural selection on colour pattern evolution in this group is difficult to identify within a broad phylogenetic comparative study.