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Accueil du site → Doctorat → Australie → 2011 → Diversification in the face of desertification : the evolution of Australian arid zone geckos

Australian National University (2011)

Diversification in the face of desertification : the evolution of Australian arid zone geckos

Pepper, Mitzy Rohani

Titre : Diversification in the face of desertification : the evolution of Australian arid zone geckos

Auteur : Pepper, Mitzy Rohani.

Université de soutenance : Australian National University

Grade : Doctor of Philosophy (PhD) 2011

My thesis concerns gecko lizards and what their genes can tell us about the historical biogeography of the Australian arid zone. In contrast to the great antiquity of the continent, this immense xeric biome is surprisingly young. Evidence of arid conditions in palaeorecords appears around 15 million years ago (mya), with the inland sand deserts thought to have formed in the last million years. Such a drastic change over the majority of the continent would undoubtedly have had a profound influence on the evolution of the Australian biota, but at present’studies are in their infancy. Why is there such high diversity of geckos in the arid zone ? When did this diversity arise ? Do some regions have more diversity than others ? How did the formation of sand deserts affect the movement and distribution of species ? To explore the evolutionary consequences of landscape and climate change following the onset and intensification of aridification, I examine genetic variation in various Australian geckos that have distributions spanning the arid regions of the continent, including topographically complex mountain ranges and the vast deserts of low-lying sand dunes. Using detailed molecular datasets, and drawing on information from geology and landscape history, I am able to shed light on the timing and mode of gecko diversification in the Australian desert. Chapter II continues to explore arid zone mountain refugia using comparative datasets of other gecko taxa. In addition, I compare how levels of diversity differ between Vll mountainous regions and surrounding low-lying desert dunes. Again, I found strong evidence for the historical persistence of taxa in mountain refugia, but in stark contrast, gecko populations in the desert regions displayed very little genetic diversity, often over extremely large geographic areas. Using molecular dating, I inferred these population expansions to have occurred extremely recently (< 2 mya), and interpret this to be a result of population bottlenecks in response to extreme arid cycling that is known to have initiated in the mid Pleistocene. It appears that aridification had little effect on population demographics of mesic-adapted taxa in refugia, but profoundly influenced genetic diversity of geckos distributed in the desert regions most heavily impacted by aridification. Chapter IV allowed me to apply a detailed focus on a poorly known region of biotic endemism in the arid zone, the Pilbara, in remote north-western Australia. To lay foundations for future molecular studies, I review the physical and biological history of the Pilbara and highlight general patterns that are beginning to emerge from the distribution of a variety of organisms. I provide a detailed review of geology, landscape history, and phytogeography to develop phylogeographic hypotheses, and I then test these predictions using large comparative datasets of seven Pilbara geckos. The molecular patterns I uncovered are consistent with those recently identified using species richness compositional patterns of invertebrates across the region. Together these concordant results across different taxa and at different levels of the genealogical hierarchy highlight the overarching (and underlying) influence of geology and landform on evolutionary processes in the Pilbara. The first chapter of my thesis investigates the presence and role of refugia in the arid zone, regions that retained climatic stability and allowed persistence of species during harsh climatic periods. Mountain ranges in central and western Australia are thought to have provided refugia for many species following widescale aridification, however this hypothesis has never been tested in a molecular framework. I assess divergence history among rock-dwelling geckos in the genus Heteronotia that have distributions confined to disjunct ranges in the arid zone. Were these gecko populations isolated from one another recently by the development of sand deserts between their rocky habitats, or were they relicts of an ancient, ancestral taxon driven into refugia by expanding arid conditions ? I found that the age of divergence among the lineages was much older than the sand deserts, and the high levels of diversity within each taxa was indicative of persistence in refugia that likely was aided by topographic complexity in these areas. The third chapter of my thesis combines molecular systematics with detailed morphological data, to revise the classification of the arid zone beak-faced gecko, Rhynchoedura ornata. Formerly considered a single, widespread species, here I describe four new species and redescribe a fifth. The distributions of these species points to ancient biogeographic separations within the arid zone that appear to relate to a topographic divide between the western Australian uplands and eastern lowlands, with divisions in the latter correlated to dryland rivers and major drainage divisions. This result provides a unique contrast to existing studies of terrestrial arid zone vertebrates, and suggests the evolutionary histories of these desert geckos was shaped by wet periods as well as dry ones.


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