Macroevolution across a changing Australian landscape

Abstract

Over geological time, the earth's surface and climate have changed, rearranging continental plates and oscillating between a hothouse and snowglobe. These changes have left lasting impressions on the diversity, richness, and distribution of earth's inhabitants. Identifying evolutionary commonalities as a result of these events is one of the main aims of the field of macroevolution. It is also the main theme that unites my thesis: investigating the influence of changes to the Australian climate and landscape on the organisms which call Australia home. Empirically, this has required extensively sampling Australian vertebrate groups for phylogenetic, distributional, ecological, and morphological trait data. Methodologically, this has required implementing and building phylogenetic comparative methods to better understand the diversity that surrounds us.

As a continent, Australia gained its independence somewhere between 40-30 million years ago when it separated from Antarctica and began drifting north towards Asia. Prior to this, the Australian plate existed alongside South America, Africa, and India, as part of the supercontinent Gondwana. In the intervening millions of years, Australia has remained isolated, and so even comparatively recent immigrant lineages have speciated in situ, resulting in a number of iconic endemic terrestrial vertebrate radiations. These radiations are great for comparative studies because they provide replicated groups which have diversified under similar environmental influences. Importantly though, they differ in absolute diversity, ecology, and behavior. My research has investigated how changes due to the isolation of the Australian plate, continental aridification, and grassland expansion have impacted the Australian fauna.

In my opening chapter I discuss how the separation of Australia from Antarctica may have precipitated a mass extinction event in a relatively understudied group of lizards, the pygopodoid geckos. Next I present evidence that the Miocene aridification of Australia likely reduced the rate of phenotypic evolution of terrestrial vertebrates by facilitating allopatric speciation and niche conservatism. In the following chapter I test the hypothesis that the diversification of macropod marsupials is linked to the Plio-Pleistocene expansion of C~4~ grasses. Finally, I present the idea that the immense disparity in body size of Australian varanid lizards is the result of character displacement and competition occurring on a continental scale.

Ultimately, the inferences we can draw about evolutionary changes occurring on deep time scales are exciting because they are often intuitive. In no place else does this seem truer than in Australia, which is a natural laboratory for macroevolutionary studies.