The Effect Of Body Size On The Origin And Diversification Of Birds

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Sixty-six million years ago, the end of the Cretaceous period was marked by one of earth's largest mass extinctions. This event resulted in the demise of three quarters of all land animals, including all of the non-avian dinosaurs and many of the birds. Only a few bird species survived, and these groups subsequently diversified into the stunning variety of species we see today. What allowed these few lucky birds to survive? The factors that predispose organisms to extinction or speciation is of the biggest questions in evolutionary biology. This is increasingly important as more species become threatened with extinction today as a result of human-caused climate change. One hypothesis, the "Lilliput effect," proposes that lower body mass increases the chances of survival during a mass extinction. This is because body size affects nearly every aspect of biology, from life span to metabolism to rate of mutations. Although many studies have sought to quantify whether body size and diversification are linked, nearly all of these have excluded a key source of data: the fossil record. This is despite the fact that mathematical simulations predict that calculating extinction rates without including extinct species invariably produces inaccurate results. Here, we seek to answer two main research questions: (1) how did body size affect speciation and extinction rates in birds over the past 175 million years? (2) how does including fossil data in our models affect our reconstructions of diversification through time? We will use measurements of fossil bird skeletons to reconstruct body mass in approximately 400 extinct avian species, including some of the largest birds ever to exist such as the dodo, great auk, and elephant bird. We will then generate a comprehensive family tree of birds including >10,000 fossil and modern species, allowing us to track their evolutionary history through time. Finally, we will use evolutionary modelling to test whether the probability of speciation and extinction are indeed affected by body mass. Using advanced statistical methods and fossil data, we will provide the first test of the hypothesis that small size was a key trait that allowed birds to diversify after the time of the dinosaurs.