The Effect Of Body Size On The Origin And Diversification Of Birds
Lead Research Organisation:
UNIVERSITY COLLEGE LONDON
Department Name: Cell and Developmental Biology
Abstract
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.
Organisations
Publications
| Description | We used a novel evolutionary analysis of a massive dataset (>10,000) that includes both extinct and extant species to characterize avian diversity dynamics throughout its ~150 Ma evolutionary history. Results from this effort illustrate the impact of body size on diversification, extinction, and survivorship at the Cretaceous-Paleogene (K-Pg) boundary, with larger-bodied species slower to speciate and more prone to extinction. Our approach is the first to examine large-scale diversification trends in birds in the context of trait-dependent factors (body size) that explicitly includes data from the deep-time record. The comparative framework and analyses developed for this paper include several novel approaches, including: (1) the integration of a large (10,316 species) dataset spanning both extant birds and representative fossils from the avian stem, (2) jointly modelling body size evolution and diversification processes, rather than simply examining correlations, and (3) explicitly incorporating the end-Cretaceous mass extinction into our diversification models. Perhaps the most striking conclusion of our study is that we find support for differential survival of large-bodied species across the end-Cretaceous mass extinction event. This suggests that the influence body size as mediated through several other associated life history traits (e.g., clutch size, metabolic rate) may have influenced survivorship and provides a counterexample to the popular hypothesis that smaller body sizes experienced a selective advantage as the K-Pg extinction event unfolded. More generally, and importantly however, our modelling results also propose the existence of an evolutionary ratchet mechanism (i.e., an irreversible trajectory toward extinction) to account for the larger-scale patterns of high species origination rates at smaller body sizes, an overall trend toward larger body sizes, and a high risk of extinction in these larger body size classes. To our knowledge, this is the first time that this type of evolutionary process has been proposed to explain patterns of trait evolution and diversity for birds. |
| Exploitation Route | We have developed a massive dataset compiling body size estimates for hundreds of fossil birds. This dataset will be important for other researchers working on body size evolution in birds and dinosaurs. We have also developed new statistical approaches for modelling speciation and extinction in fossil organisms. Other researchers will be able to use these to test complex hypotheses of diversification across the tree of life. |
| Sectors | Other |