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Understanding evolutionary rates on the Tree of Life in time and space

Final Report Summary - TOLERATES (Understanding evolutionary rates on the Tree of Life in time and space)

The diversity of life is the result of billions of years of evolutionary history that is interconnected in a Tree of Life. This diversity is uneven. Some groups are made up of many closely related species, like beetles, whereas others have few or no close relatives, like aardvarks. How and why does this imbalance arise? The palaeontologist George G. Simpson posed a question - “How fast, as a matter of fact, do animals evolve in Nature?” – suggesting that The Tree of Life is shaped by variation in the rate and limits of evolutionary change. Addressing this question has far-reaching consequences for our understanding of the origins of biological diversity. Variation in rates could apply to the birth (speciation) and death (extinction) of species but also to how their physical and behavioural features – their phenotypes – evolve. We know surprisingly little about these phenotypic rates.

This aim of this project was to determine how and why evolutionary rates vary among organisms and to test how this variation contributes to the uneven distribution of species on the Tree of Life and geographically across the globe. We used birds as a case study and collected new data on the size and shape of bird beaks using 3D scanning as well as measures of feather colour in the human-visible (red, green, blue) and ultraviolet spectrum using modified digital cameras. To do this we used specimens spanning almost all of the world’s ca. 10,000 bird species from the Natural History Museum, Manchester Museum, and the Field Museum, Chicago. We designed and built two crowd-sourcing websites – markmybird.org and projectplumage.org – where both in house experts and members of the public helped to turn the scans and images into data.

We have used the data to test ideas about how and why species diversify. For example, we have shown that the diversity of beaks, a proxy for the ecological niche, expanded into a remarkable diversity of forms early in the evolutionary history of living birds. This expansion in diversity is marked by major jumps between distinct ecological types. By using data on ecologically relevant traits from a large globally distributed animal taxon, our results help to resolve how the diversity of birds arose over millions of years. We have further found that unusual beak shapes evolve faster as do beaks shapes of species with many close relatives, suggesting an important role of ecology, via opposing effects of opportunity and competition in driving evolution. Colour also evolves at variable rates. We found that sexual selection is a likely cause for high rates of evolution. The latter result is likely to be important globally. We found that sexual dichromatism – thought to be a good measure of the strength of sexual selection - has a strong latitudinal gradient. Analyses are ongoing but this points to a role for sexual selection to determine, at least in part, the global distribution of species. Together, our results to date have begun to elucidate the patterns, causes, and consequence of variation in rates of evolution.