The macroevolutionary consequences of trait correlations

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The diversity of Life on Earth is immense. Over millions of years, evolution has generated a staggering variety of species. When we look more closely, however, we begin to notice some surprising patterns. Some groups have far more species than others, for example there are over 350,000 species of beetle but only a few species of elephant. Other groups exhibit diverse shapes and sizes, for example the Hawaiian honeycreepers, whereas others are fairly uniform, for example mice and rats. These patterns suggest that there are constraints, or limitations, on the numbers of species and their variety of form and function, i.e. biological diversity is not evenly distributed across the tree of life. Why this is so remains unresolved and is a fundamental question in evolutionary biology.Here we focus on a previously unexplored explanation for this uneven distribution of diversity: correlations among species traits. Species traits can be almost any aspect of physical appearance (such as body size or eye colour), behaviour, or life history (size vs. number of offspring). These traits are not always independent of one another, for example investing in more offspring often results in smaller offspring. In such cases we say that the traits involved are correlated. Where traits are correlated, evolution in one trait may promote or prevent evolution in another. Differences in the strength of trait correlations in different groups of species might determine variation between and among groups of related species. Where correlations among traits are strong these may prevent evolution of very different shapes and sizes. Therefore the diversity of form and function might be low, and the number of closely-related species should be few. Over millions of years these trait correlations might become weaker or even disappear, allowing species to evolve in new ways or multiply in number. These ideas suggest that trait correlations are important in our understanding of the diversity of life. Our proposal aims to understand how trait correlations can affect the processes that determine patterns of diversity (both in species number and in form and function) across the tree of life. We focus on traits related to food type and acquisition in birds. The size and shape of bird beaks can reflect dietary requirements and foraging methods. When traits are correlated, some combinations may never occur together, for example, it may not be possible to evolve beaks that are simultaneously long and curved like a curlew, and wide and deep like a duck. To test how trait correlations can affect the processes that determine patterns of diversity across the tree of life we need to measure many individuals from many different species. It would be a near-impossible challenge to do this in the field. Instead we use specimens from museums. Around the world museums house billions of specimens, often from historical collections over 100 years old. The Natural History Museum, London has around 80 million specimens. These collections are a rich source of information on nearly all species of birds (over 10,000 species). We will generate data from these collections, including 3D scans of bird beaks. We will then use these data, along with state-of-the-art analysis methods, to evaluate trait correlations among and within species, and among higher taxonomic groupings such as families and orders. By using museum specimens and a novel view of the importance of trait correlations, our research will provide new insights into how and why life diversifies. Not only will this research address a fundamental debate in evolutionary biology, but it will also create valuable bird datasets for future researchers, and highlight the importance of natural history collections for cutting edge research.