The macroevolutionary drivers of bird diversification

The diversity of phenotypic traits observed in the natural world is extensive, driven by pressures ranging from natural selection that drives adaptation to ecological niches, to social and sexual selection that determines which individuals have access to resources and mating opportunities. The relative roles of each of these broad drivers, and specific pressures within, is still a highly contested topic, and whilst this has been addressed in empirical experiments and comparative studies, a coherent theory for explaining broad-scale phenotypic evolution is still lacking for many traits.

To address the issue, this project aims to integrate global datasets of traits, environmental niches and phylogenies for all birds (>10,000 species), combined with newly proposed phylogenetic models, to help shed light on the relative roles of broad selection pressures. Birds are an ideal group for this project because they are relatively well studied, with extensive knowledge of ecological and evolutionary traits. This is further supplemented with additional unpublished data, overall contributing to a near-complete estimation of the variation in many traits, including foraging strategies, adaptive morphology, plumage dichromatism and song; there is also the opportunity to add to this data through literature reviews, museum visits and digital sound archives.

The initial stage of the project is a six-month training program, comprised of taught modules in evolutionary ecology and statistics, and a three-month project. The aim of this project is to investigate the drivers that shape the evolution of sexual dichromatism in bird plumage. This question was first addressed in a debate between Wallace and Darwin, who believed differences were a result of natural or sexual selection. There have been various attempts at explanations throughout the literature through experiments and reviews, including natural selection on females for cryptic colours, sexual selection on brightly coloured males, and social selection on females that engage in territorial behaviour. This project aims to resolve the relative input of these pressures by assessing the evolution of dichromatism across the whole clade of birds.

Completing this initial study into dichromatism will build a foundation of experience in conducting phylogenetic analyses, a more in-depth understanding of some of the main evolutionary drivers in birds, and a knowledge of the dataset, which can then be applied to other areas of evolutionary theory. This may include, amongst others, continuing research into selection pressures on females, studying the evolution of song behaviours, or studying how human-mediated changes affect the evolutionary stability of traits crucial for ecosystem services, such as beak shape. Another interest is the incorporation of phylogenetic models of birds into range maps, with the intention of assessing the rate of niche evolution, which could be crucial in assessing the future effect of climate change. Through exploring these avenues early on, potential areas of evolutionary ecology will be highlighted that are of interest, leading to more extensive research and subsequent chapters of the thesis.