EAGER: The impact of selection and temperate-tropical transitions on the evolution of New World bird diversity

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Many fundamental patterns of biological diversity have been observed, but explaining these patterns has proven challenging. In particular, the tropics (the area ~23.5° north and south of the equator) harbor many more species than the temperate zones, and many of these tropical species are significantly differentiated in their physiology, behavior, and body morphology. Some of these differences have been proposed as evolutionary drivers of species numbers; for instance, species in which females can choose from many competing males may fragment into multiple species more easily as female preferences can quickly change. Testing this and related hypotheses requires comprehensive databases of hard-to-quantify species characteristics, including social group sizes and breeding system. This project will produce the first publicly available large-scale data set that includes a metric of the importance of female choice, male testis size. These data will be compiled for a large sample of New World birds, and used to test for relationships among latitude, breeding system, social system, and the rate of speciation. Understanding these relationships is fundamental to explaining broad-scale diversity patterns, which informs conservation planning and policy.This project tests the hypothesis that temperate-tropical biogeographic transitions affect the evolution of mating and social systems in New World oscine passerine birds, and ultimately that these behavioral characteristics in turn may have affected diversification rates. Specifically, this work will focus on compilation of a data set of combined testis mass for all New World oscines based on museum specimen data, uniformly analyzed in a fashion unbiased with regard to breeding latitude. Due to the correlation of testis size with the degree of sperm competition, these data can be used as a proxy for breeding system, to test the hypothesis that tropical habitats select for genetic monogamy, favoring the evolution of sociality (in particular, cooperative breeding). New World oscines are ideal for addressing these issues as they are diverse, have transitioned between the tropics and temperate zone many times, are well-represented in museum collections, and nearly all have been included in species-level phylogenies (with the exception of wrens, for which this project will generate the first phylogeny). These data and analyses will form the basis for broader tests of the relationship among latitude, species traits, and diversification rates in New World birds.