Speciation, in which one ancestral population gives rise to two descendant species with distinct characteristics, is one of the fundamental evolutionary processes generating the diversity of life on earth. The possibility of sympatric speciation (i.e., without geographic isolation) has generated considerable controversy among evolutionary theorists. This research will examine the evolutionary history of the indigobirds, a group of African songbirds that provides perhaps the clearest example of sympatric speciation in vertebrates. As brood parasites, indigobirds reproduce only by laying eggs in the nests of other species, which sets the stage for speciation because songs learned from different host species serve as the principal criterion in indigobird mate choice. Field work in Tanzania will extend the geographic sampling of indigobird populations. In addition, a large set of molecular genetic markers will be analyzed using newly developed methods based on coalescent theory. These approaches are necessary to evaluate the intriguing possibility that infrequent interbreeding retards differentiation across most of the indigobird genome, even as natural selection results in divergence of the species at a small number of genes controlling host-specific adaptations.
The proposed research will improve our understanding of speciation, adaptation, and the processes shaping the genetic structure of populations, which are central questions in evolutionary biology. The work will also provide training opportunities for two graduate students and will advance the field of molecular ecology by demonstrating new methods of analysis. Finally, the work will continue to raise the profile of indigobirds, whose remarkable natural history is ideal for teaching evolutionary biology and fostering interest in science and biodiversity among the public.
Speciation, in which one ancestral population gives rise to two descendant species with distinct characteristics, is one of the fundamental evolutionary processes generating the diversity of life on earth. The possibility of sympatric speciation (i.e., speciation without geographic isolation) has generated considerable debate among evolutionary theorists. This project examined the evolutionary history of the indigobirds, a group of African songbirds that provides perhaps the clearest example of sympatric speciation in vertebrates. As brood parasites, indigobirds reproduce only by laying eggs in the nests of other species, which sets the stage for speciation because songs learned from different "host" species serve as the principal criterion in indigobird mate choice. This project included fieldwork in Cameroon and Tanzawnia, which extended the geographic sampling of indigobird populations and resulted in a substantial increase in the number of museum specimens available for research on these fascinating birds. Our genetic analyses of indigobird populations demonstrate that the species within each geographic region are nearly identical genetically, differing at only a small subset of genes responsible for the morphological differences between species. This supports a model of rapid speciation with ongoing genetic exchange between recently evolved populations, results that challenge fundamental concepts about species and speciation. More generally, our research has improved our understanding of the processes shaping the genetic structure of populations, a central question in evolutionary biology. Our project also contributed to the development of new methods of generating DNA sequence data for studies of natural populations that are broadly applicable to wide range of organisms, allowing more robust analyses of population structure and history (important, for example, in assessing conservation priorities) at lower cost. We have already shared our laboratory protocol and the computer programs needed to analyze these data with investigators at other institutions, thereby improving a number of other ongoing research projects and a broad range of organisms. This project provided training opportunities for several undergraduates and graduate students in fieldwork, molecular genetics, and computational methods. Likewise, postdoctoral researchers and faculty members from other institutions received hands-on training in these new laboratory and computational methods. Our contacts and assistants in Cameroon and Tanzania also benefitted from training in field ornithology, which in the case of our Cameroonian assistant contributed to his graduate training in a PhD program at the University of Yaounde. Finally, our work has improved our understanding of indigobird biology, an engaging example of fascinating behavioral and evolutionary biology for teaching and fostering interest in science and biodiversity among the broader public.
