Geographical and Genetic Heterogeneity during Speciation

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Sometimes, populations within a species begin to utilize different resources or habitats. Over time, these populations will diverge genetically if specialists on one of the possible resources leave more offspring than do more generalized individuals. If this specialized resource use alters the pattern of mate choice such that individuals tend to mate with others utilizing the same resource or habitat, these divergent populations are likely to become two different species. Although the many products of speciation in the distant past are clearly visible in the millions of species on Earth, reconstructing the process of speciation as it occurs within diverging populations is difficult because important clues to the processes disappear after speciation is complete. First, diverging populations in different geographic locations experience natural selection on the various resources semi-independently-- they may approach speciation at different rates or even evolve toward different outcomes. Secondly, some genes within incipient species diverge very rapidly under the divergent natural selection, and patterns of molecular variability at these genomic regions tell the genetic story of speciation. However, these unique patterns are obscured over time, and eventually all genes will tell the same story, so analyses of anciently diverged species cannot identify the crucial genetic changes that drive the process. Here, a well-studied set of populations of an insect species (the pea aphid) that are specialized on different host plants will be used to capture a glimpse of the early stages of speciation. Using molecular population genetics, both geographic differences between pea aphid populations across the US and the heterogeneity of divergence across the genome during speciation will be analyzed. This project will provide a high-resolution picture of speciation as it actually occurs, before crucial details are obscured by the passage of time.The pea aphid is an introduced agricultural pest. These results will not only illuminate basic evolutionary process, but will serve as a model for the study of evolution in other invasive taxa. Students and postdoctoral associates will be trained in an integrative research approach that crosses traditional scientific boundaries between ecology, microevolution and systematics. The unique genetic and ecological tools developed in this project will be widely used by members of a growing consortium of scientists working on pea aphid genetics, which is one of the species newly targeted for genome sequencing. Finally, the PI uses her research to inform her extensive efforts to educate the broader community about evolutionary biology.