Roles of isolation, secondary contact and gene flow in diversification and speciation.

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One of the most enduring goals in evolutionary biology is to understand how new species arise. The traditional view, popularized by 20th Century evolutionary biologist Ernst Mayr, emphasized a geographic view of speciation where most new species arise or at least begin through physical and thus genetic isolation of ancestral populations. Recent theoretical and empirical work has moved away from these geography-based views of speciation to ones that emphasize mechanism (e.g. selection driving ecological divergence) producing important insights into how new species arise. However, our own NSERC-funded research over the last 5 years highlights the pervasiveness of both geographic isolation and subsequent secondary contact in the histories of most vertebrate species, mirroring comprehensive reviews of the phylogeographic literature. Thus, the long-term objective of my NSERC-funded research over the next 5 years is to quantify the role of such long-term geographical isolation, and consequences of range dynamics, and especially secondary contact in the evolution and maintenance of species. My proposed NSERC research focuses on a single temperate eastern North American frog, the spring peeper (Pseudacris crucifer), which I am developing into a model system for speciation studies. Our published work suggests that the spring peeper has a dynamic evolutionary history with 6 well-supported evolutionary lineages that originated in distinct, isolated refugia between 11 and 3 million years before present. Different lineage pairs with disparate times of divergence have come into secondary contact in different parts of the species’ range, presumably within the last 10-15,000 years. In the proposed research, my students and I will combine large-scale genomic and geographic information system (GIS) data, new statistical and modeling techniques, bioacoustical analysis, and playback and hybridization experiments to address a series of research questions on speciation: (i) What is the relation of divergence times between evolutionary lineages and patterns of reproductive isolation, the hallmark of biological species? (ii) What do genomic data say about the patterns of gene flow among lineages over the entire history of this species? (iii) Does secondary contact of closely-related lineages result in more effective reproductive barriers, either to diminish acoustic interference of male advertisement calls (used by females in mate choice), or to prevent maladaptive hybridization? (iv) How might marked differences in the acoustic environment or seasonality shape the outcomes of secondary contact between diverging lineages? (v) What are the various fitness costs of hybridization between lineages? My research program helps put Canada at the forefront of speciation research, bringing together perspectives from biogeography, genomics, landscape ecology, and ecological genetics. It provides important inputs into conservation strategies for Canadian species at risk, for example how best to retain evolutionary potential for future diversification and adaptation, and whether phylogeographic divisions merit separate conservation designation. The training program that I outline will provide premier opportunities for a minimum of 4 PhD, 3 MSc and 25 BSc students in genetics, quantitative methods, GIS, and professional development. As I have done in the past, I will leverage expertise developed with my NSERC research to seek additional funding and opportunities for applied landscape genetic & conservation studies of Canadian species at risk.