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Title: Structural Variation as a Catalyst of Adaptation and Speciation: From Populations to KingdomsAbstract: In sexual species, gene flow is considered to be the greatest impediment to speciation. The main problem with gene flow is that although divergent selection facilitates the buildup of associations between alleles favored in local environments and those causing reproductive isolation, gene flow and recombination break the associations apart. However, examples of speciation in the face of gene flow began accumulating in the late 20th century, with accelerating evidence over the past two decades. How can this happen?In my talk, I will attempt to answer this question, drawing on a series of investigations of wild sunflower species, ranging from genome-wide association studies of phenotypic variation to field-based analyses of reproductive barrier strength, to functional characterization of candidate genes underlying reproductive isolation. I will show that most of the traits and genes that contribute to local adaptation and reproductive isolation in wild sunflowers are associated with recombination suppressors, mainly chromosomal inversions and other structural variants, thereby resolving the antagonism between selection and gene flow. Lastly, I present a survey of more than 1000 animal, fungal, and plant genomes showing that chromosomal inversions are abundant across the three kingdoms. I conclude by arguing that this genetic architecture offers a general solution for how speciation can occur in the presence of gene flow.

Loren Rieseberg is a University Killam Professor in the Botany Department at the University of British Columbia, Vancouver. He was born and raised in western Canada, but moved to the USA for his graduate education, receiving a PhD in Botany from Washington State University in 1987. He worked in the USA until 2006, first at the Claremont Graduate School in Southern California (1987-1993) and then at Indiana University (1993-2006), before returning to Canada.His lab employs population genomic approaches and field and greenhouse experiments to study the origin and evolution of plant species, tap wild extremophile species for crop improvement, combat invasive weeds, and conserve natural populations of plants, including their microbiomes.Rieseberg is a Clarivate Highly Cited Researcher, and his work has been recognized by MacArthur and Guggenheim Fellowships and the Darwin-Wallace Medal in evolutionary biology. He is an elected member of the Royal Societies of London and Canada, the Norwegian Academy of Arts and Letters, the American Academy of Arts and Sciences, and the U.S. National Academy of Sciences. He has served as President of the American Genetics Association (2006) and the Botanical Society of America (2017-18), Director of UBC’s Biodiversity Research Centre (2016-21), and Chief Editor of Molecular Ecology (1999-).

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Meeting ID: 926 6205 2101

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Meeting ID: 926 6205 2101

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Meeting ID: 926 6205 2101

Title: Not All Bad News: Minor Genomic Impacts of Population Declines in Brown Bears

Abstract: Brown bears (Ursus arctos) were once widespread across the Northern Hemisphere but have experienced recent and severe population declines. Present-day North American diversity and distribution of bears remain poorly understood due to conflicting hypotheses about colonization timing across Beringia and subsequent population connectivity. To address this, we generated whole-genome sequences from 268 brown bears spanning all extant populations in the contiguous United States and much of Alaska and Canada. Demographic analyses reveal three distinct waves of colonization: an initial wave during Marine Isotope Stage 4 (~57,000–71,000 years ago), a second during the Last Glacial Maximum (LGM; ~14,000–29,000 years ago), and a final wave near the end of the LGM (~12,000 years ago). Although populations were likely connected following deglaciation, populations have declined sharply over the last ~250 years. Though isolated populations show evidence of recent inbreeding, genetic load remains low. Sex chromosome analyses indicate male-biased dispersal, though anthropogenic barriers limit gene flow in both sexes. We also identify selection signals in genes associated with hibernation and diet, suggesting potential adaptive responses relevant to future persistence. Overall, our results clarify colonization history and provide a comprehensive view of the current genetic health of North American brown bears.  

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Talk title: “Predicting genotype by disentangling the underlying evolutionary forces.”

Talk Abstract:
The genotypes we observe in biology are not shaped by any one process. Alleles often experience competing selection pressures, while the mutations that introduce sequence variation differ in both their fitness effects and their probabilities of arising in the first place. How do these factors combine quantitatively to determine evolutionary outcomes? I address this question using a combination of experimental and computational approaches. In one line of work, I study mutant mitochondrial genome (mtDNA) variants that are stably co-transmitted alongside wildtype mtDNA despite harming host fitness, termed “selfish mtDNA.” Using C. elegans as a model system, I developed experiments to isolate and measure selection on mtDNA at different levels of biological organization. These measurements showed how selfish transmission within the germline combines with selection forces favoring wildtype mtDNA, revealing an astonishing diversity in the evolutionary mechanisms that maintain long-term coexistence of selfish and wildtype mtDNA. In parallel, my ongoing work combines population-genetic modeling with experimental evolution data to understand how selection interacts with the input of new mutations to shape the evolutionary paths that protein sequences follow. Together, these approaches provide a quantitative framework for disentangling the evolutionary forces underlying the genotypes we observe.

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Meeting ID: 926 6205 2101

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Meeting ID: 926 6205 2101

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Meeting ID: 926 6205 2101