%0 Journal Article %A Ken A. Thompson %A Matthew M. Osmond %A Dolph Schluter %T Patterns of speciation and parallel genetic evolution under adaptation from standing variation %D 2018 %R 10.1101/368324 %J bioRxiv %P 368324 %X When isolated populations adapt to novel environments, they may do so using the same or different alleles. Use of the same alleles—parallel genetic evolution—is increasingly likely if populations have shared ancestral standing genetic variation. Here, we investigate the conditions under which populations undergo parallel genetic evolution from standing variation and the associated implications for speciation via environment-specific hybrid fitness. Using computer simulations, we find that populations adapting to identical environments tend to fix the same alleles from standing variation unless there is a deficient or excess amount of variation in the founding population. We also find that the degree of parallel genetic adaptation decreases faster-than-linearly as selection deviates from fully parallel toward divergent. This rapid decrease in parallelism occurs because seemingly small environmental differences among populations correspond to steep reductions in the fraction of alleles that are mutually beneficial. Lastly, we show that adaptation from standing variation reduces the phenotypic segregation variance of hybrids under both parallel and divergent selection. Under parallel selection, parental populations fix the same alleles which do not segregate in hybrids. In addition, adaptation from standing variation proceeds via a greater number of alleles with individually smaller effects than when adaptation is from new mutation—this further reduces segregation variance even under divergent selection. This reduced segregation variance improves mean hybrid fitness when parents adapt to similar environments but reduces mean hybrid fitness under divergent adaptation. Therefore adaptation from standing variation forestalls speciation via parallel selection and promotes speciation via divergent selection.Impact summary Among all of the questions that evolutionary biologists have posed, some of the most lingering concern the predictability of evolution. For example, if two groups of organisms independently evolve similar traits in similar habitats, is it likely that this evolution proceeded using the same genes? If so, what can we learn about the probability that these groups have evolved reproductive barriers causing them to become distinct species? We investigate these two questions from a theoretical perspective in our article and focus on the viability of hybrids as measured in the environments to which their parents are adapted. In particular, we investigate the role of standing genetic variation—the genetic variability that exists within a population at any given time—in causing populations to evolve using the same genes. We have three main conclusions. First, we find that the greatest parallel evolution results when there is a match between the quantity of standing genetic variation and the selective demands of the environment, and significant redundancy in the standing variation readily leads to non-parallelism. Our second conclusion is that parallel evolution from standing genetic variation is likely only when different populations adapt to very similar environments; small environmental differences largely preclude parallelism at the genetic level. Finally, whereas standing genetic variation decreases the likelihood of speciation when two populations adapt to similar environments, it increases the likelihood of speciation when populations adapt to different environments. In sum, our article indicates that adaptation from standing variation can have an important role for speciation that depends on whether populations adapt to similar or different environments. %U https://www.biorxiv.org/content/biorxiv/early/2018/07/16/368324.full.pdf