“How parallel is parallel evolution? A comparative analysis in fishes”

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Krista B. Oke, Gregor Rolshausen, Caroline LeBlond, and Andrew P. Hendry

How parallel is parallel evolution in fishes? Varies greatly, revealing importance of quantifying parallelism

Just how parallel is parallel evolution anyway?

Parallel evolution is the repeated evolution of similar traits by independent populations colonizing similar environments. This parallelism is often taken as strong evidence that natural selection plays a deterministic (or “predictable” or “repeatable”) role in evolution, because, although similarities among populations could arise by chance, similarities that are repeatedly expressed in similar environments are more likely the result of selection typical of those environments. Despite many invocations of parallel evolution in the literature, differences are often evident among independent populations found in similar environments. That is, although independent populations in the same environment type are sometimes very similar, at other times they can be surprisingly different. In this study, researchers from McGill University leverage these differences to ask the basic question: just how parallel is parallel evolution? Analyzing results from 92 studies of putatively parallel evolution in fishes, the researchers find high variability in the extent to which ostensible parallel evolution is actually parallel in reality. The traits that they analyzed showed patterns ranging from highly parallel to hardly parallel at all, with everything in between. Moreover, the extent of parallelism did not differ between studies that explicitly described their findings as demonstrating parallel evolution versus studies that were not so explicit. These results highlight the importance of formally quantifying the extent of parallel evolution, rather than simply using binary parallel versus non-parallel categories. The various deviations from parallelism could be due to a number potential evolutionary nuances, including overlooked environmental variation, different genetic architectures or evolutionary histories, differences in gene flow, and others. Fortunately, the variation in the extent of parallel evolution described here reveals ample opportunities for researchers to identify the drivers of trait variation, thereby improving our understanding of the various forces shaping evolutionary trajectories in nature. Read the Article