Compiling forty years of guppy research to investigate the factors contributing to (non)parallel evolution

Background/Question/Methods

Examples of parallel evolution have been crucial for our understanding of adaptation and the deterministic role of natural selection. However, high extents of parallelism are not always observed even in seemingly similar environments where natural selection is expected to favour similar phenotypes. Leveraging this variation within well-researched study systems can provide insight into the factors that contribute to deviations from parallelism. Here, we analyze the results of 36 studies reporting 446 average trait values in Trinidadian guppies, Poecilia reticulata, from different predation regimes. We examine how the extent of parallelism is influenced by six factors: sex, trait type, rearing environment, increasing ecological complexity, increasing evolutionary history, and decreasing time since colonization.

Results/Conclusions

Our analysis revealed that parallel evolution in guppies is highly variable and weaker than in previous reviews (only 24.7% of the variation among populations was explained by predation regime on average). Levels of parallelism appeared to be especially weak for colour traits, but we did not observe any significant effects for the other factors we examined. These results point to several potential avenues for future research, including deeper investigating of the factors investigated here and other factors that might contribute to nonparallelism (e.g., gene flow, drift). Quantifying and accounting for sources of variation among evolutionary replicates will help to better understand the extent to which seemingly similar environments drive nonparallel patterns of phenotypic divergence.