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

Examples of parallel evolution have been crucial for our understanding of
adaptation via natural selection. However, strong parallelism is not
always observed even in seemingly similar environments where natural
selection is expected to favour similar phenotypes. Leveraging this
variation in parallelism within well-researched study systems can provide
insight into the factors that contribute to variation in adaptive
responses. 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 predator-driven phenotypic
parallelism is influenced by six factors: sex, trait type, rearing
environment, ecological complexity, evolutionary history, and time since
colonization. Analyses show that parallel evolution in guppies is highly
variable and weak on average, with only 24.7% of the variation among
populations being explained by predation regime. Levels of parallelism
appeared to be especially weak for colour traits, and parallelism
decreased with increasing complexity of evolutionary history (i.e., when
estimates of parallelism from populations within a single drainage were
compared to estimates of parallelism from populations pooled between two
major drainages). Suggestive – but not significant – trends that warrant
further research include interactions between the sexes and different
trait categories. Quantifying and accounting for these and other sources
of variation among evolutionary “replicates” can be leveraged to better
understand the extent to which seemingly similar environments drive
parallel and nonparallel aspects of phenotypic divergence.