Evidence of phenotypic parallelism is often used to infer the deterministic role played by natural selection. However, variation in the extent or direction of divergence is often evident among independent evolutionary replicates, raising the following question: just how parallel, overall, is parallel evolution? We answer this question through a comparative analysis of studies of fishes, a taxon where parallel evolution has been much discussed. We first ask how much of the among-population variance in phenotypic traits can be explained by different "environment" types, such as high predation versus low predation or benthic versus limnetic. We then use phenotypic change vector analysis to quantify variation in the direction (vector angles) and magnitude (vector lengths) of environment-associated divergence. All analyses show high variation in the extent of parallelism-from very high to very low, along with everything in between-highlighting the importance of quantifying parallelism rather than just asserting its presence. Interestingly, instances of low extents of parallelism represent important components of divergence in many cases, promising considerable opportunities for inferences about the factors shaping phenotypic divergence.

Shoals of golden shiners (Notemigonus crysoleucas) often swim along the perimeter of their large indoor tank at dawn and dusk, and can also be trained to anticipate food arrival by swimming directly towards the food source at midday. In this study all golden shiners in six shoals of 8-12 fish were individually marked with a visible implant elastomer, and shoal movement was video taped in order to determine whether some individuals consistently occupied front positions even when all shoal members were of similar size and experience. There were significant correlations between all three times of day (dawn, midday, dusk) in the mean position (from 1 = first at the front of the shoal to 12 = last at the back) occupied by each fish. In each shoal, 1-3 fish were leaders: all three daily times combined, they had more than twice the occupancy rate of the front two or three positions as expected from chance. In subsequent boldness tests there was a tendency (p = 0.096) for these leaders to pass through dark U-shaped tubes more readily than non-leaders. However, after being dipnetted and transferred to a refuge, leaders did not emerge earlier than non-leaders. Individual tendencies to lead may have been underlain by a motivation to feed (which may differ even in fish of similar size and experience) or by intrinsic mobility. On the other hand, a link between leadership and risk-taking remains to be established for captive golden shiners.

The Trinidadian guppy is emblematic of parallel and convergent evolution, with repeated demonstrations that predation regime is a driver of adaptive trait evolution. A classic and foundational experiment in this system was conducted by John Endler 40 years ago, where male guppies placed into low‐predation environments in the laboratory evolved increased color in a few generations. However, Endler's experiment did not employ the now typical design for a parallel/convergent evolution study, which would employ replicates of different ancestral lineages. We therefore implemented an experiment that seeded replicate mesocosms with small founding populations of guppies originating from high‐predation populations of two very different lineages. The different mesocosms were maintained identically, and male guppy color was quantified every four months. After one year, we tested whether male color had increased, whether replicates within a lineage had parallel phenotypic trajectories, and whether the different lineages converged on a common phenotype. Results showed that male guppy color generally increased through time, primarily due to changes in melanic color, whereas the other colors showed inconsistent and highly variable trajectories. Most of the nonparallelism in phenotypic trajectories was among mesocosms containing different lineages. In addition to this mixture of parallelism and nonparallelism, convergence was not evident in that the variance in color among the mesocosms actually increased through time. We suggest that our results reflect the potential importance of high variation in female preference and stochastic processes such as drift and founder effects, both of which could be important in nature.

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