Body size impacts numerous aspects of organismal biology and many factors have been invoked to explain body size distributions in a macroecological and macroevolutionary context. Body size in the freshwater fish family Percidae is strongly right-skewed (i.e, dominated by small sizes), with small body size potentially being associated with fast water habitats. We constructed a new species-level, multi-locus, time-calibrated phylogeny of Percidae, and used it to test for changes in the rate and pattern of maximum body size evolution. We also tested whether speciation rates varied as a function of body size. We found that Etheostomatinae evolved towards a smaller adaptive optimum in body size compared to the other subfamilies of Percidae, and that this shift was associated with a reduction in the rate of body size evolution. Speciation rates were associated with body size across percids, showing a peak around small to medium body size. Small body size appears to partially, but not fully, explain the diversity of small percids, as many darters fall well below the “optimum” body size. Reinforcement of selection for small body size via selection for novel morphologies or via sexual selection may help to fully explain the remarkable diversity of the darter radiation.

XML files for BEAST 2 analyses provided, as well as all genbank data for selected fasta files. Results from 3 BEAST2 runs for the constrained and unconstrained topologies given as the combined posterior distribution of trees and the maximum clade credibility chronogram.