We investigated phenotypic diversification in the species-rich and ecologically diverse turtle families Emydidae and Geoemydidae. In particular, we were interested in whether these groups, with many ecologically and morphologically similar species, show similar patterns of evolutionary radiation. We focused on directions of evolution and evolutionary allometry; we also quantitatively investigated whether two supposed morphological analogs shared within the two groups (e.g., “Box Turtles” and “Wood Turtles”) show evidence of convergence. A set of 53 three-dimensional landmarks were digitized on 1029 turtle shells representing 50 emydid species and 62 geoemydid species. These data were analyzed using standard geometric morphometric techniques. Evolutionary patterns were assessed using tests for phylogenetic signal, and the relationship between size and shell shape was determined via phylogenetic regression. Tests for phylogenetic signal in shell shape and for allometry both yielded significant results. Three hypothesis testing methods were applied to determine whether the supposed morphological analogs in the emydid and geoemydid families exhibited convergence. Although there was no evidence of shared adaptive peaks among either Box Turtles or Wood Turtles, there was evidence in both groups that their members have evolved to be more similar to each other compared to their ancestors than would be expected by chance. Thus, although both Box Turtles and Wood Turtles each show modest convergence, most of the evidence indicates a complex pattern of convergence and differentiation within both groups. Similarities in certain aspects of morphology or ecology might be overshadowing biologically meaningful variation within these morphological analogs.
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Vol. 72 • No. 2