Simultaneous integration and modularity underlie the exceptional body shape diversification of characiform fishes

Evolutionary biology has long striven to understand why some lineages
diversify exceptionally while others do not. Most studies have focused on
how extrinsic factors can promote differences in diversification dynamics,
but a clade’s intrinsic modularity and integration can also catalyze or
restrict its evolution. Here, we integrate geometric morphometrics,
phylogenetic comparative methods and visualizations of covariance to infer
the presence of distinct modules in the body plan of Characiformes, an
ecomorphologically diverse fish radiation. Strong covariances reveal a
cranial module, and more subtle patterns support a statistically
significant subdivision of the postcranium into anterior (precaudal) and
posterior (caudal) modules. We uncover substantial covariation among
cranial and postcranial landmarks, indicating body-wide evolutionary
integration as lineages transition between compressiform and fusiform body
shapes. A novel method of matrix subdivision reveals that within- and
among-module covariation contributes substantially to the overall
eigenstructure of characiform morphospace, and that both phenomena led to
biologically important divergence among characiform lineages. Functional
integration between the cranium and post-cranial skeleton appears to have
allowed lineages to optimize the aspect ratio of their bodies for
locomotion, while the capacity for independent change in the head, body
and tail likely eased adaptation to diverse dietary and hydrological
regimes. These results reinforce a growing consensus that modularity and
integration synergize to promote diversification.