Macroevolutionary Disparity as a Consequence of Developmental Conservation and Lability

Developmental systems mediate the production of heritable phenotypic variation which itself is the raw material on which natural selection operates.  It stands to reason that evolution might therefore be predicted by the ancestral variational condition.  However, comparisons between phenotypic variation as an input to natural selection and disparity as a macroevolutionary output of this process are rare and rigorous tests of the evolutionary conservation of phenotypic patterns of variation are at least as uncommon.  The fossil record offers unique opportunities to investigate how developmental processes might be linked to macroevolutionary patterns in a variety of metazoan study systems.  Here, I examine two forms of developmentally-mediated variation, dynamic trait variation across ontogeny and static trait (co)variation at morphological maturity.  In the early Cambrian trilobite genus Zacanthopsis, ontogenetic growth trajectories of multiple species are compared.  Many growth trajectories are modified to the point of divergence over an evolutionarily short timescale (about 1 million years) and on a phylogenetically local level (within a genus), with one species converging on the morphology of an older member of the genus. Across a 2–3 million year long radiation of post-Pleistocene murine rodents in southwestern Australia, patterns of static trait (co)variation in morphologically mature mandibles are compared over microevolutionary and macroevolutionary scales.  Small microevolutionary changes to the pattern of static variation are observed alongside substantial macroevolutionary modifications apparently driven by a mosaic of traits with labile or conserved developmental underpinnings.  Developmental patterns of phenotypic variation, whether dynamic or static, serve as predictors for macroevolutionary outcomes, but at least in these studies those predictors are evolutionarily short-lived and the production of novel phenotypes (and even phenotypic convergence) owes itself much more to the complex repatterning of labile developmental influences rather than adherence to an ancestral expression of phenotypic variation.