OLSON, Wendy M; HALL, Brian K; Dalhousie University, Halifax NS; Dalhousie University, Halifax NS: Evolutionary Developmental Mechanisms:Space, Time and Morphology

Embryonic development has long been recognized as the vehicle for evolutionary change. However, we are only beginning to understand the evolutionary and developmental processes that constitute evolutionary developmental mechanisms, which include processes of development that mediate descent, whether that descent involves modification or stasis. These mechanisms, which operate hierarchically during development and evolution, include cell behavior, inductive tissue interactions, changes in timing or position (heterochrony and heterotopy) and the control of morphogenesis, patterning and growth. A challenge in understanding any of these mechanisms is an adequate means of comparing ontogenetic stages and timing within and among individuals, populations and taxa. For example, given the current emphasis on modularity in evo-devo, what units of heterochrony best reveal evolutionary developmental mechanisms? Should we focus on entire ontogenies, morphological structures or developmental events? Within these broad categories, we will assess how to standardize comparisons as well as select meaningful and stable developmental (and evolutionary) landmarks. We will address these issues in several contexts using published methodologies and data we have obtained from morphological analyses of the pipid frog Hymenochirus. Our discussion will include assessment of the utility of staging tables, measures of absolute and relative time, ontogenetic trajectories, and pair-wise comparisons (within and between organ systems) in evolutionary developmental analyses. Supported by NSERC (Canada).