Developmental evolution and the origins of phenotypic variation.

Because of the variability of relevant developmental resources across different environments, and because only a portion of the genome is expressed in any individual organism as a result of its specific developmental context and experience, what is actually realized during the course of individual development represents only one of many possibilities. One conclusion to be drawn from this insight is that the origin of phenotypic traits and their variation can be traced to the process of development. In this conceptual overview, I briefly explore how recent efforts to integrate genetic, epigenetic, and environmental levels of analysis through a developmental lens is advancing our understanding of the generation of the stability and variability of phenotypic outcomes observed within and across generations. A growing body of evidence indicates that phenotypes are the outcomes of the whole developmental system, comprised of the organism, with its particular genetic and cellular make-up in its specific physical, biological, and social environments. I conclude that the emergent products of development are epigenetic, not just genetic, and evolutionary explanation cannot be complete without a developmental mode of analysis.