Changes in the expression and function of genes active during metazoan development have played a critical role in the evolution of morphological differences between species and phyla, yet the origins of these changes remain poorly understood. What roles do positive and negative selection play in the evolution of development? How do evolutionary changes accumulate given the degree to which organisms are able to buffer the effects of environmental and genetic perturbations during development? The crucial insight of the Modern Evolutionary Synthesis was that divergence between species arises from variation within populations. Following this principle, I have made use of tools from quantitative and population genetics to investigate three central questions: 1) How much genetic variation is there in the networks of genes that underlie metazoan development? 2) What affect does developmental buffering have on the accumulation of selectable genetic variation? 3) To what extent does selection act to shape patterns of genetic variation among different kinds of genes and at different stages of development? I show that developmental systems can harbor extensive levels of genetic variation, and that the amount of genetic variation in individual genes at different stages of development is related to the extent to which variation in those genes is buffered by genetic interactions. I also show that while selection plays an active role in shaping genetic variation in development, the extent to which variation in a gene is visible to selection depends in predictable ways on a) the biological function of that gene and b) whether the mutations in question influence gene expression or protein function. My results as a whole demonstrate the utility of population level approaches to the study of the evolution of development, and provide key insights into the role that selection plays in generating developmental variation.
