There are many ways that biological theory can inform ethical discussions of genetic engineering and biomedical enhancement. In this essay, we highlight some of these potential contributions, and along the way provide a synthetic overview of the papers that comprise this special issue. We begin by comparing and contrasting genetic engineering with programs of selective breeding that led to the domestication of plants and animals, and we consider how genetic engineering differs from other contemporary biotechnologies such as embryo selection. We go on to consider the implications of genetic engineering for human nature, human evolution, and persistence of the human species. Finally, we question whether genetic interventions warrant the extraordinary ethical scrutiny they are often given, and we show how the misleading "genetic blueprint" metaphor has imposed a faulty structure on the enhancement debate. We conclude by considering the nature of biological development and the sobering limits it places on what genetic engineering can reasonably hope to achieve