In the late nineteenth century, “development” and “evolution” were considered to be so intimately related that many biologists used the terms interchangeably, applying them rather indiscriminately to both the process that generates a new individual resembling its parents and the process that generates a new species different from its ancestors. For most twentieth-century biologists, continuation of that practice would have been unthinkable, because evolution and development have seemed to us to be such fundamentally different phenomena. However, history has a way of repeating itself. After a century-long estrangement that began with widespread rejection of Haeckel's dogma that “ontogeny recapitulates phylogeny,” evolution and development are now undergoing a dramatic rapprochement, with genetics acting as the broker for their remarriage. Increasingly, those investigating the mechanisms by which differentiated cells and organs arise in the course of embryonic development and those seeking to understand how morphological novelties arise in the course of evolution find themselves converging on the study of related sets of genes, and talking to one another again!

Contemporary studies of Volvox, the rolling green spheroid that first fascinated Antoni van Leeuwenhoek 300 years ago, illustrate this sort of convergence. When my wife, Marilyn, and I began to study Volvox more than 20 years ago, our objective was to capitalize on its simplicity to address a central problem of development that we had found it extremely difficult to address with any clarity by studying vertebrate embryos: How do cells with very different phenotypes arise from the progeny of a single cell?