In the nineteenth century, ideas about development, heredity and evolution were inextricably mixed up, because it seemed natural to suppose that changes that first occurred in development could become hereditary, and so could contribute to evolution. This was not only Lamarck’s view but Darwin’s, expressed in his theory of pangenesis. Weismann liberated us from this confusion, by arguing that information could pass from germ line to soma, but not from soma to germ line. If he was right, geneticists and evolutionary biologists could treat development as a black box: transmission genetics and evolution could be understood without first having to understand development. Since Weismann, developmental biology has had only a rather marginal impact on evolutionary biology. One day, we have promised ourselves, we will open the box, but for the time being we can get along very nicely without doing so. Recent progress in developmental genetics, some of which has been reviewed in the last three chapters, oblige us to reopen the question. In fact, there are three related questions, not one. The first, which is most relevant to the theme of this book, is the ‘levels of selection’ question: why does not selection between the cells of an organism disrupt integration at the level of the organism? This is the topic of section 15.2. The second is the problem of the inheritance of acquired characters. This old problem has reappeared in a new guise. We now recognize the existence of cell heredity, mediated by different mechanisms from those concerned with transmitting information between generations. In section 15.3, we discuss whether cell heredity plays any role in evolutionary change. Finally, in sections 15.4 and 15.5, we ask whether recent molecular information sheds any light on another old problem—that of the extraordinary conservatism of morphological form, maintained despite dramatic changes of function. This conservatism has led anatomists to identify a small number of basic archetypes, or bauplans. There is little doubt that conservatism is real. Consider, for example, the fact that bones and cartilages, which in humans serve in swallowing, sound production and hearing, are derived from elements of the gill apparatus whereby our fish ancestors exchanged gases with seawater, and, before that, in all probability, from elements of a filter-feeding apparatus.
The transmission of acquired characters
