Introduction

One of the pillars of developmental genetics has been description of the phenotypic consequences of mutations in particular developmental genes (see, for example, Nusslein-Volhard and Wieschaus (1980)). Many such studies have been conducted over the last couple of decades on a small but increasing range of organisms (notably Drosophila, Caenorhabditis, Danio, Xenopus and Mus). As a result, there is now a considerable body of information at our disposal on the ways in which mutations can alter individual ontogenies. Some of these alterations are trivial, while others – such as the famous four-winged Drosophila – are quite dramatic. (We looked at many examples of developmental mutations in Chapter 6, and considered a conceptual framework in Chapter 8.)

Have these various developmental mutations contributed to evolutionary change? There are two components to this question: 1. Do the mutations concerned occur spontaneously in natural populations as well as through the use of mutagenizing procedures/agents in the laboratory? 2. If they do, then are they able to spread through one or more populations, and to displace their alternative alleles?

Spontaneous occurrence in natural populations is hardly in doubt. We cannot rule out the possibility that some synthetic mutagens which an organism never encounters in the wild may produce some specific types of mutation that would not otherwise occur at all. However, in general, the difference between field and laboratory is merely one of frequency, with mutagenizing procedures simply causing particular mutations to occur more often than they would naturally, in order to facilitate their study.