Species throughout the animal kingdom share not only housekeeping but also many key regulatory genes. Nonetheless, species differ from one another developmentally and thus, also morphologically. One of the general aims of comparative developmental genetics is to understand how similar molecules can generate the known diversity of biological form. Here, we argue that gene function can change in different ways during the evolution of developmental processes. Genes can be recruited to serve completely new functions in a new regulatory linkage (co-option), they can change their molecular specificity while remaining in the original (homologous) developmental program and can, at the same time, retain other functions. We describe evidence for such evolutionary patterns based on the comparison of loss-of-function mutations of homologous genes of the two free-living nematodes Caenorhabditis elegans and Pristionchus pacificus. Ultimately, it is the interplay of conservation and change of the specificity of genes and genetic networks that generates developmental novelty over evolutionary time.