From the 1930s through the 1970s, C. H. Waddington attempted to reunite genetics, embryology, and evolution. One of the means to effect this synthesis was his model of the epigenetic landscape. This image originally recast genetic data in terms of embryological diagrams and was used to show the identity of genes and inducers and to suggest the similarities between embryological and genetic approaches to development. Later, the image became more complex and integrated gene activity and mutations. These revised epigenetic landscapes (...) presented an image of how mutations could alter developmental pathways to yield larger phenotypic changes. These diagrams became less important as the operon became used to model differential gene regulation. (shrink)

Interspecific comparisons reveal remarkable diversity in patterns of gene expression, even among closely related species. Combinatorial regulatory mechanisms could facilitate the evolution of this diversity. However, the high degree of interdependence characteristic of combinatorial networks would represent a major constraint on evolution and might generate many features that have no direct adaptive value.