Development

During the last decades of the twentieth century, evolutionary developmental biology emerged to reintegrate the two temporal processes within biology, development and evolution. Evolutionary developmental biology (Evo-Devo) is based on the recognition that all genetic changes must be expressed during development in order to produce a phenotype and thus amount to observable evolutionary changes. Development is thus the mechanism that produces the raw material of phenotypic evolution. Phenotypic evolution, in contrast, appears to be highly constrained. Of all the possible forms (the total morphospace), only a small number are actually realized. Furthermore, the diversity of life is organized in a nested hierarchy, whereby millions of species can be subsumed within a few dozen phyla, each characterized by a basic body plan (Bauplan). In other words, the many mutational changes of genotypes are translated into a much smaller number of phenotypic variants.

In addition, discoveries since the 1980s have lent further support to the idea that the number of developmental modules (transcription factors, such as Homeobox genes and regulatory networks) is relatively small. Furthermore, these developmental modules have been conserved through millions of years during evolution, in that flatworms, insects, and mammals share a number of regulatory genes. Thus, a limited "genetic toolkit of development" produces the astonishing diversity of life. These findings have serious consequences for the age-old discussions of preformism versus epigenesis. The fact that a small number of genetic elements is responsible for the enormous diversity of life indicates that development is essentially a problem of regulation and the interaction of genetic and environmental factors. In other words, the effects of genes in development are largely context dependent. Whether a specific transcription factor turns on a gene that triggers a cascade leading to the formation of an eye or whether it establishes the gradient for differentiation of the arm, for example, depends on the specific cellular and organismal context. In addition, environmental factors, which can affect developmental plasticity, are increasingly recognized as important. The current conception of development is thus largely epigenetic, within the context of inherited material genes.