Evolutionary developmental biology (often referred to as 'evo-devo' or evolution of development) is a field of biology that compares the developmental processes of different animals in an attempt to determine the ancestral relationship between organisms and how developmental processes evolved. The discovery of genes regulating development in model organisms allowed for comparisons to be made with genes and genetic networks of related organisms.
During the 1980s and 1990s more comparative molecular sequence data between different kinds of organisms has been amassed and detailed understanding of the molecular basis of the developmental mechanisms which are encoded by those genes has become clearer. Evolutionary developmental biology has arisen as a response to these growing trends.
One of the more surprising and perhaps, counter-intuitive, results of such research in evolutionary developmental biology done in this period, is that both the diversity of body plans and morphology in organisms across many phyla is not necessarily reflected in similar diversity at the at the level of the genetic sequences controlling development. Indeed as Gerhart and Kirschner (1997) have noted there is an apparent paradox: "where we most expect to find variation, we find conservation, a lack of change".
Even within a species, the occurrence of novel forms within a population points to the preexistence of genetic variation sufficient to account for morphological diversity. For example, there is significant variation in limb morphologies amongst salamanders and the differences in segment number in centipedes, even when the genetic variation is low.
A big question then, for evo-devo studies is where does the novelty come from? If the morphological novelty we observe at the level of the different clades is not always reflected in the genome, where does it come from?
Novelty may arise through several methods including gene duplication and gene regulation. Gene duplication allows fixation of a particular cellular or biochemical function at one locus, leaving the duplicated locus free to fulfill a new function. In contrast, changes in gene regulation, is a "second-order" effect of genes, resulting from the interaction and timing of the genetic network, as distinct from the functioning of the individual genes in the network
The discovery of the homeotic Hox gene family in vertebrates in the 1980s, allowed researchers in developmental biology to empirically assess the relative roles of the above two factors, with respect to their importance in the evolution of morphological diversity. Several biologists, including Sean Carroll of the University of Wisconsin suggest that "changes in the cis-regulatory systems of genes" are more significant than "changes in gene number or protein function" (Carroll 2000).
These researchers argue that the combinatorial nature of transcriptional regulation allows a rich substrate for morphological diversity, since variations in the level, pattern, or timing of gene expression, may provide more variation for natural selection to act upon, than changes in the gene product alone.