Evolutionary developmental biology

Evolutionary developmental biology (evolution of development or informally, evo-devo) 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.

Introduction
During the 1980s and 1990s more comparative molecular sequence data between different kinds of organisms was 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 in response to these data.

Development and the origin of novelty
Among the more surprising and, perhaps, counter-intuitive results of such research in evolutionary developmental biology done in this period is that the diversity of body plans and morphology in organisms across many phyla are not necessarily reflected in diversity at the level of the sequences of genes involved in the regulation of 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 do not point 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 by several means, including gene duplication, mutation-driven changes in gene regulation, and epigenetic alterations in gene regulation or morphogenesis that are later consolidated by changes at the gene level. 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 are "second-order" effects of genes, resulting from the interaction and timing of activity of gene networks, 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 gene duplication and gene regulation with respect to their importance in the evolution of morphological diversity. Several biologists, including Sean B. 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.

Epigenetic changes include modification of the genetic material due to methylation and other reversible chemical alteration (Jablonka and Lamb 1995) as well as nonprogrammed remolding of the organism by physical and other environmental effects due to the inherent plasticity of developmental mechanisms (West-Eberhard 2003). The biologists Stuart A. Newman and Gerd B. Müller (see articles in Müller and Newman, 2003) have suggested that organisms early in the history of multicellular life were more susceptible to this second category of epigenetic determination than are modern organisms.