As stated previously, evolutionary biology is required to integrate all kinds of biological information into a coherent theory of biological evolution, and of course all what is learnt from developmental biology and developmental genetics. In this essay I tried to show, with a few examples, how an evolutionary thinking, and especially asking ''why'' questions, could highlight some aspects of developmental biology. Evolutionary biology is not restricted to the task of explaining relatedness and phylogenies among species or clades. It asks general questions such as why are there multicellular organisms, why are there sexes, are similar traits homologous or convergent, etc. The answers attempt to be plausible, coherent and with a maximum likelihood. They are permanently subjected to modelisations, simulations, general confrontations and become progressively more satisfying for the scientific community.
Both disciplines now integrate genetic analysis - developmental genetics and evolutionary genetics. Up to now, there has been little overlap between these two fields, for one main reason. Developmental genetics has focused mainly on the early stages of embryonic development, unravelling the general orientation of the future organism, the origin of the segments, of the nervous system etc. Evolutionary genetics, at least in its functional aspects, has focused on complex, adult phenotypes. Adaptive genetic variations are generally described with statistical means, the responsible genes being considered as a blackbox. Only recently the search of QTL (quantitative trait loci) has become a major task for quantitative geneticists.
After unravelling the early stages of embryonic development, developmental genetics is now more concerned by later stages and the determinism of adult phenotype. Not surprisingly numerous ''early development genes'' are highly pleiotropic and expressed in various organs until the adult stage. QTL detected by phenotypic studies often turn out to be identical to previously known developmental genes. The major challenge of developmental genetics will be to understand the regulatory mechanisms of gene expression which are responsible of cell differentiation. But on the phenotypic side, we already know that QTL often imply a polymorphism not at the protein level, but in the regulatory sequences of a gene. In this respect we see a strong convergence between the two approaches. Time is coming when developmental genetics and quantitative genetics will fuse into a single discipline, tentatively called now functional genomics.