Resynthesizing evolutionary and developmental biology.

A new and more robust evolutionary synthesis is emerging that attempts to explain macroevolution as well as microevolutionary events. This new synthesis emphasizes three morphological areas of biology that had been marginalized by the Modern Synthesis of genetics and evolution: embryology, macroevolution, and homology. The foundations for this new synthesis have been provided by new findings from developmental genetics and from the reinterpretation of the fossil record. In this nascent synthesis, macroevolutionary questions are not seen as being soluble by population genetics, and the developmental actions of genes involved with growth and cell specification are seen as being critical for the formation of higher taxa. In addition to discovering the remarkable homologies of homeobox genes and their domains of expression, developmental genetics has recently proposed homologies of process that supplement the older homologies of structure. Homologous developmental pathways, such those involving the wnt genes, are seen in numerous embryonic processes, and they are seen occurring in discrete regions, the morphogenetic fields. These fields (which exemplify the modular nature of developing embryos) are proposed to mediate between genotype and phenotype. Just as the cell (and not its genome) functions as the unit of organic structure and function, so the morphogenetic field (and not the genes or the cells) is seen as a major unit of ontogeny whose changes bring about changes in evolution.