Preface

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.

The production of phenotype is regulated by differential gene expression. However, the regulators of gene expression need not all reside within the embryo. Environmental factors, such as temperature, photoperiod, diet, population density, or the presence of predators, can produce specific phenotypes, presumably by altering gene-expression patterns. The field of ecological developmental biology seeks to look at development in the real world of predators, competitors, and changing seasons. Ecological concerns had played a major role in the formation of experimental embryology, and they are returning as the need for knowledge about the effects of environmental change on embryos and larvae becomes crucial. This essay reviews some of the areas of ecological developmental biology, concentrating on new studies of amphibia and Homo. Copyright 2001 Academic Press.

Most of the morphological and functional differences between vertebrates and other chordates occur in the head and are derived embryologically from muscularized hypomere, neural crest, and epidermal (neurogenic) placodes. In the head, the neural crest functions as mesoderm and forms connective, skeletal, and muscular tissue. Both the neural crest and the epidermal placodes form special sense organs and other neural structures. These structures may be homologous to portions of the epidermal nerve plexus of protochordates. The transition to vertebrates apparently was associated with a shift from a passive to an active mode of predation, so that many of the features occurring only in vertebrates became concentrated in the head.