PUNCTUATED EQUILIBRIA AND THE ORIGIN AND EVOLUTION OF DEVELOPMENT

The evolution of animal morphology, generally thought to be based on selection of variants of small effect, does not conform to this gradualist scenario at the phyletic level by evidence from either paleontology or developmental anatomy. An alternative picture of the early evolution of animals is based on the intrinsic material properties of masses of metazoan cells. The structural motifs of animal bodies and organs, e.g., multilayered, hollow, elongated and segmented tissues, internal and external appendages, branched tubes and modular endoskeletons, can be viewed as the emergence over time, under different ecological settings, of forms inherent to assemblages of metazoan cells. These material properties derive from the recruitment of physical forces and mechanisms – adhesion, contraction, polarity, chemical oscillation, diffusion – by products of genes of the “developmental toolkit.” The toolkit genes - cadherins, BMPs, Notch, collagen, Wnt, and a few others - were partly carried over and repurposed from unicellular (holozoan) antecedents and partly metazoan novelties. Not all of them are present in all metazoan phyla but are distributed in a fashion that reflects, and indeed explains, the latters' morphological complexity. Thus, early diverging phyla such as Placozoa and Porifera have fewer toolkit genes than diploblasts, and later diverging triploblastic bilaterians have more. These genes are typically of large effect because of the abrupt transformations in tissue shape, form and pattern induced during development by the physical forces mobilized by their products. While the emergence of the genes may or may not have been gradual (the proteins specified by some of them are phylogenetically unprecedented) their morphogenetic effects and roles in phyletic divergence were saltational. Punctuated equilibria, therefore, which is exemplified by the sudden appearance of phylotypes followed by morphological stasis, rather than being an artifact of the fossil record, represents a reality of the "physico-genetic" determination of organismal form and its evolution in the metazoans.