NEWMAN, S.A.; New York Medical College: Interplay between Genetic and Physical Mechanisms in Morphological Innovation

Living tissues are chemically active, viscoelastic materials. This implies that the forms they assume are determined, in part, by mechanical, mechanochemical, and other physical processes. It is proposed that at early stages in the evolution of body plans and organ primordia physical determinants played a more prominent role in generating morphological novelties than they do in modern-day organisms. Materials molded by physical and physicochemical processes tend to exhibit a high degree of morphological plasticity and therefore capability of generating structural novelties. Then, as a result of stabilizing evolution, such plasticity would increasingly be replaced by pathways of development that were canalized, programmed, and �overdetermined.� Physical processes such as diffusion, reaction-diffusion coupling, differential adhesion, and regulation of cell-cell adhesion by biochemical oscillators, which have no morphological consequences in a world of single cells, would have begun to play unprecedented roles in the formation of novel tissue configurations�multilayering, lumen formation, segmentation, rods, nodules, tubes�once multicellularity arose. Similar novelty-generating processes would have been mobilized when the same physical processes came to act upon any neomorphic cell aggregates, such as the neural crest, or the tetrapod digital plate. The idea that morphological novelties arise from the interplay of gene products and genetically-specified cell properties with physical and physicochemical processes helps explain findings that are difficult to reconcile with the standard neo-Darwinian model. These include the burst of body plans in the early Cambrian and the punctuated character of morphological innovation.