Generic physical mechanisms of tissue morphogenesis: A common basis for development and evolution

Morphological evolution is usually considered to occur by the selection of small heritable variations in the expression of anatomical traits, on the basis of improved adaptation to new environmental conditions. An alternative mode of morphological evolution is proposed here: the production of a spectrum of forms by the action of intrinsic physical properties of cell aggregates, followed by intense selection for biochemical mechanisms that make the generation of a subset of viable morpholo-gies, and pathways of transition between morphologies, more reliable. This view provides an account of the origins of important features of metazoan body plans and organ forms, including gastrulation and other types of tissue multilayering, lumen formation, and segmentation. It also implies that most major morphological innovations would occur early in phylogeny, often more than once, with much subsequent genetic selection being of a stabilizing or canalizing variety. Consistent with recent findings, this view predicts that functional redundancy among develop-mentally important genes and genetic circuits should be prevalent