Mechanisms of Pattern Formation, Morphogenesis, and Evolution

This chapter introduces the diversity of ways in which developmental mechanisms lead to pattern formation and morphogenesis. Developmental mechanisms are described as gene networks in which at least one of the genes affects some cell behavior (cell division, cell adhesion, apoptosis, cell contraction, cell growth, signal and extracellular matrix secretion, etc.). These mechanisms mediate one of the most important processes in development: the transformation of specific distributions of cell types in space (starting with the zygote) into other, often more complex, spatial distributions of cell types (later developmental stages ending up in the adult). This chapter explains in detail why genes alone are unable to account for pattern formation and why they require cell behaviors and epigenetic factors. Three main types of developmental mechanisms are described in this respect: autonomous, inductive, and morphogenetic. This chapter also explains how these three types of mechanisms are combined in animal development and how these different combinations lead to different kinds of phenotypic variation and morphological evolution. It is concluded that understanding the mechanisms of development is crucial to have a more complete evolutionary theory in which extant phenotypes can be explained based not only on natural selection but also on what phenotypic variation can be produced by development in each generation.