Vertebrate development: an overview

Abstract

Complexity increases during development and development is hierarchical. Simon (1962), in providing an elegant exposition of why hierarchical organization is basic to what he termed ‘the architecture of complexity’, focused on two main properties of hierarchical organization. Hierarchical systems possess common properties that are independent of the specific content of the system, a theme developed by Waddington (1940, 1942) in the context of epigenetic developmental programmes that can change without every gene involved in the programme having to change (section 6.5). This has more recently been elaborated and extended by Hall (1983, 1990b), Sachs (1988) and Verraes (1989). Hierarchical systems can evolve faster than nonhierarchical ones of similar size and complexity.

Development is therefore the immediate cause of introduction of variation at the level of the individual organism... Central to understanding the role of development in evolutionary mechanisms must be the study of the emergent and epigenetic properties of developing systems and their unique role in the processes by which variation is introduced among the individual phenotypes. (Thomson, 1988, pp. 16–17.)