The idea of constancy in development and evolution - Scientific and philosophical perspectives.

The ability of developmental systems to produce constant phenotypes, even in a wide range of different environments, and the longstanding stability of species are among the most remarkable phenomena in biology. I argue that understanding the longstanding constancy and stability of species or the constant outcome of development in different environments are also prerequisites for explaining stable change (i.e., change that does not consist of random plasticity). Various approaches to account for stable changes in development are based on the causal role of genes and an organized genome, mathematical-physical-chemical models, or a combination of both. I argue that the constancy of developmental outcome and the longstanding stability of species are associated with organisms' structural and organizational hierarchies, particularly highly organized gene-regulatory networks and genetic causality, which are fundamental principles of life. Mathematical-physical-chemical models that marginalize these principles cannot convincingly account for the observed constancy in development and evolution. However, an integration of physical-chemical processes such as reaction-diffusion mechanisms and genome-based mechanisms of form generation has recently proved fruitful in explaining the development of some periodic structures. Constancy and change were also major topoi in ancient Greek philosophy, in which prominent philosophical schools such as the atomists attempted to bridge the antinomy between them by basing stable change on constant entities. I argue that the idea of change, that is, change without losing complexity or even increasing it, being based on modifications of the otherwise reliable transmission of genomes over long periods of time has a historical parallel in the writings of these ancient speculative thinkers, notwithstanding the fundamental differences between the two thought systems.