Constraint and Saltation

Developmental biology has long been a focus for evolutionary theory (Bonner 1982; de Beer 1958; Garstang 1922; Goldschmidt 1938; Gould 1977; Haeckel 1866; Raff 1996; Raff and Kaufman 1983; Waddington 1940). Evolution can be seen as a change in developmental programs that elaborate the phenotype. The effects of genes and the range of genetic variation would best be investigated on a mechanistic basis, yet until the 1990s, we had only a very small window on this enormously important developmental landscape.

Once we can understand the nature of development and how it constructs the phenotype, we confront anew some of the age-old questions of evolutionary biology. Development is legendary for its organization, sometimes appearing to be remarkably automatic and even self-organizing. The strong integration of the developmental process might not easily be breached by a mutant, which would disrupt fundamental and tightly integrated cellular and molecular processes. This would suggest a force for conservatism in evolution. On the other hand, the tremendous organization of developmental processes suggest to many that simple genetic changes might beget enormous saltatory evolutionary change.

The Janus-headed coin of development is illustrated well by the evolutionary change of the tail in ascidian tadpole larva, which has been lost in evolution several times independently (Jeffery 1997).