variation, which selection thereafter converts to evolutionary and genetic change in the population. Thus, we call it a theory of facilitated phenotypic variation.
We will discuss the means by which animals have generated developmental and physiological variation since Cambrian times. In the course of their descent from a common ancestor, animals have diverged in their anatomy and physiology by the gradual accumulation of selected heritable modifications, their phenotypic variations. Although such variation is indispensable to evolution, Darwin conceded that “our ignorance of the laws of variation is profound” (Darwin, 1859b, p. 167), and 150 years later the mode of its generation remains largely unknown. Phenotypic variation is thought to affect all aspects of an animal’s phenotype and to be “copious in amount, small in extent, and undirected” with regard to selective conditions (Gould, 2002). Most of these characterizations go back to Darwin himself. As Gould has noted (2002), they accord well with selection’s primacy as the creative force in evolution, refining chaotic, profligate variation into exquisite adaptations. However, they afford little insight into the generation of phenotypic variation, and they raise questions about how copious, small, and undirected variation really is. Although small in extent, heritable phenotypic variations need be significant enough to be selected, and, if complex change entails numerous sequential phenotypic variations, evolution may be impeded. An example we will pursue later is that of the species of Darwin’s finches that diverged in the Galapagos from a common ancestor. The beaks of some species are large and nutcracker-like, and those of others are small and forceps-like. As Darwin did, we too might imagine that many small heritable beak variations accrued slowly in the different species to create large observable differences. Small variations are arguably the only viable and selectable ones, because they would allow the upper and lower beaks, the adjacent skull bones, and head muscles to coevolve with each other in small selected steps, thereby maintaining viable intermediate beaks along the paths to the nutcracker and forceps forms. Repeated selections would be needed to coordinate the numerous, small, independent beak and head changes, all requiring genetic change. Is this an accurate appraisal of the paths of change? Or might the finch’s own means of beak development coordinate many changes, allowing larger viable variations and a simpler, more rapid beak evolution? Insight into the mode of generation of variation could answer such questions about the size, abundance, and directedness of phenotypic variations.
Research of the modern era has revealed that heritable phenotypic variation requires genetic change, that is, DNA sequence change. Changes