Important theoretical concepts tend to resist satisfactory definition (cf. Stigler, 1957). Such concepts are in the service of the expansive ambitions of the theories in which they occur, and must accordingly respond flexibly to the changing requirements for maintaining order in a changing intellectual empire. The term ‘evolution’ — obviously important in biology, but also in the physical and social sciences — provides a good illustration of this principle. A prominent biologist and author of a highly expansive treatise on biological evolution had the following to offer in his glossary:
. Any gradual change. Organic evolution, often referred to as evolution for short, is any genetic change in organisms from generation to generation, or more strictly, a change in gene frequencies within populations from generation to generation (Wilson, 1975).