Coevolution of functionally constrained characters: prerequisites for adaptive versatility.

One of the major problems of organismic evolution theory is to explain how complex organisms were able to evolve by random mutations in spite of the severe functional constraints that canalize their route of change. The problem is discussed on the basis of a quantitative genetic model. How the degree of genetic variation influences the adaptation speed of functionally coupled but genetically uncorrelated characters is examined. It was found, that if more than three independent characters contribute to the variation of a functionally constrained system, optimal degrees of genetic variation exist. Higher degrees of variation lead to decreasing adaptation rates. Conversely, functional constraints do not limit the degree of adaptely reasonable genetic variability as long as the number of independent characters is not higher than three. The conclusion is drawn that there is no need to develop a genetic correlation between functionally coupled characters as long as not many more than three characters are integrated into a functional system. This explains the fact that there is no genetic coupling between the inherited signal sender and receiver mechanisms in orthopterians, even though there is a strong functional coupling between them.