2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 2
Presentation Time: 8:15 AM


MCSHEA, Daniel W., Department of Biology, Duke Univ, Box 90338, Durham, NC 27708-0338, dmcshea@duke.edu

A simple principle predicts a tendency, or vector, toward increasing organismal complexity in the history of life. As the parts of an organism accumulate variations in evolution, they should tend to become more different from each other. In other words, the variance among the parts, or what I call the “internal variance” of the organism, should tend to increase spontaneously. It is obvious that internal variance must increase initially, when the parts are identical. But I present a model showing that increase is more likely than decrease even later on, after considerable differentiation has occurred. Further, I argue that internal variance is a type of complexity, albeit complexity in a purely structural sense, divorced from any notion of function. If the principle is correct, then this increasing tendency should exist in all evolutionary lineages, and therefore any trend in complexity that results will be “driven.” More precisely, it will be driven by statistical necessity, a kind of constraint, not by natural selection. The principle has roots in the metaphysical writings of Herbert Spencer and is present in other forms in contemporary evolutionary thought. However, since Spencer, to my knowledge it has not been stated explicitly and applied to explain the evolution of organismal complexity. Importantly, the principle might be correct, but an actual evolutionary trend in complexity still might not result, because the predicted tendency could be strongly opposed by selection. The tendency might also be negated if variations producing certain kinds of developmental truncations are especially common in evolution. Testing for this tendency ought to be difficult – in that it requires examining cases of evolutionary change in which selection was absent, or nearly so – but may not be impossible.