The Evolutionary Biology of Constraint Limits : The Biological Reasons

Abstract

parameters?the rate at which the generating curve expands, the rate of its translation down the axis of coiling, and the rate of its motion away from the axis (Fig. 1). Variation in these parameters can "convert" a snail into a clam into an ammonite into a brachiopod. If these three parameters are considered as axes of a cube, the potential morphological space of coiled objects can be defined. When all actual species are plotted onto this reference scheme, we find that living forms occupy rather little of the potential volume (Fig. 2). The history of evolutionary thought has been dominated by two major tra ditions for the explanation of such limited occupancy. In one, the adaptationist program1 that has prevailed under the "modern synthetic"2 theory of evolution, constraints are imposed primarily by working solutions: all parts of the cube can be reached, and unoccupied regions represent ill-adapted forms. In the second