Character Displacement in Evolutionary Time

Abstract

Patterns of distribution of morphology in the fossil record should be examined more rigorously to evaluate the relative effectiveness of alternative evolutionary models and to integrate the dimension of true evolutionary time more directly with evolutionary theory. Grant (1972) has discussed three main problems in the detection of character displacement: we must assume that sympatry followed allopatry, that the state in allopatry represents the static, pre-contact state, and that there are no other reasons accounting for differences within a species in allopatric and sympatric zones. These problems can be largely avoided with suitable paleoiitological data. In an example involving the coincident 10 million-year histories of two congeneric species of Devonian trilobites, the one known instance of sympatry reveals marked divergent shifts in one species and a “mixed” reaction of convergent, divergent, and neutral shifts in the other species. The characters undergoing these shifts are largely those which (i) serve as specific differentia , and (ii) show the most change within one of the species over its entire history. This suggests that character displacement may be simply a magnified microcosm of a general pattern of interaction between two species even when allopatric, provided that allopatry is sustained through competitive exclusion.