A UTILITARIAN APPROACH TO EVOLUTIONARY CONSTRAINT

The concept of evolutionary constraint is central to current perceptions of the evolutionary process, but operationally, it is difficult to apply. Much of this difficulty stems from conceptual ambiguity and inconsistent usage. The broadest conception of constraint as ''historical contingency'' has little pragmatic value because it universalizes constraint to a property of life. Likewise, equation of stabilizing selection with constraint creates unnecessary conceptual redundancy. Conceptions of constraint that emphasize mechanism over phenotypic or phylogenetic patterns tend to eliminate redundancy and restrict constraint to a force that shapes the action of natural selection, and which may oppose it. Constraint is a property of characters, not lineages, and at this level is always negative in the sense of limitation. However, character constraint is neutral to organismal adaptation and, therefore, can have either negative or positive evolutionary effects at the lineage level (i.e., hamper or promote organismal adaptation). Constraint hypotheses can be framed from either a posteriori or a priori perspectives that show that constraint is sensible only when bounded within a relative time-frame. Both stasis and parallelism have been invoked as phylogenetic manifestations of constraint and these alternate conceptions can lead to opposing hypotheses about the time and place of action of constraint. Disallowing pattern as prima fade evidence of constraint avoids this conflict. Homology and Bauplan might reflect the action of constraint at the level of one and many characters, respectively, but the failure to evolve at any given hierarchical level should not be taken as direct evidence of constraint.