ON THE CIRCUMSTANCES OF OPPORTUNITY IN ESCALATION: THE COMPLEMENTARITY PRINCIPLE

Opportunities for enemy-related adaptation are most likely in a permissive environment, in which there is a surplus of biologically accessible raw materials and energy (Vermeij 1987, 2004). This underlying tenet of the escalation process has been typically ignored in tests conducted at the lineage-level, which trace adaptations related to passive defenses, escape performance and feeding behavior among other energy-intensive functions within physically similar environments through time. But it has played a central role in recent tests concerned with understanding the circumstances surrounding the origins and evolutionary fates of functionally important minor adaptive innovations, which open up previously unavailable ecological possibilities.

Despite its importance, we still know very little about whether the circumstances surrounding innovation and improvements that streamline or elaborate existing enemy-related adaptations are complements. A literature review of lineage-level tests of the escalation hypothesis suggests that the circumstances for improvement in existing adaptations are not always coincident with intervals thought to be conducive to minor innovation. This result implies that complementarity is not perfect; it is possible to have improvement without innovation. In other words, the circumstances for improving existing adaptations should be more common and less different from normal conditions than are the circumstances associated with the evolution of minor innovations.

An understanding of the nature of this environmental variation will be necessary for a full characterization of opportunity in escalation. An important question is: how do improvements in existing enemy-related adaptations during normal conditions influence the fate of innovations in more permissive environments? This kind of information is critical to conservation efforts aimed at facilitating organismal adaptation to human-induced environmental change.