Coevolutionary alternation as an ecological cause for stasis

Abstract A primary tenet of punctuated equilibria (PE) is that stasis, that is, little to no net morphological change, characterizes the histories of species. In the past ~50 years since PE was proposed, stasis has been recognized in the evolutionary histories of many species, but consensus has not been reached concerning its causes. One unresolved issue is whether viable ecological mechanisms for stasis exist. We argue that a promising potential ecological explanation for stasis is coevolutionary alternation, which addresses how antagonists (e.g., predators or parasites and their groups of victims) coevolve over eco-evolutionary time across broad spatial scales. Coevolutionary alternation predicts different patterns of predator preferences and prey defenses within different populations and alternation of high and low levels of prey defenses as predator preferences evolve. The geographic structure of populations experiencing different environmental pressures and coevolutionary dynamics can yield stasis in such traits on the scale of entire species. We suggest that predator–prey coevolutionary alternation could be modeled using coupled stochastic differential equations (SDEs), which have been used to study correlative and causal connections among time series. SDEs can handle irregular sampling intervals, measurement uncertainty, and feedback loops between time series and can incorporate environmental proxies and time series from multiple geographic locations. We advocate developing this approach further to test the role of coevolutionary alternation in stasis and make recommendations for how SDEs might be used to model the coevolutionary feedback of predator(s) and multiple prey populations evolving in response to one another across space in a constantly changing environment.