Does Phenotypic Integration Promote Convergent Evolution?

Phenotypic integration is often perceived as being able to produce convergent evolution in the absence of selection, but specific mechanisms for this process are lacking and a connection has never been empirically demonstrated. A new model of the effect of integration on convergence provides such a mechanism, along with other predictions about the influence of integration on evolutionary patterns. I use simulations and data from three empirical systems-turtle shells, characiform fish, and squirrel mandibles-to investigate the degree to which evolutionary integration is associated with high levels of convergent evolution. Levels of integration were varied in Brownian motion simulations and the resulting amounts of stochastic convergent evolution were quantified. Each empirical system was divided into modules, and the strength of integration, average amount of convergence, phenotypic disparity, and rate of evolution in each module were measured. Results from the simulations and from all three empirical systems converge on a common result: higher levels of phenotypic integration are indeed associated with higher levels of convergence. This is despite a lack of consistent association between the strength of phenotypic integration and evolutionary rate or disparity. The results here are only correlational. Further studies that more closely examine the influence of within-population drivers of evolutionary integration-for example, genetic or developmental integration-on convergence are required before it is possible to definitively establish when phenotypic integration can cause evolutionary convergence. Until then, however, the results of this study strongly suggest that phenotypic integration will often promote convergent evolution.