Related Papers

Calsbeek, R., and R.M. Cox. 2010. Experimentally assessing the relative importance of predation and competition as agents of selection. Nature 465: 613-116.

Figure 1. Photos of female brown anoles, illustrating typical variation within and between the three dorsal pattern morphs. From Calsbeek et al. (2009).

How do competition and predation shape the adaptive landscape?

Despite a burgeoning literature describing the strength and form of natural selection in wild populations, our current view of this process is based almost entirely on correlations. This presents a major impediment to a true causal understanding of the most important force in evolution. We take an experimental approach to the study of natural selection by manipulating both the phenotypic targets of selection and the ecological agents that shape selection in wild populations.

One example of this approach is a recent study in which we manipulated the presence of both bird and snake predators across replicated island populations of anoles (Calsbeek & Cox, 2010). Although predators had a dramatic effect on the perching behavior and overall survival of anoles, selection on anole morphology and performance did not differ across predation treatments (Fig. 1). However, as the density of lizards increased, the strength of natural selection also increased to favor large size, long limbs, and high stamina (Fig. 1). These results suggest that competition may be more important than predation in shaping natural selection on several key traits in the adaptive radiation of Anolis lizards.

Figure 1. Left panels show average selection differentials across replicate islands from three predation treatments: no predators, bird predators, and bird + snake predators. Selection on morphology and performance did not differ across predation treatments. However, the panels on the right show that selection differed as a function of our manipulations of lizard density, suggesting that competition is more important than predation in generating selection on anoles.