Correlated responses to selection. Correlated response to selection is a third critical evolutionary process that my students, collaborators, and I have investigated. A correlated response to selection is when one trait responds to direct selection on a second trait and is necessarily dependent on a genetic correlation between the two traits. In addition, genetic correlations and correlated responses to selection can actually constrain evolution when, for example, two traits share a positive genetic correlation with each other, but one is positively and one is negatively correlated with Darwinian fitness. The evolution of the integrated phenotype can be strongly influenced by such constraints.

Using the model system of mice, we have tested for a variety of correlated responses to selection on activity behavior in an attempt to elucidate the complexities of the evolution of the integrated phenotype. We have measured correlated responses in aging (Bronikowski et al., 2006), behavioral ontogenies (Morgan et al 2003), behavior (Bronikowski et al., 2001; Carter et al., 2000; Koteja et al., 1999; Klomberg et al., 2002), food consumption, energy budgets, and body mass (Koteja et al., 2001; Swallow et al., 2001; Koteja et al., 2000; Rhodes et al., 2000; Koteja et al., 1999; Swallow et al., 1999), life history traits (Girad et al., 2002) muscle traits (Zhan et al., 1999; Garland et al., 2002), metabolic traits (Swallow et al., 1998), and anti-oxidant enzyme activities (Thomson et al., 2002).

In summary, the emerging picture of the evolution of the integrated phenotype in this model system of mice is remarkable. Selection on activity behavior has resulted in the correlated evolution of several different behaviors, some-but not all-metabolic traits, tumor occurrence, survivorship, and anti-oxidant enzyme function. In addition, multiple constraints on the evolution of wheel running have been identified.