UNPREDICTABILITY OF CORRELATED RESPONSE TO SELECTION: PLEIOTROPY AND SAMPLING INTERACT

Given a set of loci that contribute additive genetic variation for a trait being selected, the pleiotropic effects of these loci on a second trait may vary. I simulated selection on genetic systems having different combinations of pleiotropic effects to investigate the variability of correlated responses to selection. The simulation shows that there are many possible combinations of pleiotropic effects that are characterized by the same value of the genetic correlation; the genetic correlation does not uniquely determine a set of pleiotropic effects. Furthermore, for a given value of the genetic correlation, differences in pleiotropic effects have a substantial impact on the variation in correlated responses. Some combinations of pleiotropic effects constrain correlated response to a narrow range of possible values; others allow a wide range, including some correlated responses in a direction opposite the sign of the genetic correlation. The genetic correlation is not a reliable predictor of pleiotropic constraint. Whereas it has been previously established that genetic correlations are not necessarily constraints, the alternative is also true: correlated response can be strictly constrained despite a genetic correlation of zero. Given the frequency of correlated responses in a direction opposite to the one predicted by the genetic correlation, it follows that correlated response is not a reliable predictor of genetic correlation in the base population.