Wright's Shifting balance theory is a theory by Sewall Wright used to model evolution using both drift and selection. Shifting balance theory: 1. A species divided into subpopulations small enough that random genetic drift important 2. Drift in one subpopulation carries it past an adaptive valley to a new higher adaptive peak 3. This subpopulation has increased fitness. It makes more offspring, which spread the new adaptation to other populations through increased gene flow