Shifting balance theory

Shifting balance theory is a theory by Sewall Wright used to model evolution using both drift and selection. Wright assumes that when populations are very small, genetic drift and epistasis are very important. Shifting balance theory:
1. A species is divided into subpopulations small enough that random genetic drift is 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