Shifting balance theory
![Sketch of a fitness landscape. The arrows indicate the preferred flow of a population on the landscape. The red ball indicates a population that moves from an adaptive valley to the top of an adaptive peak. Under a strict regime of natural selection (which usually acts to increase fitness in a population), it is not possible for a population at peak A to reach peak B because this requires descending into an adaptive valley. Shifting balance theory aims to explain how this may be possible.](/Images/godic/202502/10/Fitness-landscape-cartoon2710.png")
The shifting balance theory is a theory of evolution proposed in 1932 by Sewall Wright, suggesting that adaptive evolution may proceed most quickly when a population divides into subpopulations with restricted gene flow. The name of the theory is borrowed from Wright's metaphor of fitness landscapes (evolutionary landscapes), attempting to explain how a population may move across an adaptive valley to a higher adaptive peak. According to the theory, this movement occurs in three steps: