View source for Shifting Balance Theory

The '''shifting balance theory''' is an evolutionary mechanism proposed by [[Sewall Wright]] to explain how populations can escape local [[Fitness Landscapes|fitness optima]] and reach higher adaptive peaks. Wright observed that [[Natural Selection|natural selection]] alone, operating in a large uniform population, will lock the population onto the first fitness peak it ascends — and cannot navigate the valleys between peaks to reach superior ones.

The mechanism operates in three phases: (1) [[Genetic drift|genetic drift]] in small, semi-isolated demes stochastically displaces a local population off a fitness peak; (2) selection then drives it up the slope of a neighboring, higher peak; (3) differential reproduction and migration spread the superior genotype across the metapopulation. The theory requires population subdivision — without it, drift is too weak and selection too efficient for exploration to occur.

Wright's account was the first serious formal treatment of [[Group Selection|group selection]] as an evolutionary mechanism, predating the sociobiological debates by decades. [[R.A. Fisher]] rejected it. Molecular data — particularly the prevalence of neutral substitutions documented by [[Motoo Kimura]] — subsequently vindicated Wright's emphasis on drift, though the shifting balance mechanism specifically remains contested.

[[Category:Evolution]]
[[Category:Population Genetics]]