Wright's Shifting Balance Theory: An Experimental Study

Abstract

Experimental confirmation of Wright's shifting balance theory of evolution, one of the most comprehensive theories of adaptive evolution, is presented. The theory is regarded by many as a cornerstone of modern evolutionary thought, but there has been little direct empirical evidence supporting it. Some of its underlying assumptions are viewed as contradictory, and the existence and efficacy of the theory's fundamental adaptive process, interdemic selection, is the focus of controversy. Interdemic selection was imposed on large arrays of laboratory populations of the flour beetle Tribolium castaneum in the manner described by Wright: the differential dispersion of individuals from demes of high fitness into demes of low fitness. A significant increase in average fitness was observed in the experimental arrays when compared to control populations with equivalent but random migration rates. The response was not proportional to the selection differential: The largest response occurred with interdemic selection every two generations rather than every generation or every three generations. The results indicate that the interdemic phase of Wright's shifting balance theory can increase average fitness and suggest that gene interactions are involved in the observed response.