PERSPECTIVE: A CRITIQUE OF SEWALL WRIGHT'S SHIFTING BALANCE THEORY OF EVOLUTION.

Area of Science:

• Evolutionary Biology
• Population Genetics

Background:

• Sewall Wright's shifting balance theory proposes a three-phase model for evolution.
• This theory involves genetic drift and selection acting on subdivided populations.

Purpose of the Study:

• To critically evaluate the theoretical underpinnings of Wright's shifting balance theory.
• To examine empirical data from natural and laboratory settings relevant to the theory's phases.

Main Methods:

• Theoretical analysis of evolutionary mechanisms.
• Review of natural and laboratory-based empirical data on population genetics and adaptation.

Main Results:

• Phases I and II (peak shifts) can occur, but genetic drift is often not essential.
• Phase III (species-wide adaptation spread) faces significant theoretical hurdles, including gene flow barriers and assembly of complex traits.
• Empirical data and artificial selection experiments do not strongly support the shifting balance theory over simpler models like mass selection.

Conclusions:

• The shifting balance theory, particularly its third phase, presents substantial theoretical and empirical challenges.
• The complexity and difficulty in testing the role of genetic drift make it an unlikely primary driver for the evolution of complex adaptations.
• The theory is unlikely to be a major explanation for the evolution of adaptations.