Evolution: Shifting Balance Theory

Stephen J Tonsor, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
Francisco B‐G Moore, Michigan State University, East Lansing, Michigan, USA

Published online: April 2001

DOI: 10.1038/npg.els.0001781

Abstract

The shifting balance is an evolutionary process by which: 1) Drift and selection in small populations drive among‐population epistatic differentiation. 2) The epistatic combinations differ in fitness, and their mean fitnesses are dependent on the joint frequencies of the various epistatic combinations. 3) Epistatic differentiation among populations drives among‐population selection when migration levels depend on population mean fitness. This leads to the replacement of the ancestral epistatic combination throughout the metapopulation, under some circumstances.

Keywords: epistasis; adaptive topography; fitness surface; peak shift; random genetic drift

Figure 1.

A two‐locus genotypic fitness surface in which the fitness of the genotypes is determined by formulae first published by Crow et al., and configured as used in Moore and Tonsor . In this kind of epistatic interaction, there are nine possible two‐locus, two‐allele combinations, involving 0, 1 or 2 copies of mutant ‘A’ allele at one locus, and of the mutant ‘B’ allele at a second locus (the alternatives are the wild type ‘a’ and ‘b’ alleles, respectively). Three fitness phenotypes are produced by the nine genotypes: 1) Individuals with at least one copy of A and one copy of B. These have a relative fitness of 1 + ks. 2) Individuals homozygous for both a and b alleles. These individuals have a relative fitness of 1‐s. 3) Individuals heterozygous at one locus but homozygous wild type (aa or bb) for the other locus. These have an intermediate relative fitness of 1. In this figure, k = 4 and s = 0.025.

Figure 2.

An ‘adaptive topography’ for the two‐locus two‐allele interaction whose genotypic fitnesses are illustrated in Figure . Based on Moore and Tonsor .
Figure 3.

The frequency of SBP through Phase III for the epistatic interactions used Figures and . Ten thousand demes with average N = 30, simulations run for 12 000 generations.

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References

Coyne JA, Barton NH and Turelli M (1997) Perspective: a critique of Sewall Wright's shifting balance theory of evolution. Evolution 51: 643–671.

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Further Reading

Charlesworth B and Rouhani S (1998) The probability of shift peaks in a founder population. II. An additive polygenic trait. Evolution 42: 1129–1145.

Coyne JA, Barton NH and Turelli M (1997) Perspective: a critique of Sewall Wright's shifting balance theory of evolution. Evolution 51: 643–671

Lande R (1985) Expected time for random genetic drift of a population between stable phenotypic states. Proceedings of the National Academy of Sciences of the USA 82: 7641–7645.

Peck SL, Ellner SP and Gould F (2000) Varying migration and deme size and the feasibility of the shifting balance. Evolution 54: 323–327.

Phillips PC (1993) Peak shifts and polymorphism during phase three of Wright's shifting balance process. Evolution 47: 1733–1743.

Wade MJ and 529: Goodnight CJ (2000) The ongoing synthesis: a reply to Coyne, Barton and Turelli. Evolution 54: 317–324.

Wade MJ and Goodnight CJ (1991) Wright's shifting balance theory: an experimental study. Science 253: 1015–1018.

Related Sub-Topics:

Selection and Variation | Evolution: Theories and Ideas | Population Structure and Genetics | Evolutionary Ecology | Population Ecology
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Article Title: Evolution: Shifting Balance Theory
 
How to Cite close
Tonsor, Stephen J, and Moore, Francisco B‐G(Apr 2001) Evolution: Shifting Balance Theory. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0001781]