2006 The Society for the Study of Evolution. All rights reserved.
Evolution, 60(3), 2006, pp. 448459
STOCHASTICITY, COMPLEX SPATIAL STRUCTURE, AND THE FEASIBILITY OF THE
SHIFTING BALANCE THEORY
BRENDAN O'FALLON1,2 AND FREDERICK R. ADLER1,3,4
1Department of Biology, University of Utah, Salt Lake City, Utah 84112
3Department of Mathematics, University of Utah, Salt Lake City, Utah 84112
Abstract. Sewall Wright's shifting balance theory of evolution posits a mechanism by which a structured population
may escape local fitness optima and find a global optimum. We examine a one-locus, two-allele model of underdom-
inance in populations with differing spatial arrangements of demes, both analytically and with Monte Carlo simulations.
We find that inclusion of variance in interpatch connectivities can significantly reduce the number of generations
required for fixation of the more favorable allele relative to island and stepping-stone models. Although time to fixation
increases with migration rate in all cases, the presence of one or two relatively isolated demes may reduce the number
of generations by 80% or more. These results suggest that the shifting balance process may operate under less restrictive
conditions than those found with a simple spatial arrangement of demes.
Key words. Fitness landscape, Sewall Wright, shifting balance theory, spatial structure, underdominance.
Received July 19, 2005. Accepted January 5, 2006.