Evolution__3rd_Edition

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Mean population fitness ( w ) –
0 p'
1
Frequency of A allele ( p)
Random drift may take a population
away from a local peak
Wright’s shifting balance theory is
concerned with evolution on
complex adaptive topographies
CHAPTER 8 / Two-locus and Multilocus Population Genetics 217
Figure 8.9
A two-peaked fitness surface with local and global maxima.
Natural selection will take a population with gene frequency
p′ towards the local peak, away from the peak with highest
average fitness.
Figure 8.7a), the organisms with the better genotype will also be better adapted. But
when fitness is frequency dependent or when group and individual adaptations conflict
(Chapter 11), the maximum of mean population fitness may not correspond to the best
adaptation.)
Wright was interested in how evolution could overcome the tendency of natural
selection to become stuck at local fitness peaks. When fitness peaks correspond to
optimal adaptations, the question is relevant to the evolution of adaptation; but when
they do not, the question still has a technical interest in population genetics. Wright
suggested that random drift could play a creative role. Drift will tend to make the population
gene frequencies “explore” around their present position. The population could,
by drift, move from a local peak to explore the valleys of the fitness surface. Once it had
explored to the foot of another hill, natural selection could start it climbing uphill on
the other side. If this process of drift and selection were repeated over and over again
with different valleys and hills on the adaptive topography, a population would be
more likely to reach the global peak than if it was under the exclusive control of the
locally maximizing process of natural selection.
Wright’s full shifting balance theory includes more than just selection and drift
within a local population. He also suggested that populations would be subdivided into
many small local populations, and drift and selection would go on in each. The large
number of subpopulations would multiply the chance that one of them would find the
global peak. If members of a subpopulation at the highest peak were better adapted,
they could produce more offspring and more emigrants to the other subpopulations.
Those other subpopulations would then be taken over by the superior immigrant
genotypes. Thus the whole species would evolve to the higher peak. Wright’s theory is
thus an attempt at a comprehensive, realistic model of evolution. Everything is
included: multiple loci, fitness interactions, selection within and between populations,
drift, and migration. (The theory of adaptive peaks is also relevant to speciation:
Section 14.4.4, p. 394.)
The question of how important the shifting balance process is in evolution is long
standing, dating back to Wright’s publications in the 1930s. Coyne et al. (1997) recently
reopened the controversy, arguing that we have no good reason to think that the shifting
balance process has contributed much to evolution. The full controversy has looked
at many topics. Here are four of them.