Evolution__3rd_Edition

214 PART 2 / Evolutionary Genetics
. . . and neutral linkage
disequilibrium are all possible
Figure 8.7
A fitness surface, or adaptive
topography, is a graph of the
mean fitness of the population
as a function of gene, or in some
cases genotype, frequency. (a) If
allele A (frequency p) has higher
fitness than a (frequency 1 − p),
mean population fitness simply
increases as the frequency of
A increases. (b) With
heterozygous advantage
(fitnesses of genotypes
AA : Aa : aa are 1 − s :1:
1 − t) mean population fitness
increases to a peak at the
intermediate frequency of
A at which the proportion of
heterozygotes is the maximum
possible.
on average have lower fitness than if linkage equilibrium existed between the A and B
loci because the increase in the A′B haplotype reduces the proportion of B/b heterozygotes.
Natural selection will favor recombinant individuals that do not have the
A′B haplotype.
A third possibility is for linkage disequilibrium to be selectively neutral. An example
of this was provided by the hitch-hiking of an allele at a neutral polymorphic locus with
a selectively advantageous mutant at a linked locus. While the mutant is being fixed,
linkage disequilibrium temporarily builds up between it and the alleles it happens to be
linked to at nearby loci. It disappears when the mutant reaches a frequency of one.
The distinction between advantageous and disadvantageous linkage disequilibrium
is crucial to understanding one of the major problems of evolutionary biology:
why recombination, and sexual reproduction, exists. We look at that problem in
Sections 12.1–12.3 (pp. 314–27). We finish this chapter by looking at another influental
multilocus population genetic concept a one that is so influential that it is part of the
language of evolutionary biology.
8.12 Wright invented the influential concept of an adaptive
topography
Wright’s idea of an adaptive topography (or adaptive landscape) is particularly useful for
thinking about complex genetic systems; but it is easier to begin with the simplest case.
This is for a single genetic locus. The topography is a graph of mean population fitness
(d) against gene frequency (Figure 8.7). (Adaptive topographies can also be drawn for
fitness in relation to genotype frequencies. They can even be drawn with phenotypic
variables on the x-axis; see, for example, Raup’s analysis of shell shape, in Figure 10.9
(p. 278). Figure 10.4 (p. 267), used in Fisher’s theory of adaptation, is also similar.) We
have repeatedly met the concept of mean fitness; it is equal to the sum of the fitnesses of
each genotype in the population, each multiplied by its proportion in the population.
In a case in which the genotypes containing one of the alleles have higher fitnesses than
those of the alternative, the mean fitness of the population simply increases as the
frequency of the superior allele increases and reaches a maximum when the gene is
fixed (Figure 8.7a). That is fairly trivial. When there is heterozygous advantage, mean
Mean population fitness ( w ) –
(a) (b)
0 1 0 1
Frequency of A allele
..