Evolution__3rd_Edition

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p'
0 1 0
Frequency of A allele
Summary
CHAPTER 8 / Two-locus and Multilocus Population Genetics 219
Mean population fitness ( w ) –
1
Frequency of B allele
1 Population genetics for two or more loci is concerned
with changes in the frequencies of haplotypes,
which are the multilocus equivalent of alleles.
2 Recombination tends, in the absence of other
factors, to make the alleles of different loci appear in
random (or independent) proportions in haplotypes.
An allele A 1 at one locus will then be found with alleles
B 1 and B 2 at another locus in the same proportions as
B 1 and B 2 are found in the population as a whole. This
condition is called linkage equilibrium.
3 A deviation from the random combinatorial proportions
of haplotypes is called linkage disequilibrium.
Figure 8.10
As extra gene loci are considered (extra axes in the adaptive
topography) it becomes increasingly likely that what appeared
to be a local maximum in fewer dimensions will turn out to be
a hillside or saddle point in more dimensions. In this case, the
fitness surface for the A locus at a gene frequency of zero for the B
allele is the same as in Figure 8.9; but at other B gene frequencies,
the local peak in the A locus fitness surface disappears. If the
population started in the valley at B gene frequency = 0 and
A gene frequency = p′, natural selection would initially move
gene frequencies to the local peak, but they would eventually
reach the global peak by continuous hill climbing.
with the operation of selction, that adaptations evolve by selection within a population,
and that adaptive evolution can proceed smoothly up to the highest fitness peak.
Wright thought that populations are small, drift and epistatic fitnesses are important,
and that adaptive evolution is liable to become stuck at a local optimum. Biologists
today rarely count themselves simply as members of one school or the other, but the
controversy between these two views has inspired, and continues to inspire, important
evolutionary research.
4 The theory of population genetics for a single locus
works well for populations in linkage equilibrium.
5 Linkage disequilibrium can arise because of low recombination,
non-random mating, random sampling,
and natural selection.
6 For selection to generate linkage disequilibrium,
the fitness interactions must be epistatic: the effect
on fitness of a genotype (such as A 1 /A 2 ) must vary
according to the genotype it is associated with at other
loci.
7 Pairs of alleles at different loci that cooperate in
their effects on fitness are called coadapted. Selection