Evolution__3rd_Edition

220 PART 2 / Evolutionary Genetics
acts to reduce the amount of recombination between
coadapted genotypes.
8 When selection works on one locus, it will influence
gene frequencies at linked loci. The effect is called
hitch-hiking.
9 The spread of a favorable mutation causes a reduction
in diversity in neighboring regions of the DNA.
The process is called a selective sweep. Local reductions
in genetic diversity, in DNA sequence data, can
provide evidence of selection.
10 The mean fitness of a population can be drawn
graphically for two loci; the graph is called a fitness
surface or an adaptive topography.
11 Wright suggested that real adaptive topographies
will have many separate “hills,” with “valleys” between
them. Natural selection enables populations to climb
Further reading
the hills in the adaptive topography, but not to cross
valleys. A population could become trapped at a local
optimum.
12 Random drift could supplement natural selection
by enabling populations to explore the valley bottoms
of adaptive topographies.
13 It is questionable whether real adaptive topographies
do have multiple peaks and valleys. They might
have a single peak, with a continuous hill leading to it.
Natural selection could then take the population to the
optimum without any random drift.
14 Two-locus population genetics uses a number
of concepts not found in single-locus genetics. The
most important are: haplotype frequency, recombination,
linkage disequilibrium, epistatic fitness interaction,
hitch-hiking, and multiple peaked fitness surfaces.
Population genetics for two loci, as for one locus, is introduced in such standard textbooks
as Hartl & Clark (1997) and Hedrick (2000).
The multilocus genetics of mimicry in Papilio memnon and Heliconius is explained
by Turner (1976, 1984). Turner & Mallett (1996) discuss the puzzle of diversity in
Heliconius, with the shifting balance process as one possible explanation. The HLA loci
are introduced from a more evolutionary perspective by Hughes (1999), and a more
molecular genetic perspective by Lewin (2000). Wolf et al. (2000) is a multiauthor book
on epistasis and evolution. Wade et al. (2001) distinguish two meanings of epistasis,
which differ between two-locus population and quantitative genetics.
Linkage disequilibrium in the human genome is described by Reich et al. (2001): for
humans, linkage disequilibrium also matters for locating disease genes, and changes in
population size need to be considered. Kohn et al. (2000) describe another example of a
selctive sweep, like Sdic a the gene for warfarin resistance in rats. Gillespie (2001) looks
at the effect of population size on hitch-hiking, and draws the subversive conclusion
that population size may have little effect on evolution because its effect on hitchhiking
is the opposite of its effect at any one site.
On Wright’s shifting balance theory, see Wright’s four-volume treatise (1968–78),
particularly volumes 3 and 4 (1977, 1978) and Wright (1986). Wright (1932) is a short
and accessible paper from earlier on. See also Lewontin (1974, final chapter) and
Provine (1986, chapter 9). For the modern controversy, see the exchange in Evolution
(2000), vol. 54, pp. 306–27, including the references there.
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