Evolution__3rd_Edition

210 PART 2 / Evolutionary Genetics
Natural selection at one locus can
cause evolution at linked loci
Natural selection at one locus tends
to reduce diversity at linked loci
interaction between loci that must go on during the development of a complex, organic
body is so high that they would expect epistatic fitness interactions to be common.
Such is the assumption of the school of thought that follows Wright, whose ideas we
shall discuss at the end of the chapter.
8.9 Hitch-hiking occurs in two-locus selection models
When a gene is changing frequency at one locus over time, it can cause related changes
at linked loci; conversely, events at linked loci can interfere with one another. Suppose,
for instance, that directional selection is substituting one allele A′ for another (A) at one
locus, and there is a neutral polymorphism (B, B′) at a linked locus. Then whichever of
B and B′ happened to be linked with A′ when it arose as a mutant will have its frequency
increased. If the new mutant A′ happened to arise on a B-bearing chromosome, B will
eventually be fixed together with the selectively favored allele A′ unless recombination
splits them before A has been eliminated. The increase in the B allele frequency is due to
hitch-hiking.
Another possibility is for the polymorphism at the B locus to be a selectively “balanced”
polymorphism, due to heterozygous advantage. Suppose again that a selectively
favored mutation A′ arises at a linked locus, and that it happens to arise on the same
chromosome as a B allele. Now the polymorphism at the B locus will interfere with the
progress of A′. As A′ increases in frequency by directional selection it will increase the
frequency of B with it. Because A′ is linked to B, it will be more likely to be in a body
with a B/B homozygote than will its allele A, and less likely to be in a B/b heterozygote.
B/b has higher fitness than B/B and the selection against B/B individuals will also work
against the A′ gene. Depending on the selection coefficients at the two loci, and the rate
of recombination between them, the heterozygous advantage at the B locus can slow
the rate at which A′ is fixed. The A′ gene will then have to wait for recombination
between the two loci before it can progress to fixation.
8.10 Selective sweeps can provide evidence of selection in
DNA sequences
One consequence of hitch-hiking is that when natural selection fixes a new, favorable
gene, the amount of genetic variation is reduced in the neighboring regions of the
DNA. When a favorable mutation arises, it will initially be on a chromosome which has
a particular sequence of nucleotides. As the mutation is fixed, it carries with it the
nucleotides that are linked to it. Other nucleotide variants at neighboring sites in the
DNA are eliminated, along with the inferior alleles at the locus where selection is acting.
The result is reduced genetic diversity. (Genetic diversity can be measured by sequencing
the DNA of many chromosomes from many fruitfly individuals, and counting the
fraction of nucleotide sites that differ between two randomly picked chromosomes.)
The sweep reduces genetic diversity most at the locus where selection is acting.
Nearby in the DNA, diversity will be reduced; further out, diversity will still be reduced
..