Evolution__3rd_Edition

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Evidence for linkage disequilibrium (χ 2 )
0.05
7
6
5
4
3
2
1
21.2 10.9 7.6
0 0 0.10 0.20
Linkage disequilibrium has been
measured in microbes, ...
. . . and in fruitflies
Recombination rate
CHAPTER 8 / Two-locus and Multilocus Population Genetics 209
Figure 8.5
Linkage disequilibrium (on the y-axis) among gel electrophoretic
samples of pairs of genes in Drosophila. It is plotted as a χ 2 value.
The χ 2 value indicates how strong the evidence is for linkage
disequilibrium: the higher the χ 2 , the more linkage disequilibrium
between that pair of genes. (The χ 2 value corresponding to a
statistical significance of 0.05 is indicated.) Most of the gene pairs
have insignificant or low linkage disequilibrium (i.e., low χ 2 ).
The x-axis is the rate of recombination between the pairs of genes.
Redrawn, by permission of the publisher, from Langley (1977).
disequilibrium in a population can be measured. If it is high, then epistatic selection
may be common. The argument works in one direction but not the other: because there
are several possible causes of linkage disequilibrium (Section 8.7), its existence does not
demonstrate epistatic selection. However, if linkage disequilibrium is absent or low, we
can infer that epistatic selection is unimportant in nature.
A few general surveys of the extent of linkage disequilibrium in natural populations
have been made. One by Maynard Smith et al. (1993) for bacteria found high levels of
linkage disequilibrium in some species, such as Escherichia coli (which lives in our, and
other mammals’, guts), but low levels in other species, such as Neisseria gonorrhoeae.
The reason why many bacteria show linkage disequilibrium is that they reproduce
asexually, and there is no recombination to break the linkage disequilibrium down.
But some bacteria do sometimes exchange genes between individual cells, though not
by the kind of sexual processes that eukaryotes use. N. gonorrhoeae presumably has
enough genetic exchange between individuals to produce linkage equilibrium.
In eukaryotic organisms that are known to reproduce sexually, the evidence suggests
that there is little deviation from linkage equilibrium in nature. The main evidence has
historically come from surveys of protein polymorphisms, to see directly whether genes
at different loci are associated. Figure 8.5 illustrates some comprehensive results for the
fruitfly Drosophila. Some evidence of linkage disequilibrium is found, but the results
suggest the level is low and most loci are in linkage equilibrium. DNA sequence
evidence is now also becoming available and shows much the same pattern. Epistatic
interactions are undoubtedly important in particular cases, like Papilio, but they may
not be common for polymorphic loci in sexually reproducing species.
Not all biologists agree with this conclusion. They might be unconvinced by the evidence
of Figure 8.5, perhaps calling it “limited,” or “for a single species.” The amount of