Evolution__3rd_Edition

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Observed heterozygosity
1.0
0.8
0.6
0.4
0.2
0 0 0.2 0.4 0.6 0.8 1.0
The nearly neutral theory invokes
an effect for population size
Expected heterozygosity
CHAPTER 7 / Natural Selection and Random Drift 171
Figure 7.5
Observed levels of genetic variation (measured as
heterozygosities) are too constant between different species,
with different population sizes, than the netural theory predicts.
Each point gives the observed heterozygosity (y-axis) for a species
(total 77 species), plotted against the “expected” heterozygosity
from estimates of the population size and generation length
of the species and assuming a neutral mutation rate of 10 −7 per
generation. Species with large population sizes appear to have too
little genetic variation, relative to the neutral theory’s prediction.
Redrawn, by permission of the publisher, from Gillespie (1991).
Yet another problem for the neutral theory appeared in the “McDonald–Kreitman
test,” which we look at later in Section 7.8.3.
7.5.2 The nearly neutral theory of molecular evolution posits a
class of nearly neutral mutations
In response to the factual difficulties we have just looked at, Ohta developed a modified
version of the neutral theory. The modified version a the nearly neutral theory a grew
in popularity until the 1990s. It is now a widely (though not universally) supported
explanation for much of molecular evolution.
Kimura’s original, “purely” neutral theory explained molecular evolution by exactly
neutral mutations. For exactly neutral mutations, we can ignore population size.
For purely neutral mutations, the rate of evolution equals the neutral mutation rate.
Population size cancels out of the equation (Section 6.3, p. 144). Population sizes are
difficult to measure, and it is a great advantage if we can ignore it. However, the purely
neutral theory appears not to fit all the facts. The nearly neutral theory can explain a
greater range of facts, by bringing population size back into the theory.
Population size only cancels out for purely neutral mutations. For a nearly neutral
mutation, the relative power of drift and selection depends on population size. Nearly
neutral mutations behave as neutral mutations in small populations, and their fate is
determined by random drift. They behave as non-neutral mutations in large populations,
and their fate is determined by selection. To see why, consider a slightly disadvantageous
mutation a one with a very small selective disadvantage. If it were purely neutral, its
chance of eventually being fixed would be 1 /2N. If it is slightly disadvantageous, its
chance of being fixed by random drift is slightly less than 1 /2N. In a small population,
of 100 or so, the mutation has a fairly high chance (slightly less than one in 200) of
ultimately being fixed by drift. But in a large population, of a million or so, the chance
of being fixed by drift is negligible (slightly less than one in 1,000,000). This is just to
restate the fact that drift is more powerful in small populations (Section 6.1, p. 138).
A slightly advantageous mutation, with a selective advantage of s relative to the other
allele at the locus, has some chance of being lost by random accidents even though it is