Evolution__3rd_Edition

174 PART 2 / Evolutionary Genetics
. . . and non-effects ...
. . . but the explanations are
controversial
because the mutational process is influenced by generation length. DNA is copied
fewer times per year in human gonads than in rat gonads. But why should there be less
of a generation length influence (or even no influence) on the rate of evolution at nonsynonymous
sites? We begin by assuming that many amino acid-changing mutations
are slightly disadvantageous. In a species with a long generation length, such as a whale,
we now have two factors to consider: (i) DNA is copied slowly per year, which reduces
the mutation rate per year; and (ii) population sizes are small, which makes drift more
powerful than selection. Slightly disadvantageous mutations are less likely to be eliminated
by selection, and are more likely to be fixed by drift. Factor (i) slows the rate of
evolution; factor (ii) speeds it up.
Fruitflies, by contrast, have large population sizes but short generation times.
They have a larger supply of mutations, because they copy their DNA more per year.
But their population sizes are large, making fewer of the non-synonymous mutations
effectively neutral. In all, generation length has two opposing influences on the rate of
evolution for sites where many mutations are nearly neutral. Ohta suggests that the two
effects could approximately cancel out, and the rate of evolution per year would be
much the same whatever the generation length. That is her explanation for the possible
absence of a generation time effect on the rate of amino acid substitutions. She may be
right, but critics such as Gillespie argue that the two influences are unlikely to cancel
each other out exactly. Then, some generation length effect would still be expected on
the nearly neutral theory.
By this stage, we are at the frontiers of research, both for the facts and the theories.
The nearly neutral theory can in principle account for what is known about molecular
evolution, but that is not to say it has been shown to explain molecular evolution. The
main conceptual difference between the nearly neutral theory and Kimura’s original,
purely neutral theory is in the use of population size. Population size does not affect the
rate of evolution for purely neutral mutations. But it does affect the rate of evolution
for nearly neutral mutations. This gives the nearly neutral theory great flexibility,
because a wide variety of facts can be accounted for by assuming an appropriate history
of population sizes. But the use of population sizes also make the theory difficult to test,
because population sizes are difficult (and historic population sizes impossible) to
measure. Kimura’s original purely neutral theory, by contrast, was much more testable
because its predictions did not require us to know anything about population sizes.
In summary, Ohta modified the purely neutral theory by positing a class of nearly
neutral mutations. The relative power of selection and drift on these mutations depends
on population sizes. The nearly neutral theory, by plausible arguments about population
size, can account for several observations that present problems for Kimura’s
purely neutral theory.
7.5.4 The nearly neutral theory is conceptually closely related to
the original, purely neutral theory
The nearly neutral theory makes use of natural selection. In some circumstances (large
population size), the theory draws on natural selection; in other circumstances (small
population sizes), it does not. Nearly neutral theory might be thought to blur the distinction
between “selectionist” and “neutralist” explanations of molecular evolution.
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