Evolution__3rd_Edition

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Two extreme views b selectionist
and neutralist b can be
distinguished
CHAPTER 7 / Natural Selection and Random Drift 157
What frequency of advantageous, disadvantageous, and neutral mutations do we
expect there to be? Consider the nucleotide sequence of a gene in a living organism. The
gene codes for a reasonably well adapted protein: the protein is unlikely to be a dud if
the organism containing it is alive. Now consider all the mutations that can be made
in the gene. You could work down the gene, altering one nucleotide at a time, and ask
for each change whether the new version was better, worse, or equally as good as the
original gene. In a population of organisms in nature, mutations will be occurring and
causing these kinds of change, in certain frequencies.
Many mutational changes will be for the worse, and will have negative selection
coefficients. Adaptation is an unlikely state of nature, and a random change in an
adapted protein is likely to be for the worse. The disagreement has been about the relative
frequencies of the other two classes of mutations: the neutral and the selectively
advantageous. If natural selection has produced most evolutionary change at the
molecular level, many advantageous mutations must have occurred, but few neutral
mutations. If neutral drift has produced most evolutionary change at the molecular
level, the relative frequencies are the other way round. Figure 7.1 illustrates two extreme
views, in which most molecular evolution will be driven by selection (Figure 7.1a) or by
drift (Figure 7.1b). The difference between the two is in the relative heights of the graph
in the 0 and + regions. The high frequency of mutations in the – region is common to
the two. Kimura’s original neutral theory of molecular evolution implied something
like Figure 7.1b.
At this point, it is worth pointing out two things that Kimura was not saying, and
his modern followers are still not saying. The neutral theory says that the majority of
molecular evolution is driven by neutral drift a but that does not mean the majority
of mutations are neutral. Figure 7.1c illustrates what Kimura (1983) called “panneutralism,”
in contrast with his own ideas. Pan-neutralism mean that almost all
mutations are neutral. Then, almost all evolution would be by neutral drift, just as in
(a) Frequency
(b) Frequency
(c)
of mutation
of mutation
Frequency
of mutation
– 0 + – 0 + – 0
+
Selection coefficient Selection coefficient Selection coefficient
Figure 7.1
The neutral and selectionist theories postulate different
frequency distributions for the rates of mutation with various
selection coefficients. (a) According to the selectionists, exactly
neutral mutations are rare and there are enough favorable
mutations to account for all molecular evolution; whereas
(b) neutralists believe there are more neutral, and hardly
any selectively favored, mutations. (c) The theory of
pan-neutralism, according to which all mutations are
selectively neutral.