Evolution__3rd_Edition

158 PART 2 / Evolutionary Genetics
Neutralists do not claim that all
mutations are neutral ...
. . . or that neutral drift explains
adaptation
the neutral theory. But if most evolution is by neutral drift, that does not mean most
mutations are neutral. Evolution is not the same as mutation. In Figure 7.1b, all the
mutations that may end up contributing to evolutionary change are neutral, but the
majority of mutations are disadvantageous and will be selected against. Disadvantageous
mutations disappear from the population before they have any chance to show
up as evolution. The neutral theory therefore does not rule out natural selection. It
simply has a different use for it than has the selectionist theory of molecular evolution.
The selectionist theory uses natural selection to explain both why mutations are lost
(when they are disadvantageous) and are fixed (when they are advantageous). The neutral
theory uses selection only to explain why disadvantageous mutation are lost; it uses
drift to explain how new mutations are fixed.
Pan-neutralism is almost certainly false. We have strong evidence against it. For
example, pan-neutralism has difficulty in explaining why different genes, and different
parts of genes, evolve at different rates (Section 7.6 below). Nor is it theoretically
plausible. It is absurd to suggests that hardly any mutations are disadvantageous.
Organisms, including their molecules, are adapted to their environments; we only need
reflect on the efficiency of digestive enzymes a or any other biological molecule a in
supporting life to realize that. If molecules are adaptive, many (or most) changes in
them will be for the worse.
The other thing that the neutral theory of molecular evolution does not claim is that
all molecular evolution is driven by neutral drift. It says that most molecular evolution
is by neutral drift. An important fraction of molecular evolution is almost certainly
driven by selection: the fraction of molecular evolution that occurs during the evolution
of adaptations.
Biological molecules are well adapted for their functions. Hemoglobin carries
oxygen; enzymes catalyze biochemical reactions. These adaptive functions did not
evolve by accident. Random drift will not have contributed much, if at all, to adaptive
evolution. The evolutionary events that gave rise to the adaptive functions of the
modern molecules of life were almost all powered by selection.
Selectionist and neutral theories of molecular evolution agree that selection drives
adaptive evolution. The disagreement is over what fraction of molecular evolution
is adaptive. To see the point, imagine a gene of about 1,000 nucleotides (corresponding
to a protein of about 300 amino acids). There are 4 1,000 or about 10 600 possible
sequences of the gene. The protein encoded by the gene will have some function, for
example carrying oxygen in the blood (actually done by hemoglobin, which is made
up of four polypeptides of slightly less than 150 amino acids each). The neutral theory
suggests that, of the 10 600 possible molecules, the great majority would fail to carry
oxygen at all, and many would do so poorly. Then there would be a minority, of
maybe a few hundred different sequences, all very similar to one another, all of
which would code for proteins that carried oxygen equally well. What we observe
as evolution consists of shuffling round within this limited set of equivalent sequences.
The selectionist alternative is that the few hundred variants are not equivalent, but
that one works better in one environment, another in another environment, and so
on. Evolution then consists of the substitution of one variant for another when the
environment changes.
As the chapter unfolds, we shall see how the original neutral theory (illustrated
in Figure 7.1b) has been modified in two ways. One is the development of the “nearly
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