Biology Reference
In-Depth Information
7
Single base-pair
substitutions
7.1 Single base-pair substitutions in evolution
7.1.1 The selectionist and neutralist perspectives
Nowadays a certain number of believers in evolution do not regard natural
selection as a cause of it. We must therefore carefully distinguish between two
quite different doctrines which Darwin popularised, the doctrine of evolution,
and that of natural selection. It is quite possible to hold the first and not the
second.
J.B.S. Haldane
The Causes of Evolution
(1932)
The vast majority of mutations that occur are either neutral with respect to
fitness
(defined as the individual's ability to survive and reproduce) or are disadvanta-
geous. If they are disadvantageous, they will tend to be removed from the popula-
tion since their bearers will be less likely to survive and/or reproduce (
negative
or
purifying selection
). Occasionally, a new mutation confers a selective advantage and
increases the fitness of individuals bearing it so that it will eventually reach
fixa-
tion
(the point at which the allele frequency in the population becomes 100%).
This is termed
positive selection
. Selection is thus nothing more than the differen-
tial and nonrandom reproduction of genotypes resulting from the superior or
inferior fitness of their associated phenotypes. In vertebrates, direct evidence at
the molecular level for the occurrence of selection, whether positive or negative,
has however often been hard to obtain and there are as yet relatively few good
practical examples (reviewed in Chapter 2, section 2.3.7 and Sections 7.1.2 and
7.5.2).
Gene evolution does not however invariably require selection since changes in
allele frequency can also occur by chance owing to random sampling of gametes
(
genetic drift
). Whereas selection implies directed change, genetic drift may be
viewed as a stochastic process of undirected change. Genetic drift can cause rapid
changes in small populations but its effect will be fairly minimal in large ones. This
is one of the central conclusions of the
neutral theory of molecular evolution
(Kimura,
1983). The importance of this theory to our understanding of the evolutionary
process cannot be understated. Its major points may be summarized as follows:
