Biologo

The Nearly Neutral Theory of Molecular Evolution
Tomoko Ohta
Annual Review of Ecology and Systematics, Vol. 23. (1992), pp. 263-286.
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Annu. Rev. Ecol. Syst. 1992. 23:26346
Copyright O 1992 by Annual Reviews Inc. All rights reserved

THENEARLYNEUTRAL
THEORYOFMOLECULAR
EVOLUTION
Tomoko Ohta
National Institute of Genetics, Mishima 41 1, Japan
KEY WORDS: motecular evolution and polymorphism, random drift, weakselection

INTRODUCTION
For a long time the study of evolution has been based on morphology; the
long neck of a giraffe, the human brain, a bird's wing, and so on. Morphological
change in evolution is explained by Darwin's theory of natural selection, but
this theory is largely qualitative rather than quantitative. Population genetics
started more than half a century ago as an attempt tounderstand evolutionary
change quantitatively. Because evolution must take place in all individuals of
a species, the change of gene frequency in the population has been analyzed.
However, so long as the facts of evolution are based on morphological traits,
evolutionary change is very difficult to connect with gene frequency change
except in relatively few circumstances.
The remarkable progressof molecular biology has made it possible to apply
population genetics theory to real data. We now know that genetic information
is stored in linear sequences of DNA which are stably transmitted from
generation to generation, and we can compare the linear sequences of DNA
and amino acids among species. It is also possible to compare secondary and
tertiary structures of proteins and nucleicacids from various sources.
Because of such progress, some aspects of traditional neo-Darwinism are
beginning to need revision. The first step in such a revision is the neutral
mutation-random drift hypothesis put forward by Kimura (47) in 1968. In the
next year, King & Jukes (53a) published a similar idea, though from a more
biochemical point of view than that of Kimura. This theory statesthat most
evolutionary changes at the molecular level are caused by random genetic
drift of selectively neutral or nearly neutral mutations rather than by natural
selection. Because this theory was contrary to the neo-Darwinian view at that
time, it provoked much controversy. A complete review of the theory is found
263

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OHTA

in Kimura's book (49), so I shall give only a briefoutline here, together with
the basic concepts of the stochastic theory of population genetics.
The second step in the revision is to clarify the interaction of natural
selection and random drift at the molecular level. Natural selection cannot be
so simple as to be "all or nothing." There are numerous types of mutations,
whose behavior is influenced by both selection and random drift. In this...