The Neutral Theory of Molecular Evolution (Kimura[9])
states, in essence, that most of the variation seen at the molecular level
is selectively neutral-- that is, there are no important fitness
advantages or disadvantages associated with particular alleles-- and that
genetic drift, rather than natural selection, dominates the dynamics. This
does not mean that mutations, when they occur, are all neutral, or
that the genes themselves are unimportant. On the contrary, it is thought
that most mutations are deleterious to the organism, and thus are unlikely
to remain in the population long enough to contribute measurably to the
``standing" variation. Only those mutations that do not have a harmful
effect have an appreciable chance of sticking around long enough for us to
see them. The Neutral Theory hypothesizes that this class of ``allowable"
mutations is composed entirely of selectively neutral
variants. The alternative viewpoint (much simplified)
is that advantageous mutations, while perhaps exceedingly rare, do
play a major role in evolution, and that polymorphism at the molecular
level can best (or, at least, possibly) be explained by natural selection.
For now, we develop some of the quantitative aspects of neutrality,
considering selection separately. We are particularly interested in
describing the dynamics and fate of a new (neutral) mutation. The natural
measure of the state of the allele is its population frequency, and it is
this that we consider in light of a specified model for mating and
reproduction.