Evolution__3rd_Edition

172 PART 2 / Evolutionary Genetics
Nearly neutral evolution is
controlled by drift in small, and by
selection in large, populations
Nearly neutral mutations can be
exactly defined
advantageous. The mutation might provide an advantage in the adult stage, but if the
individual who contains the mutation has an accident while young the mutation will
be lost. The chance that a slightly advantageous mutation is fixed by selection can be
calculated and it is roughly 2s. The mutation has a 1 − 2s chance of being lost by random
factors. Thus, if a mutation increases the fitness of an organism by 1%, the chance that
the mutation is lost by accident is 98%. (Graur & Li (2000, p. 54) give a simple derivation
of this classic result.)
The 98% chance of being lost by accident is for any one copy of a mutation that has a
selective advantage of 1%. An advantageous mutation is more likely to be present in
one unique copy in a small, than a large, population. In a small population, an advantageous
mutation may arise once but then be lost by chance. In a large population, the
same mutation may occur several times and be present in multiple copies. (We assume
the same mutation rate per gene in small and large populations.) Any one copy of the
mutation may be lost by chance, but there are so many copies that one of them is likely
to survive and be fixed by selection.
Evolution, therefore, is arguably dominated by drift in small populations and by selection
in large populations. We can be more exact. For mutations in populations where:
1
> 2s
2N
random drift is more important than selection in deciding that mutation’s evolutionary
fate. Therefore mutations that satisfy the inequality:
s
<
N
1
4
or 4Ns < 1
behave as effectively neutral even if they have a non-zero selection coefficient.
The inequalities are often expressed in the approximate form:
s <
N
1
or Ns < 1
These are not strictly speaking accurate, but the four can often be dropped because the
arguments in this area are often inexact.
A mutation that satisfies the inequality 4Ns < 1 (or Ns < 1) is a nearly neutral
mutation. The class of nearly neutral mutations includes purely neutral mutations
(s = 0), together with mutations that have small non-zero selection coefficients. The
conceptual interest of nearly neutral mutations is that they evolve by random drift
rather than natural selection.
The number of mutations that satisfy the inequality will depend on the population
size. If N is large, only mutations with small s will satisfy the inequality and behave as
neutral. As N decreases, more and more mutations, with higher and higher s, will satisfy
the inequality and be dragged into the effectively neutral zone. The realized rate of
neutral mutation therefore goes up as population size goes down. The number of
mutations per gene is unchanged as population size decreases, but the fraction of them
that behave as neutral will be higher if N is lower.
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