Evolution__3rd_Edition

268 PART 3 / Adaptation and Natural Selection
For populations not close to their
adaptive optima ...
. . . three factors influence the
magnitude of their genetic steps
Genetic evidence bearing on
Fisher’s theory ...
10.5.2 An expanded theory is needed when an organism is not near an
adaptive peak
Fisher’s assumption that living things are close to their adaptive optima may not always
be met. Then, mutations of larger phenotypic effects have a better chance of improving
adaptation. Small mutations are still more likely to be improvements than are large
mutations, but Kimura (1983) pointed out that this may be overridden by a second
factor. A large mutation, when it is advantageous, may have a larger selective advantage
than a small mutation, because it moves higher up towards the peak (Figure 10.4a).
The full contribution of any class of mutations (e.g., the class of mutations with small
phenotypic effect or the class of mutations with large phenotypic effect) to evolution
depends on three factors:
Chance that the chance that chance that the selective
mutations are = the mutations × mutations are × advantage of
substituted arise advantageous the mutations
(The equation is not mathematically correct. For instance, we saw in Section 7.5.2
(p. 172) that the chance that a selectively advantageous mutation is fixed is about 2s,
where s is the selective advantage. The third term in the equation here should be 2s, not
simply s. However, the equation identifies the three factors at work, and the three are
approximately multiplicative.)
To find the relative contribution of small and large mutations to adaptive evolution,
we need to know the relative size of all three factors for the two classes of mutations.
Fisher’s argument only looks at the second factor.
No rigorous results are available for the first factor, but theory and evidence both
indicate that mutations of small effect are more frequent than mutations of large effect.
Orr (1998) looked mathematically at the second and third factors together, combining
Fisher’s argument and Kimura’s conjecture. In Orr’s model, the mutations that were
substituted showed a certain frequency distribution of phenotypic effects (negative
exponential, to be exact). A small number of mutations of large effect were substituted
initially, followed by an increasing number of mutations with smaller effects. The reason
is that, in a population away from a peak, somewhat large mutations are initially
fixed. Then there is a phase of fine tuning in which many small mutations are fixed.
Therefore, it is theoretically possible that some adaptive evolution is by large mutations,
particularly in poorly adapted populations.
10.5.3 The genetics of adaptation is being studied experimentally
So far we have been looking at theory. What do the facts tell us about the genetics of adaptation?
Two kinds of evidence are available. One comes from crosses between different
forms within a species, or between closely related species. Orr & Coyne (1992) reviewed
eight such crosses for cases in which the two crossed forms differed in an unambiguously
adaptive character. In five or six of these, the difference was controlled by a single
gene with major effect. Orr and Coyne concluded that Fisher’s theory is poorly supported.
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