Evolution__3rd_Edition

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Fitness
(a) (b)
Small mutation
Large mutation
Character (x)
Figure 10.4
(a) A general model of adaptation. For a trait (x), the fitness of an
individual has an optimum at a certain value of x, and declines away from
that point. There is then a hill of fitness values. A mutation which changes
the value of x also changes its bearer’s fitness. A mutation of small effect is
more likely to improve its bearer’s fitness if the bearer is somewhere near
the adaptive peak. (b) Fisher’s calculations concerning the chance that a
mutation improves fitness, depending on the magnitude of the mutation’s
phenotypic effect. The y-axis units refer to a particular model, but the
general shape of the graph will be the same in any model like (a).
Fisher’s microscope analogy
Wright’s shifting balance theory is
another alternative
CHAPTER 10 / Adaptive Explanation 267
Probability of improvement
chance of improvement decreases as the phenotypic effect of the mutation increases
(Figure 10.4b). A macromutation has zero chance of being advantageous.
Fisher’s argument can be explained less formally. Any well adjusted machine is more
likely to be improved by fine tuning than by gross insult. If your radio wanders out of
tune, you can recover the station by a small adjustment to the tuning knob. Taking a
hammer to the machine is much less likely to help. Fisher used the analogy of focusing a
microscope. If the microscope is already fairly well focused on its object, most focusing
movements will be small, fine adjustments, taking the lens up or down a tiny amount. A
big jerk on a randomly picked part of the microscope is unlikely to improve the focus.
The main assumption of Fisher’s argument is that the organism is near the adaptive
optimum. We should also notice a second, related assumption, which is that the
adaptation has a single peak. If Figure 10.4a had multiple peaks, separated by valleys,
a macromutation might have some chance of improving things by taking the organism
to another peak. In Section 8.13 (p. 216), we saw that fitness surfaces with multiple
peaks were the basis of Wright’s shifting balance theory of evolution. Wright’s theory
is, with Goldschmidt’s, a second alternative to Fisher’s. Wright did not invoke macromutations.
He argued that adaptive evolution was facilitated by random drift in small,
subdivided populations. As we saw, Fisher doubted whether real adaptive surfaces have
multiple peaks and judged Wright’s shifting balance process unnecessary. But if real
adaptive surfaces are sometimes multipeaked, it could complicate the details of Fisher’s
calculations. Fisher’s basic point, however, that large changes in well adjusted systems
are usually for the worse, still stands. Adaptive evolution is usually by piecemeal
reform, not revolution.
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Magnitude of change