Evolution__3rd_Edition

218 PART 2 / Evolutionary Genetics
The importance of Wright’s shifting
balance theory is controversial, in at
least four respects
Fisher and Wright schools can be
distinguished
1. What facts are better explained by the shifting balance process than by simple
natural selection within one population? For instance, the passion flower butterflies
(Heliconius, Section 8.2 above) have many morphs, each mimicking a different
model. Each morph probably occupies an adaptive peak. An adaptive valley separates
each peak, because intermediate forms would be poorly adapted to mimic
any model and would be eaten. How can one morph evolve into another? On the
shifting balance view, a morph can originate by drift within a local population, and
then spread if it is advantageous. Alternatively, however, evolution in the mimetic
form of the butterflies may be driven by changes in the abundance of local models. If
a model with a certain coloration becomes locally common, perhaps because its
resources are locally abundant, then the mimic species will evolve to match that
coloration. Thus, although the species now shows a multipeaked fitness surface, the
peaks may not have been separated by valleys in the past. The Heliconius example, as
with all others that have been discussed in the controversy, is inconclusive.
2. Can genetic drift drive populations across real adaptive valleys? Genetic drift is
powerful when it is not opposed by selection: that is, when drift is between different
neutral forms. However, in Wright’s theory, drift has to work in opposition to
selection. This is a much more difficult process, and critics doubt whether it occurs.
The selective disadvantages in the valleys between different morphs of Heliconius,
for example, correspond to 50% fitness reductions. Random drift could not establish
forms that have such large disadvantages.
3. Do populations have the structure proposed by Wright? Are populations subdivided
into many small subpopulations? If populations are large, all the main possible
genotypes will be present in it including the best genotype a the one correpsonding
to the highest adaptive peak. It can be fixed by normal natural selection within
the population. The shifting balance process only helps if populations are so small
that the best genotype happens never to have arisen in many local subpopulations.
Supporters suggest that real populations are often as Wright suggested; critics
doubt it.
4. Do real fitness surfaces have multiple peaks? Fisher, for instance, doubted whether
natural selection would actually confine populations to local peaks. Fisher was preeminently
a geometric thinker and he pointed out that, as the number of dimensions
in an adaptive topography increases, local peaks in one dimension tend to
become points on hills in other dimensions (Figure 8.10). In the extreme case, when
there are an infinity of dimensions, it is certain that natural selection will be able to hill
climb all the way to the global peak without any need for drift. Each one- (Figure 8.7)
or two-dimensional peak (Figure 8.8) will be crossed at the peak by an infinity of
other dimensions, and it is highly implausible that the fitness surface will turn
downhill in all of them at that point. This is a highly interesting argument, though it
is, of course, purely theoretical. It refutes Wright’s theoretical claim that natural
selection will get stuck at local peaks, but leaves open the empirical question of how
important selection and drift have been in exploring the fitness surfaces of nature.
The importance of the shifting balance process remains undecided, but the controversy
has a broader interest. Biologists distinguish between a “Fisher” and a “Wright”
school of evolutionary thought. Fisher maintained that natural populations are generally
too large for drift to be important, that epistatic fitness interactions do not interfere
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