Evolution__3rd_Edition

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Figure 10.5
A cross between the two
modern species would show
that the large beak size
difference is controlled by
a single gene of large effect
(A vs A 4 ). However, the modern
difference has evolved by a
number of small stages in the
past. The Fisherian evolution
is invisible in a modern cross.
(0.5 in ≈ 1.25 cm.)
. . . is hard to interpret
Some experimental work supports
and expands it
Time
Present
Past
Species 1
1 cm beak
genotype AA
Ancestor
1 cm beak
genotype AA
1.1 cm beak
genotype A1 A1
CHAPTER 10 / Adaptive Explanation 269
1.2 cm beak
genotype A2 A2
1.3 cm beak
genotype A3 A3
Species 2
1.4 cm beak
genotype A4 A4
There is an inherent difficulty in testing theories about the evolutionary past using
genetic crosses between modern forms. The genes in modern species will only reflect
the way evolution proceeds if no genetic change has occurred since the adaptations
originally evolved. Figure 10.5 illustrates the problem. Two species have diverged in a
series of small steps, but the modern species differ by one gene with large effect. The
problem does not invalidate Orr and Coyne’s conclusion that the evidence for Fisher’s
theory is poor. But it would be a mistake to turn their evidence round and count it
against Fisher’s theory. In practice, more extensive genetic crosses are needed. In the
African swallowtail Papilio dardanus, for instance, an initial cross suggests mimetic
polymorphism is due to a single gene of large effect. But further crosses between apparently
similar morphs from different regions of Africa show that several genes are at
work (Turner 1977, p. 184).
Burch & Chao (1999) pioneered a second kind of experiment. They knocked a bacteriophage
away from its adaptive peak by allowing deleterious mutations to accumulate.
They then measured the mutational steps by which the phage population evolved back
to its former level of adaptation. The result depended on population size. In small
populations, the phage evolved back to its peak in many small mutational steps. In large
populations, they evolved back in some large, and some small, mutational steps. Their
explanation is that large advantageous mutations are rarer than small advantageous
mutations. In a small population, no large advantageous mutations may arise and
adaptive evolution proceeds using mutations of small effect. In large populations, a few
large advantageous mutations may be present, and they contribute to adaptive evolution.
Burch and Chao’s results fit with the basic theory of Sections 10.5.1 and 10.5.2
here, and show that population size also matters. Their work also shows how microbial
systems can be used to test themes about the genetics of adaptation.