Evolution__3rd_Edition

..
Manchester
The fitness estimates have been
repeated many times
Other factors have been suggested
Dorset
Birmingham
N
100 km
60 miles
East Anglia
CHAPTER 5 / The Theory of Natural Selection 113
Figure 5.5
Frequency of melanic and light-colored forms of the peppered
moth in different parts of Britain when the frequency of the
melanic form was near its peak. The green part of each pie
diagram is the frequency of the melanic form in that area.
Melanic moths are generally higher in industrial areas, such
as central England; but note the high proportion in East Anglia.
Melanic frequencies have subsequently decreased (see Figure 5.6,
for instance). Redrawn, by permission of the publisher, from Lees
(1971).
Cook (2000) reviewed about 30 experimental fitness estimates, done by several
teams of biologists, 3 and they all gave similar results. The fitness estimates for the two
forms of the peppered moth are about the most repeated result in evolutionary biology,
and do not depend on the details of any particular experiment. The repeated results
amount to an almost overwhelming case that the rise and fall of the melanic form of the
peppered moth depended on air pollution. The evidence that air pollution exerted its
effect via bird predation is also strong, if not overwhelming.
Evidence has also been put forward for other factors, in addition to bird predation.
Migration is one extra factor. The geographic distribution of the two forms does not
exactly fit Kettlewell’s theory. The melanic form, for example, had a frequency of up to
80% in East Anglia, where pollution is low (Figure 5.5). And in some polluted areas, the
dark form did not seem to have a high enough frequency. It never exceeded about 95%
even though it was clearly better camouflaged and ought for that reason to have had a
frequency of 100%. However, male moths can fly long distances to find females, and a
male peppered moth mates on average 1.5 miles (2.5 km) away from where it is born.
3 It has even been suggested that Kettlewell faked his results. The charge has only been supported by indirect
evidence that is open to innocent interpretations. But however that may be, Kettlewell’s explanation for evolution
in the peppered moth a bird predation a does not depend on Kettlewell’s own research. His results have
been independently repeated.