Evolution__3rd_Edition

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Mark–recapture experiments
suggest s ≈ 0.57
CHAPTER 5 / The Theory of Natural Selection 111
Table 5.6
Frequencies of melanic and light peppered moths in samples recaptured at two sites in the
UK: Birmingham (polluted) and Deanend Wood, Dorset (unpolluted). The observed
numbers are the actual numbers recaught; the expected numbers are the numbers that
would have been recaught if all morphs survived equally (equals proportion in released
moths times the number of moths recaptured). The recaptured moths at Birmingham
were taken over a period of about 1 week, at Deanend Wood over about 3 weeks. Data
from Kettlewell (1973).
Light moths Melanic moths
Birmingham (polluted)
Numbers recaptured
Observed 18 140
Expected 36 122
Relative survival rate 0.5 1.15
Relative fitness 5/1.15 = 0.43 1.15/1.15 = 1
Deanend wood (unpolluted)
Numbers recaptured
Observed 67 32
Expected 53 46
Relative survival rate 1.26 0.69
Relative fitness 1.26/1.26 = 1 0.69/1.26 = 0.55
5.7.3 A second estimate of the fitnesses is made from the
survivorship of the different genotypes in mark–
recapture experiments
The estimate of fitness can be checked against other estimates. The gene frequency
change was (and still is) thought to be produced by survival differences between the
two forms of moth in nature, rather than differential fertility. We can measure the
rate of survival of the two forms in nature, and see how they differ. Kettlewell (1973)
measured survival rates by mark–recapture experiments in the field. He released
melanic and light-colored peppered moths in known proportions in polluted and
unpolluted regions, and then later recaught some of the moths (which are attracted to
mercury vapor lamps). He counted the proportions of melanic and light-colored
moths in the moths recaptured from the two areas.
Table 5.6 gives some results for two sites, Birmingham (polluted) and Deanend
Wood, an unpolluted forest in Dorset, UK. The proportions in the recaptured moths
are as we would expect: more light-colored moths in the Deanend Wood samples and
more melanic moths in the Birmingham samples. In Birmingham, melanic moths were
recaptured at about twice the rate of light-colored ones, implying s = 0.57. This is a
higher fitness difference than the s = 0.33 implied by the change in gene frequency.