Evolution, 4th Edition

104 CHAPTER 5

Natural Selection and Evolution in Real Time

The English have long had an interest in (some might say an obsession with) but-

terflies and moths. For hundreds of years, amateur collectors have roamed the coun-

tryside in search of specimens. They were surprised when a melanic (dark) color

form of the peppered moth (Biston betularia) was first discovered in 1848 and then

rapidly spread in central England (FIGURE 5.1). Recent research shows that the dark

coloration results from a single mutation that likely occurred around 1819 [20].

Why did the melanic mutation in the peppered moth suddenly spread? During

the mid-1800s, England was transformed by the Industrial Revolution, which was

powered by burning vast quantities of coal. Before then, peppered moths had been

well camouflaged: their light coloration was a beautiful match to the bark and lichen

on the trees where the moths rest during the day. But the burning of coal produced

soot that killed the lichens and blackened the tree trunks, suddenly making moths

with the typical coloration very conspicuous. Moths with the melanic allele, however,

were camouflaged in the new environment. That increased their survival, and caused

the melanic allele to spread nearly to fixation by the late 1800s [40, 41]. In 1924, J. B. S.

Haldane calculated from the speed of its spread that the melanic mutation must have

given moths a 50 percent survival advantage [17]. Experiments done by Bernard Ket-

tlewell in the 1950s suggested that predation by birds was responsible.

Starting in the 1960s, coal burning decreased dramatically and trees slowly lost

their coatings of soot. The change provided an unintended experiment that verified

the hypothesis that pollution gave melanic moths a survival advantage. Between

1960 and 2000, the frequency of melanic moths fell from nearly 100 percent to nearly

0 percent throughout much of central England (see Figure 5.1B) [12, 30].

Several scientists raised questions about whether bird predation was really the

cause of the melanic mutation’s rise and fall. Michael Majerus took up the chal-

lenge of answering this question in the 2000s. Over 7 years, he did a series of

experiments involving nearly 5000 moths. Tragically, Majerus died before the work

was published, but his colleagues finished the job [12]. The results decisively con-

firmed Kettlewell’s conclusions: bird predation was the overriding reason for the

rapid evolution of melanism in the 1800s and its decline 100 years later.

The spread of melanism in the peppered moth was the first time that evolution

had been documented in real time, and Haldane’s calculations showed that natural

selection acting on the melanic allele can explain its rapid spread. The dozens of

FIGURE 5.1 Industrial melanism in the

peppered moth is the most famous

example of evolution by natural selection

directly observed in the wild. (A) The pale

gray “typical” form and the melanic form

on a tree trunk darkened by air pollution

(left) and on a normal, nonblackened trunk

(right). (B) The decline in the frequency of

the melanic form in three British localities,

indicated by dots of different colors, as air

pollution decreased during the late twenti-

eth century. (B after [11].)

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Evolution4e_05.01.ai Date 12-29-2016 01-06-17

Frequency of melanic form

1960 1970 1980

Year

1990 2000

0.5

0.4

0.3

0.2

0.1

0.6

0.7

0

1.0

0.9

0.8

(A) (B)

The melanic form of

Biston betularia declined

in frequency over more

than 40 years in 3 different

locations in Britain.

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Sinauer Associates

Troutt Visual Services

Evolution4e_05.02.ai Date 12-14-2016

(A) Wild type

(B) Glucose-averse

Fructose Caffeine Glucose

Bitter

Time

Amplitude

Sweet Bitter Sweet Bitter Sweet

Fructose Caffeine Glucose

Bitter

Time

Amplitude

Sweet Bitter Sweet Bitter Sweet

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