Geography Reference
In-Depth Information
England's skies cleared as industry moved beyond coal as a fuel and regu-
lations reduced air pollution. Soot and smoke stains eroded from buildings.
Lichens reclaimed tree trucks. Birds found dark morphs of moths more easily
than light ones, so again the population of peppered moths adapted. Begin-
ning in the late 1960s dark moths decreased in frequency by about 12 percent
per year in some areas, while remaining common in others. Today, this con-
temporary evolution has restored white peppered moths to Manchester's
streets.
Pigeons, in contrast, are still mostly dark. Dark pigeons may have once
benei ted from camoul age, but for reasons not entirely understood, they also
have a slight reproductive advantage over light-colored forms. Their repro-
ductive powers keep them common today despite being conspicuous to pred-
ators. In fact, being of a common shade has some advantages. Many raptors
actively select the odd bird from a l ock of prey even if that bird better matches
the environment's color. In cities, this would include the rare pigeon with
white markings in a coal-colored urban l ock. Regardless of the city's hue, pi-
geons, it seems, may stay forever dark—a reminder to future generations of the
perils of a coal economy.
The soot of coal may no longer be an evolutionary force in most modern
cities, but the novelty of the urban environment remains a potent driver of
adaptive change. The antibiotics we engineer to i ght disease quickly select
for resistance in their bacterial targets. Three short years after penicillin was
discovered, bacteria were resistant to it. Similar responses to next-generation
antibiotics have given us superbugs like MRSA (methicillin-resistant
Staphy-
lococcus aureus
bacteria), which kill tens of thousands of people a year. The
pesticides we spray in cities to kill mosquitoes or caterpillars are no match for
the heritable variation hidden in insect populations. After the development of
DDT in 1939, bugs evolved resistance to it by 1946. Birds unfortunately did
not and would continue to lay thin-shelled, easily broken eggs in the United
States because of DDT if not for the actions of scientists who successfully
