April 29, 2017 at 2:51 am

Curtis, Helena. Biology. New York: Worth

Publishers, 1989.

Futuyma, Douglas J. Evolutionary Biology.

Sunderland, MA: Sinauer Associates, 1998.

Gould, James L., and William T. Keeton, with

Carol Grant Gould. Biological Science,6th ed.

New York: W. W. Norton & Company, 1996.

Patterson, Colin. Evolution, 2nd ed. Ithaca, NY:

Comstock, 1999.

Ridley, Mark. Evolution. Boston: Blackwell

Scientific, 1993.

One well-studied example of genetic variation

in populations is that of Biston betularia, the

peppered moth.

There are three color morphs in the peppered

moth: a light morph, a dark or melanistic

morph, and an intermediate morph. Before the

Industrial Revolution, the light morph was the

most common form, although melanistic moths

were also seen occasionally. However, by the

end of the nineteenth century, the melanistic

morph had become much more common, and

had practically replaced the light morph in

certain areas.

Biologists traced this shift to industrial

pollution in urban areas. Without camouflaged

resting places, the light moths became easy

targets for bird predators. This explained both

the prevalence of melanistic moths in polluted

urban environments, and of light moths in

comparatively pristine country habitats.

The puzzling aspect of the peppered moth

story is that genetic variation was not entirely

eliminated in populations. In urban areas, for

example, melanistic moths make up only from

90 to 100 percent of the total population,

despite very strong selection. Apparently there

are forces other than predation pressure at

work. It was hypothesized briefly that

heterozygote advantage might be the

explanation, but that theory was ultimately

rejected. It is now believed that gene flow

between country and urban areas, and

frequency-dependent selection are viable

alternatives. However, much work remains to

be done on this historic system