An example of
evolution resulting from natural
selection was discovered among "peppered" moths living near English industrial
cities. These insects have varieties that vary in wing and body coloration from
light to dark. During the 19th century, sooty smoke from coal burning furnaces
killed the lichen on trees and darkened the bark. When moths landed on these trees
and other blackened surfaces,
the dark colored ones were harder to spot by birds who ate them and, subsequently, they
more often lived long enough to reproduce. Over generations, the environment
continued to favor darker moths. As a result, they progressively became more
common. By 1895, 98%
of the moths in the vicinity of English cities like Manchester
were mostly black. Since the 1950's, air pollution controls have significantly reduced the amount of
heavy particulate air pollutants reaching the trees, buildings, and other
objects in the environment. As a result, lichen has grown back, making trees
lighter in color. In addition, once blackened buildings were cleaned
making them lighter in color. Now, natural selection favors lighter moth varieties so they have
become the most common. This trend has been well documented by field
studies undertaken between 1959 and 1995 by Sir Cyril Clarke from the University
of Liverpool. The same pattern of moth wing color evolutionary change in
response to increased and later decreased air pollution has been
carefully documented by other researchers for the countryside around Detroit, Michigan.
While it
is abundantly clear that there has been an evolution in peppered moth
coloration due to the advantage of camouflage over the last two centuries, it
is important to keep in mind that this story of natural selection in action
is incomplete.
There may have been additional natural selection factors
involved.