The Evolution of the Peppered Moth

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A random genetic mutation gave rise to the dark phenotype of the peppered moth. The variation within the peppered moth population allowed the species to survive under the changing environmental conditions. Charles Darwin said that the process that eliminates creatures with inferior traits and preserves those with superior traits is known as natural selection.  The case of the peppered moth is an example of how environmental changes brought about by human activities can affect the course of evolution.

 

Chuckie D.

The Theory of Natural Selection as put forth by Charles Darwin suggests that "those individuals who possess superior physical, behavioral, or other attributes are more likely to survive than those which are not so well endowed."

 

Those organisms that survive are more likely to produce offspring with the same advantages for survival thus increasing the population of the organism. An organism that produces a large number of offspring is deemed fit. (Fitness has nothing to do with the size or strength of the critter-it is all about passing genes on. Frogs are fit. The Pope is not fit.)

 

 

 

Prior to the industrial revolution, the dark phenotype was rare.

The case of the Peppered Moth in Manchester, England is a well-documented study demonstrating the value of camouflage in Natural Selection and the continuing process of evolution. This case involves two variations of the same organism, a white-colored phenotype and a black-colored phenotype.

Scientists have determined that a single gene controls the body color of the peppered moth. The dark phenotype is controlled by a dominant allele, which means that a moth possessing at least one such allele will have a dark body. (Each individual will have alleles - one from each of its parents.) To have a light body, the moth must have both alleles for light body color.

 

 

The white moth is hidden on the lichens within the red circle.

Prior to the industrial revolution, the white moths comprised 99% of the moth population. The white moths were protected by their ability to blend with the light-colored lichens, which lived on the bark of trees.

The black moths were visible against the light background and therefore more susceptible to being eaten by birds. In fact, the dark moths only comprised 1% of the population. Consequently, the black variety did not have as great a chance to reproduce and increase their numbers. The white variation was more fit. Thus the frequency of the dark allele was very low (about .001%), maintained primarily by spontaneous mutation from light to dark alleles.

 

 

During the later part of the 19th century (as the Industrial Revolution began), smoke particles produced by developing industry began to gradually darken the trunks of the trees on which the moths rested. In addition, the light-colored lichens covering the trees were killed by sulfur dioxide emissions from the new coal burning mills and factories.

This change in the environment caused the white moths to become more visible and more likely to be eaten by birds, while the black form became better camouflaged. This situation led to a decrease in the population of white moths while the black moths were better able to breed and therefore increase in number.

In 1848, the dark moths comprised 1% of the population and by 1959 they represented ~90% of the population. So, in 100 years the frequency of dark moths increased by 1000 fold! 

 

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