In pre-industrial England, a species of moth, the peppered moth (Biston betularia) occurred. Almost all peppered moths were gray with dark flecks, but very rarely a black moth was observed. The color forms are illustrated here; note that they belong to the same species; they reproduce with each other, and the color difference depends primarily on alternate alleles for a single gene. The black moths were presumably produced by mutation: biochemical change in DNA that makes up the genes for a trait.
Peppered moths rest during the day on tree trunks, where they are vulnerable to being eaten by birds. In pre-industrial England, tree trunks were gray. As illustrated here, gray moths are well camouflaged on gray tree trunks; black moths stand out.
It has been shown experimentally (by Kettlewell) that in areas with gray tree trunks, black moths are much more likely to be eaten by birds than are gray moths (presumably because black moths are much easier for the birds to see.)
When industry developed in England, pollution from factories turned tree trunks in forests in industrial areas black. As illustrated here, on black tree trunks, black moths are well camouflaged; gray moths stand out.
It has been shown experimentally (by Kettlewell) that in areas with black tree trunks, gray moths are much more likely to be eaten by birds than are black moths (presumably because gray moths are much easier for the birds to see.)
In industrial areas, because black moths avoided being eaten by birds, they survived better and therefore reproduced more (had higher fitness.) As a result, each generation, more and more of the offspring born came from black parents and inherited the black coloration, since the color differences between moths (gray versus black) are genetic. After many moth generations, almost all the moths in industrial areas were black. Black coloration is an adaptation to an environment with black tree trunks; that is, it has evolved through natural selection because black individuals have higher fitness in forests with black tree trunks than do gray moths.
Note that this example illustrates Darwin's four postulates:
Note also that an individual moth does not adapt -- it is genetically either black or gray and can not change. Individuals do not adapt. The population of moths adapts because those individuals with lower fitness traits do not reproduce as much, so their genes are not carried on from generation to generation as much as the genes for higher fitness traits, so the traits coded for by those genes (in this case the trait of being a gray moth) die out over time, and traits with high fitness (in this case the trait of being a black moth) become common.
Return to main lecture introducing evolution and natural selection