case2102: Moth Coloration and Natural Selection
Description
This data was collected by J.A. Bishop. Bishop selected seven
locations progressively farther from Liverpool. At each location,
Bishop chose eight trees at random. Equal number of dead (frozen)
light (Typicals) and dark (Carbonaria) moths were glued
to the trunks in lifelike positions. After 24 hours, a count was
taken of the numbers of each morph that had been removed---presumably
by predators.source
Ramsey, F.L. and Schafer, D.W. (2002). The Statistical Sleuth: A
Course in Methods of Data Analysis (2nd ed), Duxbury.Details
Population geneticists consider clines particularly favourable
situations for investigating evolutionary phenomena. A cline is a
region where two colour morphs of one species arrange themselves at
opposite ends of an environmental gradient, with increasing mixtures
occurring between. Such a cline exists near Liverpool, England, where
a dark morph of a local moth has flourished in response to the
blackening of tree trunks by air pollution from the mills. The moths
are nocturnal, resting during the day on tree trunks, where their
coloration acts as camouflage against predatory birds. In Liverpool,
where tree trunks are blackened by smoke, a high percentage of the
moths are of the dark morph. One encounters a higher percentage of
the typical (pepper--and--salt) morph as one travels from the city
into the Welsh countryside, where tree trunks are lighter.
J.A. Bishop used this cline to study the intensity of natural selection.References
Bishop, J.A. (1972). An Experimental Study of the Cline of Industrial
Melanism in Biston betularia [Lepidoptera] Between Urban
Liverpool and Rural North Wales, Journal of Animal Ecology
41: 209--243.