England’s peppered moths, like Darwin’s finches, demonstrate the cyclic effects of natural selection as conditions change. The issue involved moths with wings mottled in a light coloured way and moths that were darkly mottled. For major problems with the experimental design which was assumed to demonstrate the action of natural selection in the moths as the environment changed from pristine to polluted air, and back to cleaner air again, see Let’s Mothball the Peppered Myth
Aside from the problems in the experimental study in England, a similar reversal from dark to light populations of moth has been documented in a region in Michigan which never had changes in lichens covering the tree trunks. In England, it was the replacing of light coloured lichens with black soot on the trees that was believed to have camouflaged the darker moths and kept them from being eaten by birds. However, in the US, there never was a decline in the light coloured lichens on the tree trunks.
American ecologists thus report: “Although the evolution of melanism [darker wings] in this southern Michigan population seems to have paralleled the changes in British populations in both directions, common causes for the change are not obvious. Clearly, more extensive sampling of the American Biston is needed.” [B. S. Grant, D. F. Owen and C. A. Clarke. 1995. Decline of Melanic Moths (correspondence). Nature 373 Feb. 16 p. 565.] For whatever reason, the system in Michigan is oscillating in similar fashion to the British moths, and nobody has any good ideas why this is so. This lack of correlation between frequency of dark specimens and lichen cover is serious for a theory that industrial melanism results from the ability of moths to avoid being eaten because of their ability to blend into their background. Since neither in England nor in the U.S. have the moths been found to settle on tree trunks anyway, these studies did not demonstrate anything of substance. Another textbook demonstration of “evolution in action” needs to be discarded!