Editing Natural experiment (section)

===Industrial melanism===

With the [[Industrial Revolution]] in the nineteenth century, many species of moth, including the well-studied [[peppered moth]], responded to the [[atmospheric pollution]] of [[sulphur dioxide]] and [[soot]] around cities with [[industrial melanism]], a dramatic increase in the frequency of dark forms over the formerly abundant pale, speckled forms. In the twentieth century, as regulation improved and pollution fell, providing the conditions for a large-scale natural experiment, the trend towards industrial melanism was reversed, and melanic forms quickly became scarce. The effect led the evolutionary biologists L. M. Cook and J. R. G. Turner to conclude that "[[natural selection]] is the only credible explanation for the overall decline".<ref name=CookTurner2008>{{cite journal |last1=Cook |first1=L. M. |last2=Turner |first2=J. R. G.|title=Decline of melanism in two British moths: spatial, temporal and inter-specific variation |journal=Heredity |date=2008 |volume=101 |issue=6 |pages=483–489 |doi=10.1038/hdy.2008.105 |pmid=18941471 |doi-access=free |bibcode=2008Hered.101..483C }}</ref>