<P> From around 1962 to the present, the phenotype frequency of carbonaria has steadily fallen . Its decline has been measured more accurately than its rise, because of more rigorous scientific studies being conducted . Notably, Bernard Kettlewell conducted a national survey in 1956, Bruce Grant conducted a similar one in early 1996, and L.M. Cook in 2003 . </P> <P> Similar results were found in America . Melanic forms have not been found in Japan . It is believed that this is because peppered moths in Japan do not inhabit industrialised regions . </P> <P> In modern times, because of cleaner air standards in Europe and North America, the dark - bodied moth is becoming less frequent, again demonstrating the adaptive shifts in the peppered moth population . </P> <P> J.W. Tutt was the first to propose the "differential bird predation hypothesis" in 1896, as a mechanism of natural selection . The melanic morphs were better camouflaged against the bark of trees without foliose lichen, whereas the typica morphs were better camouflaged against trees with lichens . As a result, birds would find and eat those morphs that were not camouflaged with increased frequency . </P>

The process that has led to the changes in the moth population