 This cline in morph colour, from a dominance of lighter morphs in the west of England (which did not suffer as heavily from pollution), to the higher frequency of melanic forms in the north, has slowly been degrading since limitations to sooty emissions were introduced in the 1960s.Clines generated through this mechanism have arisen through the joining of two formerly isolated populations which differentiated in allopatry, creating an intermediate zone.This secondary contact scenario may occur, for example, when climatic conditions change, allowing the ranges of populations to expand and meet.