What was the variation in 1850 in the peppered moth species?

In 1850, the peppered moth (Biston betularia) exhibited a notable variation primarily between its two color morphs: the light-colored, speckled form and the darker, melanic form. This variation was influenced by environmental factors, particularly the industrial pollution in certain regions of England, which darkened tree bark and favored the survival of the darker moths. The observation of these color variations became a key example in studies of natural selection and adaptation, particularly during the Industrial Revolution.