An increase in the amount of the pigment melanin in a population which lives in an area that has been darkened with pollution by human industrial activities. The extra melanin helps the individuals of the population to better camouflage themselves against predators in the darker, sooty surroundings.
Industrial melanism occurs because lighter-colored individuals (ones without extra melanin) are found and eaten at a higher rate than darker-colored individuals.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.