The process in which individuals at one extreme of a range of phenotypes are favoured, and the beneficial genetic variants (alleles) carried by those individuals tend to become established in the population. Hence the overall make-up of the population shifts in a particular direction, often in response to changing environmental conditions. An example of positive selection is the increase in darker forms of the peppered moth (Biston betularia) that occurred in industrial areas, where the moths with darker wing coloration were better camouflaged than those with lighter wings against polluted tree trunks (see industrial melanism). The concept is central to the theory of natural selection as first proposed by Charles Darwin and Alfred Russel Wallace in 1858. It is now thought that most mutations are selectively neutral and undergo genetic drift (see neutral theory of molecular evolution), or are harmful and are eliminated by purifying selection. Compare disruptive selection; stabilizing selection.