Melanism

Melanism is the increased development of the dark pigment melanin in the skin, hair, feathers, scales, or cuticle of an organism. In animals, melanism is typically produced by dominant alleles at one or a few loci, making it a genetically simple trait with large phenotypic effects. It occurs naturally in many species as a polymorphism maintained by balancing selection, frequency-dependent selection, or mutation-selection balance. But melanism also responds rapidly to environmental change — most famously in the case of industrial melanism, where pollution-darkened environments reversed the selective advantage of light morphs.

The genetic architecture of melanism is remarkably convergent across taxa. In vertebrates, mutations in the melanocortin receptor pathway are common. In insects, regulatory changes in pigmentation genes like cortex and optix produce similar phenotypic outcomes from different starting points. This convergence suggests that melanism is not merely a color change but a developmental module — a relatively independent subunit of the phenotype that can be modified without catastrophic pleiotropic consequences. The modularity of melanism is what makes it evolutionarily labile, and what makes it a recurrent target of both natural and artificial selection.