Industrial Melanism

Abstract

Industrial melanism refers to the evolution of dark body colours in animal species that live in habitats blackened by industrial soot. The phenomenon has been documented in numerous species that hide from predators by blending in with their backgrounds. Peppered moths provide one example. Before the industrial revolution, peppered moths in the UK were pale grey, but after their habitats became polluted with soot from coal‐fired industries, melanic (black) phenotypes became numerous and spread to other regions. Away from industrial centres, the pale phenotype remained common. Following clean air legislation a century later, the atmosphere improved, soot‐damaged habitats gradually recovered, and the pale phenotype returned as the predominant form. Parallel changes have occurred in America. The melanic and pale phenotypes are determined by genes, and the changes in their percentages in populations reflect natural selection. Experiments identify bird predation on the moth phenotypes as the agent of selection. Key Concepts: The natural colour patterns of animals are adaptations produced by natural selection. A change in frequency (percentage) of genetically determined phenotypes in natural populations is direct evidence of evolutionary change. Mutations introduce new genetic variation to a population, but recurrent mutations occur too rarely to bring about rapid changes in the frequency of genes. Random changes in the frequency of genes (genetic drift) are irregular and unpredictable in direction. Directional, rapid changes in the frequency of genetically determined phenotypes in populations result from natural selection. Historical records on phenotypic frequencies from population samples allow the assessment of natural selection. Gene flow (migration) retards genetic differentiation among geographically widespread populations. Clines indicate different selection pressures along environmental gradients; when selection is removed, migration homogenises the differences along a cline. Parallel evolution is ‘nature's replicate experiment’.