The blocking of fixation of a genotype among two or more competing genotypes possessing similar fitness within a population.

This occurs only if the population is and may also be dependent upon a relative absence of within the population's environment. The result is an inefficiency in the evolution of toward increased fitness.

More generally:

The frequency of beneficial mutations, and thus the intensity of interference, depends on several factors, among which some of the most relevant are the , the error rate, and the degree of adaptation of the population . In large populations a high number of genomes replicate simultaneously, increasing the probability of generating beneficial mutations that can compete for fixation. In addition, the time necessary to fix any substitution is longer in large populations than in smaller ones, which also increases the opportunities for interference . The increase of the error rate is associated with higher numbers of both deleterious and beneficial mutations per genome. In this situation, the fixation of beneficial mutations can be delayed not only by the coexistence of several lines carrying different mutations but also because at high error rate beneficial mutations arise in deleterious backgrounds, thus hampering the action of selection .

See also hitchhiking, linkage, and .