The Red Queen hypothesis, also referred to as Red Queen's, Red Queen's race or the Red Queen effect, is an evolutionary hypothesis which proposes that organisms must constantly adapt, evolve, and proliferate not merely to gain reproductive advantage, but also simply to survive while pitted against ever-evolving opposing organisms in an ever-changing environment, and intends to explain two different phenomena: the constant extinction rates as observed in the paleontological record caused by co-evolution between competing species, and the advantage of sexual reproduction (as opposed to asexual reproduction) at the level of individuals. Leigh Van Valen proposed the hypothesis to explain the "Law of Extinction", showing that, in many populations, the probability of extinction does not depend on the lifetime of the population, instead being constant over millions of years for a given population. This could be explained by the coevolution of species. At other times, established species have evolved cooperatively by assuming adaptive coevolutionary dependencies. These complementary relationships develop through graduated symbiosis, directing punctuated advantages specialized enough to ensure a greater survivability and fitness rate for both species, coupled. Indeed, however, an adaptation in a population of one species (e.g. predators, parasites) may change the natural selection pressure on a population of another species (e.g. prey, hosts), giving rise to common antagonistic coevolutions. If this positive feedback occurs reciprocally, a potential dynamic coevolution may result. In another idea, the Red Queen hypothesis is used independently by Hartung and Bell to explain the evolution of sex, by John Jaenike to explain the maintenance of sex and W. D. Hamilton to explain the role of sex in response to parasites. In all cases, sexual reproduction confers species variability and a faster generational response to selection by making offspring genetically unique. Sexual species are able to improve their genotype in changing conditions. Consequently, co-evolutionary interactions, between host and parasite for example, may select for sexual reproduction in hosts in order to reduce the risk of infection. Oscillations in genotype frequencies are observed between parasites and hosts in an antagonistic coevolutionary way without necessitating changes to the phenotype. In multi-host and multi-parasite coevolution, the Red Queen dynamics could affect what host and parasite types will become dominant or rare.

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