The Red Queen's Hypothesis, aka "Red Queen's race" or "Red Queen Effect" is an evolutionary hypothesis explaining the advantage of sex at the level of individuals (mixing of genes), and the constant evolutionary arms race between competing species. In the first (microevolutionary) version, by making every individual an experiment when mixing mother's and father's genes, thus enabling sex to be a microbiological defense mechanism. This may allow a species to adapt quickly just to fight the constant evolution of dangerous pathogens such as influenza and countless other naturally occuring theats. This allows an organism to hold onto the ecological niche that it already has in the ecosystem. In the second (macroevolutionary) version, the probability of extinction for groups (usually families) of organisms is hypothesized to be constant within the family and random between families.
The term is taken from the Red Queen's race in Lewis Carroll's Through the Looking Glass. The Red Queen said, "It takes all the running you can do, to keep in the same place."