The Red Queen's Hypothesis

The Red Queen's Hypothesis, also referred to as Red Queen, Red Queen's race or Red Queen Effect, is an evolutionary hypothesis. The term is taken from the Red Queen's race in Lewis Carroll's Through the Looking-Glass, and What Alice Found There. The Red Queen said, “It takes all the running you can do, to keep in the same place.”. [see Wonderland Looking-Glass: Chapter 2: The Garden of Live Flowers] The Red Queen Principle can be stated thus:

In reference to an evolutionary system, continuing adaptation is needed in order for a species to maintain its relative fitness amongst the systems being co-evolved with.

The hypothesis is intended to explain two different phenomena: the advantage of sexual reproduction at the level of individuals, 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, sexual reproduction may allow a species to evolve quickly just to hold onto the ecological niche that it already occupies 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 group and random among groups. Its counterpart is the Court Jester Hypothesis, which proposes that macroevolution is driven mostly by abiotic events and forces.

You see? That's why you need to have sex with me.

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Wikipedia Red Queen's Hypothesis