The Red Queen or "Red Queen's Race" is an evolutionary theory explaining the advantage of sex. By making every individual an experiment when mixing mother's and father's genes, sex may allow a species to adapt quickly just to hold onto the ecological niche that it already has in the ecosystem.
Accordingly, the metaphor Red Queen represents the situation in nature where creatures must adapt quickly to changing environmental threats just to survive from generation to generation. In Lewis Carroll's Through the Looking Glass, from which the metaphor is originally derived, Alice complains that she has to run just to stay in the same place. The character of the Red Queen appears as a representation of a chess piece. She has become a popular object of metaphor as a result of the dream-like scene in which she seizes Alice by the shoulder in order that they can run together very fast in a desperate attempt to keep up with the landscape. She then advises Alice that in order to get anywhere else, as opposed to merely remaining in the same place, one must run at least twice as fast.
The metaphor has been found appropriate for the descriptions of arms races. These comparisons may be made in the fields of military hardware, competition in commercial product markets and competition between people in society for wealth or esteem, but, most famously, the idea was introduced by Leigh van Valen to the discussion of arms races in the biological theory of evolution by natural selection.
The recognition of a simple example of a biological arms race (from Richard Dawkins) can be achieved by considering the contrast between two adaptations of the polar bear. This animal has a coat of hair which is thick to help the bear survive the cold of the arctic and white in order that the bear can stalk seals for food. For the first case the selection pressure is likely to be constant or subject to random change, in the second case the selection pressure is likely to increase steadily as selection for cautiousness in seals makes the average seal harder and harder for the bear to stalk successfully. As a result both the bear and the seal find themselves running a red queen's race over evolutionary time, each becoming better and better adapted (to stealth and caution respectively) but neither becoming any more successful (as they are engaged in a zero sum game).
Science writer Matt Ridley wrote a book The Red Queen in which he discussed the debate in theoretical biology over the adaptive benefit of sexual reproduction to those species in which it appears. The connection of the Red Queen to this debate arises from the fact that the traditionally accepted theory (The Vicar of Bray) only showed adaptive benefit at the level of the species or group, not at the level of the gene. By contrast, a Red-Queen-type theory that organisms are running cyclic arms races with their parasites can explain the utility of sexual reproduction at the level of the gene by positing that the role of sex is to preserve genes which are currently disadvantageous, but which will become advantageous against the background of a likely future population of parasites.
Sex is an evolutionary puzzle. In most sexual species, males make up half the population, yet they bear no offspring directly and generally contribute little to the survival of offspring. In addition, males and females must find each other to mate, and sexual selection often favors traits that reduce the survival of organisms. Thus, sex is highly inefficient.
One possible explanation for the fact that nearly all vertebrates are sexual is that sex increases the rate at which adaptation can occur. This is for two reasons. First, if an advantageous mutation occurs in an asexual line, it is impossible for that mutation to spread without wiping out all other lines, which may have different advantageous mutations of their own. Second, it mixes up genes. Some genes might be advantageous only when paired with other genes, and sex increases the likelihood that such pairings will occur.
For sex to be advantageous for these reasons requires constant selection for changing conditions. One factor that might cause this is the constant arms race between parasites and their hosts. Parasites generally evolve quickly, due to their short lifespans. As they evolve, they attack their hosts in a variety of ways. Two consecutive generations might be faced with very different selective pressures. If this change is rapid enough, it might explain the persistence of sex.