Red Queen Theory
In Lewis Carroll's classic Through the Looking-Glass, the Red Queen, a living chess piece that Alice meets, has to run in place as quickly as she can to simply stay in the same place. In order to get anywhere else, she says, you must run twice as fast.
This character's race is used as a metaphor in the Red Queen Theory (also called the Red Queen's Race or Red Queen Effect, or simply Red Queen) to explain the advantage of sex and the constant race between competing species. Through the mixing of chromosomes that occurs in sexual reproduction, each new individual becomes a genetic experiment, where different genes can come together to allow quick adaptation and change to hold onto or adapt to ecological niches.
In nature, a species may have to adapt quickly to its changing environment in order to simply survive. In adaptive scenarios, the capacity to adapt quickly confers a huge advantage. Simplistically put, consider a predator-prey adaptive relationship. In order for the predator species to continue preying on his primary food source, he may have to adapt to capture the continually-adapting prey.
The main flaw in the Red Queen Theory is that when a species reproduces sexually, half the species does not bear offspring directly and often does not contribute to the survival of the young. However, the increased rate of adaptation makes up for the loss in fecundity, and the combination of genes helps ensure that less-adaptive genes for one environment may be saved to come in useful in another environment. In particular, less-adaptive genes may have a special use when the species needs to compete with a particular parasite; the relationship between sickle-cell anemia and malaria is a good example.
Web Resources On Red Queen Theory
PBS: The Red Queen
The Red Queen: Sex and the Evolution of Human Nature
Book Resources On Red Queen Theory
The Red Queen: Sex and the Evolution of Human Nature by Matt Ridley
The Selfish Gene by Richard Dawkins