The Red Queen hypothesis states that the primary driver behind evolution is the struggle to survive rather than environmental forces. Predator and prey constantly evolve in competition with one another, with each evolutionary advantage shifting the balance one way or the other.
An example of this principle is a plant that evolves a toxin to kill insects that feed on it. The toxin will kill off much of the insect species, but a certain percentage will be naturally immune and breed while the others die off. Eventually, all the insects will be immune, rendering the plant's evolutionary advantage useless.
The hypothesis got its name from Lewis Carroll's "Through the Looking-Glass," in which the Red Queen declares, "Now, here, you see, it takes all the running you can do, to keep in the same place."