A hypothesis, proposed by L. M. Van Valen in the early 1970s, that describes how the coevolution of competing species creates a dynamic equilibrium, in which the probability of extinction remains fairly constant over time. Hence, evolution is seen neither as ‘progressive’—with a species’ chances of survival improving over time—nor as ‘escalatory’—with increasing vulnerability to extinction over time. Instead, as one species evolves improvements that make it more competitive, its competitors experience selection pressures that force them to evolve in order to keep pace with it. Ones that lag too far behind will become extinct. The hypothesis is named after the remark made by the Red Queen in Lewis Carroll’s Through the Looking Glass: “Here, you see, it takes all the running you can do, to keep in the same place.”