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"In an evolving universe, who stands still moves backward."
-- Robert Anton Wilson
A term used in evolutionary theory
In an evolutionary system, fitness is not something that is measured against an objective standard. A life form doesn't reach a point where it is fit to survive in the system, then always manages to stay alive.
Instead, fitness is relative. There is a fixed amount of resources available to support life, and when multiple species are competing for the same resources, an evolved advantage to one suddenly makes all the rest of them less well adapted, as they will find their share reduced.
Thus, when multiple species are mixed in the same environment, each must continue to evolve to adapt to changes in the environment and changes in other species. A species must continue to change just to stay in the same place in the environment. Failure to do so can lead to fewer available resources - or even extinction.
The term was taken from Lewis Carroll's book Beyond the Looking-Glass, where the red queen says "in this place it takes all the running you can do, to keep in the same place."
Failure to keep up can lead to something being "red queened".