survival of the fittest noun [U]
the principle that animals and plants suited to the conditions they live in are more likely to stay alive and produce other animals and plants than those which are not suited

[TahlilGaran] Advanced Learner's Dictionary ▲

survival of the fittest
the idea that the most able or fit will survive
It was a matter of the survival of the fittest in the jungle.

[TahlilGaran] English Idioms Dictionary ▲

survival of the fittest
n. phr. The staying alive or in action of the best prepared; often: idea that those living things best able to adjust to life survive and those unable to adjust die out.
Life in the old West was often a case of survival of the fittest.
With changes in the world's climate, dinosaurs died but many smaller animals lived on. It was survival of the fittest.
On the 50-mile hike it was survival of the fittest; only 12 out of 25 Scouts finished.

[TahlilGaran] English Idioms Dictionary ▲