“Survival of the fittest” is probably the single best-known phrase from On the Origin of Species, though Darwin didn’t use it until the fifth edition, after it was coined by the sociologist Herbert Spencer.
What does 'survival of the fittest' mean?
Fitness remains a concept central to evolutionary biology, but its meaning is very different to our usual understanding of the word.
'Survival of the fittest', or 'natural selection' as it sometimes called, describes not the physical condition of an individual, but how well it is suited to its environment, as measured by its reproductive success over its lifetime - the ‘fittest’ being the species that in successive generations will leave most copies of itself.
Organisms that are better adapted to their surroundings are most likely to both survive and pass on the most useful and strongest adaptations to the next generation of plants or animals.