Survival of the fittest

This is Charles Darwin. He was a naturalist, a theorist, trained as a minister and a man who married his first cousin. He wrote The Origin of Species and the Descent of Man, he was an epert on earth worms and barnacles. His theory of natural selection coupled with Mendelian genetics is the basis of modern evolutionary biology.

This is Herbert Spencer. He was a philosopher, economist, and social scientist and one time railroad employee who wrote The Man Versus the State and Principles of Sociology. He coined the phrase "survival of the fittest," in conjunction with liberal market ecnomics.

Herbert Spencer, not Charles Darwin.

Darwin later incorportated the phrase into the 5th edition of Origin (citing Spencer as the souce) as a synonym for natural selection due to it's popularity, not as an actual theoretical frame. Survival of the fittest when applied to market economics may have some validity. But market economics have NOTHING to do with either natural selection narrowly, or evolution more broadly. Do not describe a competitive social situation as Darwinian--it is not. Darwin does not justify free market capitalism. I'm not saying that free markets capitalism is wrong or bad, I'm just saying that it has nothing to do with natural selection. There may be a superficial mechanical analogy there, but this leads to the naturalistic fallacy: that because a thing seems natural it is therfore right.
Also, while Spencer is often credited with the origination of "social Darwinism" Spencer's idea was that evolutionary principles in the market would lead to greater cooperation and ultimate equilibrium, not more ruthless competition.