Darwin did have quite explicit "social evolution" views, indeed since he didn't know about genes, he saw them as counterexamples to his ideas.
Darwin wondered what happened if you put an
(obviously superior) white bloke on an island of negroes. Since he believed that inhgeritance was a quality not unlike mixing two liquids, he couldn't see how this superiority would not be diluted away. Same applies to a favourable change, who does it survive ?
Things like height and skin colour do appear to work on the mixing principle, so he wasn't being very stupid.
Evolutionary theory can be applied to many domains, not just changes in animals. It is used as a programming technique for solving very nasty problems in circuit design and optimising share portfolios.
Dawkins applies it to groups of ideas like religions. This nicely handles the way they share "survival" characteristics (evengelism, absence of birth control for believers, and control over who you are allowed to mate with).
No reason you can't apply it to the way people prosper or fail in a society.
But you have to be careful about the term "survival of the fittest", it should be taken as "best fit", not strongest. In standard biology we observe that strong tigers are nearly extinct, but rats are doing quite well in a world influenced by humans.
In various concentration camps run by the Europeans and Japanese in WWII, big people tended to die first because they needed more food.
It is assumed by many that our society favours stupid people who breed a lot, and that this damages the gene pool.
Evolution of course doesn't have the idea of good or bad, and if rapid breeding is the optimum, well, it worked for rabbits.
But of course a complex modern society offers ways that less intelligent people can get themselves or their children killed. Homeopathy, crystal healing and other stupid forms of fake medicine are obvious examples of this.
Some western religions forbid the use of life saving techniques on children. Technically Christian scientists et al forbid them for adults as well, but whereas an adult can choose to sin, the child gets a painful death infliced upon them.
The vast increase in diabetes may be bad for overall health, but may be doing good things for IQ. It has long been known that there is a strong correlation between survival rates of diabetics and intelligence. Unlike illnesses such as cancer or AIDS, the patient has an actice and critical role in managing the illness. Poor judgement or sloppliness, can make them sick or dead. Living diabetics may actually be on average the smartest group on Earth.