Ironically, these are the same people who buy into Darwin and survival of the fittest.
The whole 'survival of the fittest' phrase is so ambiguous, but widely used because it is easy for public consumption.
There is a survival of the fittest among species. So far we haven't gone extinct.
Then there is continuation of the genes and traits of the individuals of a species. Those that are most successful usually mate the most. Therefore traits that enable 'surviving' spread around. This causes an evolution in the physical characteristics of the offspring.
I have not mentioned evolution as it applies to how life went from prebiotic soup to present day humans, because I don't think that is what Darwin was addressing, and it's way too complicated to even get started on (at this late hour).