To me "survival of the fittest" essential means the individual lives long enough to reproduce and spread its genes (including any mutated ones). A mutation that conferred super strength to an individual might allow it to survive long beyond any of its fellow species members, but it would be useless in terms of aiding in transforming that species unless the super strong individual mated and produced progeny with the same gene and so on and so on.
I have a difficult time understanding though how a, let's call it a "neutral" mutation gets passed on enough to become common to the species in the absence of an "advantageous" mutation.
For discussion purposes lets say that a mutation that resulted in small red dots on a human's elbows appeared. I can't imagine this creating an advantage or disadvantage in terms of the ability to reproduce. Let's also say it's a dominant gene. The human with the mutation has ten children who all carry the gene and all of their offspring carry it as well. Even if they are fantastically prodigious in terms of reproduction the "red elbow dot" gene isn't going to become common among homo sapiens, unless they also carry a gene that gives them an advantage in terms of reproduction.
With the exception of a mutation that somehow provides immunity to a given plague or radiation, I just don't see any giving the necessary advantage to individuals who will pass it on and transform the human species.
As our ability to tinker with genes grows, even "disadvantageous" genes may not create an actual disadvantage as they will be neutralized.
Humans only have to live to puberty to pass on their genes, and as we can see all around us there are individual humans who are far less than perfect specimens passing on their genes. Modern medicine keeps alive an untold number of children who would have died only a hundred years ago.
In the absence of an apocalypse that returns us to the same evolutionary playing field as all other species, I think our evolution through natural selection has come to an end.