Evolution theory identifies natural selection as an existing mechanism in nature. It did not invent it. It does not praise it. It does not pass any kind of moral judgement upon it. Evolution theory only describes it. Don't shoot the messenger.
Nobody's shooting the messenger. However, you have delivered the death blow to Objectivism, and on libertarian ideas based on Rand's ideas. Objectivism, after all, prides itself on a strict adherence to operating within the bounds of objective reality. However, objective reality includes "survival of the fittest," or "might makes right." The problem with an insistence on "rationally-derived absolutes," such as "it is wrong to initiate force," is that such claims simply cannot survive contact with survival of the fittest.
If the principle of non-initiation of force is indeed absolute, the origins of its absoluteness cannot come from objective reality, as defined by Rand and friends -- the counter-examples are too numerous.
Skipping to the end of the argument, the choices boil down to "because God said so;" or "the moral absolutes defined by Rand, or libertarians who follow her reasoning, cannot be obtioned through application of reason alone."
Rand (and many FR libertarians) reject God. As such, their moral reasoning cannot be supported by anything other than "because I said so."
As a libertarian, I don't reject God, I reject corrupt organized religion.