Is natural selection the only means of evolution?

No, don't forget about genetic drift, mutation, and gene flow.

Explanation:

Genetic drift - when a smaller population gets sectioned off from the rest (maybe they move to an island or something) and the disparity in allele frequencies is just compounded. For example, if 100 people move to an island to have a population, and 50 of them have an eye disorder, they might evolve into a species with natural eye disorders... whoa!

Mutation - when an error occurs in the actual process of DNA exchange in meiosis (or the forming of a new organism). This is rare, because most mutations are lethal.

Gene flow - if two populations get separated by some factor and there's no "gene flow," or exchange of alleles, they may diverge into separate species and evolve newly!

There's also non-random mating, if you want to get into that. It basically just changes the equilibrium so that allele frequencies don't stay the same, which can create evolution.

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".