How Can Natural Selection Work if Mutations Can't Cause Species to Change?

Speciation is macroevolution and we cannot see that. We can only infer it from the fossil record because the process just takes too much time to observe. The slight variations within a species are microevolution (fluctuating allele proportions) which we can directly observe (ex: domestication). Sometimes allele proportions become so dramatically different between populations that they can no longer interbreed but again this takes millions of years.