From a purely pragmatic and objective point of view, since humans are part of the natural ecosystem, it is impossible for us, or any animal, to "interfere" with natural selection. We are simply part of it. Humans cannot destroy the ecosystem, but we are a catalyst for change. So are many other things. There have been mass extinctions throughout history which completely changed the ecosystem. One could even argue that by trying to preserve the current variety of species on earth, we are interfering in the natural changing and adapting of the ecosystem.
Ultimately we are part of nature, not above or separate from it. Preserving a species or driving one to extinction is not inherently positive or negative. Nature doesn't care, and we just play our part. And weather we, a asteroid strike, a gamma ray burst from a nearby supernova, or an alien species wipe out all life on earth, all is equally natural. Objectively is doesn't matter, and whatever we decide to do is fine.