I don't dispute evolution per se, but "natural selection" in the circumstance you're talking about may only a mechanism for filtering a pre-existing gene pool.
That by itself doesn't mean you have a new species.
Let me be really clear... I'm not making the claim that I've disproven evolution.
I'm only suggesting that our handle on the causes of it aren't quite as firm as is frequently claimed.
1. Genetic Drift - chance changes in the gene pool of a small population. This includes something called the Founder Effect where a new colony is established in a new habitat by only a few individuals and is not representative of the original population, and the Bottleneck Effect where a small number of individuals of a species survive, but again are not representative of the original population.
2. Gene Flow - a gain or loss of alleles by migration of fertile individuals between populations.
3. Mutations - changes in an organisms DNA, a relatively rare event occurring once per every 10e5 or 10e6 gametes.
4. Non-Random Mating - including inbreeding and assortative mating (based on phenotype).
5. Natural Selection - differential success in reproduction (often due to changes in environment).