We KNOW mutations are possible, we KNOW there IS such a thing as good mutations, we KNOW natural selection is taking place, we can SEE and measure the changes of natural selection, we have even made whole new subspecies (dogs are a good example... I searched and found out that they're not a new species but subspecies). So we KNOW "microevolution" exists. It doesn't take a rocket scientist to see how after a few thousand years, mutations build up and ta-da! you have a new species. Do you question if we can see and measure evolution in action?
Actually, William R. Rice and George W. salt (lol... rice and salt) "have recorded a speciation event, or very nearly so, in their extended experiments on fruit flies. From a small stock of mated females they eventually produced two distinct fly populations adapted to different habitat conditions which the researchers judged 'incipient species'" (national geographic magazine quote)
Peter and Rosemary Grant have done their research in the wild (galapagos) and have seen how the beak changes in the finches over time.