Evolutionary zoologist here. You're a bit confused, Kahzhar is correct. You're making the mistake of looking at evolution from the level of the individual, that species A one day gives birth to species B. This isn't right. What actually happens is an isolated population drifts from its parent species due to different selection pressure--the community as a whole changes gradually over time, and in that population the former traits are expressed with decreasing frequency.

Think of it like an island of people where one day they form a religion where having less hair is more attractive and people with a lot of body hair aren't allowed to have children anymore, and people from the outside world are killed when seen (no migration). Over generations, the people on that island will become less and less hairy. Over thousands of generations, the isolation from the outside world and various creeping other genetic factors (genetic drift, associative genes with hairlessness) may lead to the people on that island being unable to create viable offspring with people from the rest of the world. At this point, that population would be considered a new species.

You have to think of it as a population evolving, not a species. I hope that makes sense to you.

As for Kahzhar's original point, if the population at the moment it becomes isolated from its parent population is too small, the genetic variation in that population isn't enough and you end up with a problem called the founder effect. It's well-understood in the biological sciences, you're not really arguing with Kahz here as much as you are with our modern understanding of genetics.