[AntiGuv]

The key point here to keep in mind is the concept of genetic drift. The longer two populations reproduce in isolation from one another, you will have more and more genetic drift. You can clearly see the evidence in that - for whatever reason - Africans drifted toward darker skintypes while Asians drifted toward slanted eyes and Europeans drifted toward aquiline features. At some point, if you isolate two populations of the same species long enough, they will drift apart to the point where the sperm of one will not fertilize the egg of another, or where females of one can no longer carry infants of the other to term. Whenever this juncture is reached, then you will have yourself two or more divergent species. There won't even be a "bright line" between the two. You probably won't be able to tell the one species from the next just by looking at them. That's why there are many closely related species that look almost indistinguishable from one another - but they've drifted apart just enough that they can no longer mate with each other.

[gore3000]

The key point here to keep in mind is the concept of genetic drift.

Actually no. Genetic drift is a false proposition. Neutral mutations do not spread. Reason is quite simple, in any population the genes of each individual will reproduce at the same rate as that of other individuals. Since the original mutation occurred in only a single individual a neutral mutation will only have the same number of descendants as those of any other individual in the population. What this means is that if there were 1000 individuals in the population, a neutral mutation will always be present in only 1/1000 of the population no matter how much the population increases and no matter how many generations one goes forward. Further, because a new mutation is only present in a single individual, the chances of its being lost by either random accident to the individual carrying it or by a short string of unfavorable odds (for example if one only has two quarters and consistently bets on heads coming up, chances are that after enough tries one will lose both quarters). So neutral mutations will not spread and most likely will dissappear from the population entirely.