Joined: Mar. 2005
Re Lemma 1: the conclusion doesn't follow from the premise. Variation is a prerequisite for speciation, in that without it speciation can't occur. Certainly variation is not sufficient for speciation (as you pointed out), because there are other factors (such as isolation of one of the varieties).
Re Lemma 2: the conclusion doesn't follow from the premise. A species that's recovered form a recent population bottleneck will have much less variety than a species that's had a large population continuously for a long time, so variety is not proportional to population size.
Re Lemma 3: I doubt that said probability is completely independent of population, but there's too many variables to expect much of a correlation between them. Besides which, even if Lemma 3 were true it wouldn't be an argument against evolution having happened.
Re Lemma 4: Maybe but so what? Sure there may be some species with lots of variety that don't speciate. But there also may be some with relatively little variety that do.
Conclusion: your conclusion does not follow from the premises.