I actually have no idea if the 'pros" make this argument on this issue per se, but it goes like this.
If I take 10 coins, lay them down, 5 heads, and 5 tails and sample them at a 90% rate (take 9) discard the other, duplicate the 9 (I now have eighteen) and keep doing that I will end up with a population of all heads or all tails at some point.
When that happens, the trait (heads or tails) is called "fixed".
Fixed is essentially the trait being all, or the trait being extinct.
In this case (genetics) sampling=fertility. We need not bring Darwin and fittest into this at all. Lightning bolts kill folks dead, things happen, sampling is never 100%. The greater the sample, the less the drift. But it will drift, traits will become fixed (either extinct or 100%). It is just gonna happen.
Mutation and selection confuse matters, it is the play between hard math (drift) and crazy biology (mutation and selection) that gets us here.
I think being able to come up with a circumstance of multiple independent origin lineages extant would be one of remarkably tortured numerology. Regardless what happened, it is most likely in the absence of some extreme exception that what we see today is from one.
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