[Sharlin]

Your understanding of selection and evolution is inadequate. As I said, the majority of mutations are neutral. Of the rest, the vast majority have some small effect on the reproductive fitness of the organism. More often negative than positive, yes. But it's about "on average x±ε offspring instead of x" and not "99.999% likely to cause death pre reproduction, 0.001% likely to make the gene code for hemoglobin when it previously did something completely different". A gene that fickle would never get selected for in the first place; any line carrying such a ticking timebomb would go extinct very quickly. The biochemistry of genetics is itself subject to selection; the most critical genes are selected for being extra robust against harmful mutations. You proposed a mechanism for the same gene to separately evolve more than once, but that argument is irrelevant because the real world doesn't work like that.

[lkrubner]

Neutral mutations can simply be ignored for answering the question posed above. Gort asked:

"isn't it possible, at least in theory, that mutation happened more than once?"

The answer is obviously "yes". Even taking your own words, the answer is clearly "yes", so I'm not clear why you are arguing. Even in the extreme case, where mutations lead to death 99.9999% of the time, the answer would remain "yes". Your own math shows the answer is "yes". I'm sure you know what convergent evolution is:

http://www.zo.utexas.edu/courses/thoc/convergence.html

As it says there:

"Molecules can evolve convergently"

I think you wanted to make the point that this is rare. You should have said so. You could have answered gort by saying "Yes, but this is rare." Instead, you've taken an indefensible position.