PaulK writes:
To put the obvious first it's so clear that he meant 1/(3 * 10^7) that I'd have asked him straight out if it was a typo. If he insisted then I'd point out that 3 base pairs per codon for 10^7 codons is obviously 3*10^7 base pairs. If that didn't work I'd ask him to explain himself to find out just what absurdity he had in mind.
I think he may really have meant (1/3)
107, because 10
7 is the number of base pairs in his genome. I would argue that he was thinking, "There are three wrong bases for any position, and the probability of getting one of those three wrong bases is 1/3. So the probability of the first base pair in the genome being wrong is 1/3, and the probability of the second base pair being wrong is 1/3, and so forth, so multiple 1/3 by itself 10
7 times."
Which is, course, completely bogus.
What he really wanted to do if he was determined to use that style of approach was properly figure out the probability of an error for a base pair, and 10
-8 seemed to be an acceptable figure to him. This means that the probability of all the base pairs being right would be:
(1-10-8)107
This happens to be around .9, which means there's about a .1 probability of one or more mutations.
This is so similar to his (1/3)
107 that it makes it seem likely that he was trying to use this particular approach, but what he did was both wrong (the 1/3) and too simplistic (this has to be calculated, it isn't something simple that you write down off the top of your head).
So the probability of three mutations at specific predesignated positions in a single reproductive event is:
(1-10-8)(107-3) * 10-83 = 9.08*10-25
I'm on the edge of my competency here, so let me know if I've gone wrong. Anyway, it is indeed a very small number. As I told Deadlock, predesignating the positions isn't the way evolution operates. Natural selection doesn't sit around waiting for beneficial mutations, it just works on what it has. And if you apply 10
6 Bernoulli trials like this you still get a very small number:
C(106,1) * 9.08*10-25 * (1 - 9.08*10-25)(106-1)) = 9.08*10-20
This is unsurprising because of the unlikelihood of three mutations in predesignated locations in a single reproductive event. As I explained to Deadlock several times, this isn't the way evolution works because of multiple offspring and generations, and because of the
post facto fallacy (is that the proper name for this fallacy, calculating the odds of what really happened, like winning a lottery, when all the outcomes were equally unlikely but one of them has to happen?).
Anyway, that's my attempt at things, not the same answer you got, but I had a different interpretation of this problem. What do you think?
--Percy