stevestory
Posts: 13407
Joined: Oct. 2005
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(for reasons I don't understand) Mike Dunford posts some stuff at UD.
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Mike Dunford
01/24/2007
12:37 pm
DaveScot: | Quote | | Haldane used the maximum amount of hard selection practical to arrive at the highest speed of fixation possible (300 generations for humans). |
I’m sorry, but that just isn’t correct. First, and on a more minor count, Haldane’s calculations were not human-specific, and 300 generations was a general approximation. Second, that figure is not the highest speed of fixation possible. It is the highest speed of fixation practical in a hard selection scenario. Under soft selection, fixation can take place more rapidly than 300 generations.
Sal: | Quote | | But let’s look at it briefly, and one will see it is only a tautologous version of Kimura’s neutral theory, with the “beneficial” label retrodictively (after-the-fact) added to the subsitutions that make it to fixation! |
Hardly. The scenario that you sketched was neutral, because the trait was totally invisible in the population. That doesn’t mean that all such scenarios will be - think about dominant traits, for example. We can identify regions in the genome that are, or have recently been, under selection provided that we can find the right sorts of markers nearby because the gene frequencies in those areas deviate from the neutral expectation in specific ways.
DaveScot: | Quote | | What doesn’t make much sense is trying to say Haldane is wrong about the cost and speed of beneficial substitutions. |
I’m not sure that anyone is making that argument. Haldane was right about the cost and speed of beneficial substitutions in the specific case of that sort of hard selection. The error only comes in extrapolating the results obtained using Haldane’s specific set of assumptions to cover all cases, including those which do not meet those assumptions.
A lot of what I’m trying to say will come through more clearly if I take a simulation through a few steps. Unfortunately, I’m not a good enough programmer (read: not a programmer at all) to set up some process to do it by hand. I won’t have the time or inclination to do so today - I’ve already got several hours of math on the table, and am not voluntarily going to add more, but I’ll take a swing at it tomorrow. I’ll post it on my own blog - that will make it easier for me to do the formatting and save drafts - and post a link here when I’m done. |
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