Bravo, David!

"But his main point is that there will be selection in favour of closer linkage between favourable gene combinations on the same chromosomes, and it is therefore a puzzle why recombination is as frequent as it is. I think this remains a problem."

I think it is only a problem if adaptation is slow and that new good alleles are rare. Here is how I see it:

The advantage of recombination combining new good alleles on the same chromosome segment outweighs the disadvantage of recombination breaking old good linkages. I.e., when two good alleles are present at low frequencies on competing chromosome segments then a high recombination rate increases the chance of a new chromosome segment with both good alleles. The new chromosome segment with both good alleles begins sweeping the populace, picking up more good alleles along the way. (I use "chromosome segment" rather than chromosome since the human recombination rate is so high that the unit of inheritance is typically smaller than a chromosome. There are between ten and twenty crossovers for each chromosome pair during meiosis.)

A high rate of recombination favors adaptation when many good alleles are circulating in the populace on competing chromosome segments. This should occur when the environment changes rapidly or when a large population is generating many new good alleles. E.g., the last fifty thousand years of human history.

PS I second the "Bravo!". Excellent post.

Bravo, David!

sir,

i can but give a nod to the refinement and generosity of spirit you reflect upon yourself and your lineage in such a comment of praise. were that mine RSS was fleet enough that i might have had pride of place in offering congratulations to mr. burbridge.

yours truly,
c. v. snicker

Thanks. I just found this paper - much more sophisticated analysis of 'additive' variance than mine, but I think the conclusion is much the same: except in rather exceptional circumstances, most of the 'epistatic' variance will be 'additive'.

http://www.plosgenetics.org/ arti...al.pgen.1000008