epistasis - senecio7の日記

Hurst LD 2000 Epistasis and the evolutionary process. Heredity 85:625-626.

review of: Wolf JB, Brodie ED III & Wade MJ, eds. 2000 Epistasis and the evolutionary process. Oxford UP. ISBN:9780195128060

Brodie suggests that Fisher, when analysing adaptive evolution, considered a model in which mutants at a locus went through to fixation so fast that no interacting allelic variants at other loci were around to disrupt the process
this defends the use of single locus models but does not explain why the assumption was made
most consider that Fisher was imagining a model in which, because of recombination, the background that any given allele would sit in was constantly churning so the average effect was all that we needed to consider
the average might well approximate to multiplicativity if ε is greater then zero, as often as it is less than zero

Phillips, Otto and Whitlock make, I think, a most telling point
even if Fisher was right that on average ε is about zero, the little studied variance in ε still matters

is epistasis everywhere?
many studies say not
but here Templeton provides possibly the most incisive of the chapters in arguing that standard modes of analysis have such a baggage of Fisherian assumptions
it is actually biased against finding epistasis
it simply being what is left over when all other sources of variance are removed

Wright's view, antithetical to that of Fisher, was that epistasis was important because after population sub-division, the epistatic effects would allow different phenotypes in different sub-populations
without needing to evoke selection and local adaptation

the finding that additive variation increases after bottlenecking, reviewed separately by Meffert and Goodnight, is strong support for Wright's position
(or at least some version of it)
this result is quite the opposite of what the simple Fisherian models would predict

the Wrightian ancestry of many involved in epistasis research also goes a long way to explaining why all but one of the 24 authors is at a North American institute
(and the other is an American abroad)

as Brodie makes clear, effects that are additive for phenotype can be non-additive for fitness
and vice versa

the introductory chapter by Brodie attempts to do the same graphically but leaves more problems than answers

without exception the chapters are well considered and reach guarded conclusions
in this context it was rather fun to see Wade cite a particularly strong and unguarded assertion from Brian Charlesworth
in a letter to Norm Johnson he posits that 'this relentless and futile search for intraspecific epistasis needs to be abandoned!'
the book convinced me that Charlesworth is wrong on this one