epistasis
Crow JF 2008 Maintaining evolvability. J Genet 87:349-353.
- long-term selection experiments show that populations continue to retain seemingly undiminished additive variance despite large changes in the mean value
- there are several reasons for this
- intermediate-frequency alleles increase in frequency towards one, producing less variance (as p → 1, p(1 − p) → 0)
- others that were originally near zero become more common and increase variance
- a roughly constant variance is maintained
- only a case-by-case analysis will provide the answers
- despite the difficulties that complex interactions cause for evolution in Mendelian populations, such populations nevertheless evolve very well
- long-lasting species must have evolved mechanisms for coping with such problems
- a large and important part, perhaps the most important part, of evolution is quantitative
- much of evolutionary change consists of making continual adjustments to the deteriorating effects of an ever-changing environment
- most such changes are likely quantitative
- at least that is the assumption of this article
- although interactions slow the progress of selection, the process still works at a rate mainly determined by Fisher's 'Fundamental theorem of natural selection'
- only additive × additive and additive × additive × additive, etc. components contribute to changes of a trait under selection
- this is only part, perhaps a small part, of the genetic variance
- a two-locus model of complete dominance, complementary epistasis, and allele frequencies of ½ shows only 0.064 of the variance due to additive × additive epistasis
- there is also reason to think that even additive parts of the epistatic variance may not make a significant contribution
- Kimura was the first to show that, with loose linkage, the population rather quickly reaches a state where the linkage disequilibrium variance exactly cancels out the epistatic variance
- gene-frequency change by selection may well be determined entirely by additive variance
- I suspect that Kimura's principle of quasi-linkage equilibrium is more often applicable than has been traditionally assumed
