A GENERAL-MODEL FOR THE EVOLUTION OF RECOMBINATION

A general representation of multilocus selection is extended to allow recombination to depend on genotype. The equations simplify if modifie r alleles have small effects on recombination. The evolution of such m odifiers only depends on how they alter recombination between the sele cted loci, and does not involve dominance in modifier effects. The net selection on modifiers can be found explicitly if epistasis is weak r elative to recombination. This analysis shows that recombination can b e favoured in two ways: because it impedes the response to epistasis w hich fluctuates in sign, or because it facilitates the response to dir ectional selection. The first mechanism is implausible, because epista sis must change sign over periods of a few generations: faster or slow er fluctuations favour reduced recombination. The second mechanism req uires weak negative epistasis between favourable alleles, which may ei ther be increasing, or held in check by mutation. The selection (s(i)) on recombination modifiers depends on the reduction in additive varia nce of log(fitness) due to linkage disequilibria (v(1), < 0), and on n on-additive variance in log(fitness) (V'(2), V'(3),.. for epistasis be tween 2, 3.. loci). For unlinked loci and pairwise epistasis, s(i) = - (v(1) + 4V(2)/3)delta r, where delta r is the average increase in reco mbination caused by the modifier. The approximations are checked again st exact calculations for three loci, and against Charlesworth's analy ses of mutation/selection balance (1990), and directional selection (1 993). The analysis demonstrates a general relation between selection o n recombination and observable components of fitness variation, which is open to experimental test.