  In large populations, multiple beneficial mutations may be simultaneously
spreading. In asexual populations, these mutations must either arise on the
same background or compete against each other. In sexual populations,
recombination can bring together beneficial alleles from different backgrounds,
but tightly linked alleles may still greatly interfere with each other. We show
for well-mixed populations that when this interference is strong, the genome
can be seen as consisting of many effectively asexual stretches linked
together. The rate at which beneficial alleles fix is thus roughly proportional
to the rate of recombination, and depends only logarithmically on the mutation
supply and the strength of selection. Our scaling arguments also allow to
predict, with reasonable accuracy, the distribution of effects of fixed
mutations when new mutations have broadly-distributed effects. We focus on the
regime in which crossovers occur more frequently than beneficial mutations, as
is likely to be the case for many natural populations.
