Haldane's sieve and adaptation from the standing genetic variation

We consider populations that adapt to a sudden environmental change by fixi ng alleles found at mutation-selection balance. In particular, we calculate probabilities of fixation for previously deleterious alleles, ignoring the input of new mutations. We find that "Haldane's siere"-the bias against th e establishment of recessive beneficial mutations-does not hold under these conditions. Instead probabilities of fixation are generally independent of dominance. We show that this result is robust to patterns of sex expressio n for both X-Linked and autosomal loci. We further show that adaptive evolu tion is invariably slower at X-linked than autosomal loci when evolution be gins from mutation-selection balance. This result differs from that obtaine d when adaptation uses new mutations, a finding that may have some bearing on recent attempts to distinguish between hitchhiking and background select ion by contrasting the molecular population genetics of X-linked vs. autoso mal loci. Last, we suggest a test to determine whether adaptation used new mutations or previously deleterious alleles from the standing genetic varia tion.