Probability of detecting polymorphism in the absence of selection, as a function of N . Two different ranges of detectable polymorphism were used. Dashed curve: (0.1, 0.9); dotted: (0.3, 0.7). We use μ = 7.9 × 10-4. The horizontal bar indicates the observed fraction of polymorphic populations (0.074) from de Boer et al. .