a, Nucleotide polymorphism (π). The solid curves give the recombination rate (cM Mb−1). b, Divergence (k) for D. simulans (light green) and D. yakuba (dark green). c, Polymorphism to divergence ratio (Pol/Div), estimated as 1 − [(π0-fold/π4-fold)/(k0-fold/k4-fold)]. An excess of 0-fold divergence relative to polymorphism (k0-fold/k4-fold) > (π0-fold/π4-fold) is interpreted as adaptive fixation whereas an excess of 0-fold polymorphism relative to divergence (π0-fold/π4-fold) > (k0-fold/k4-fold) indicates that weakly deleterious or nearly neutral mutations are segregating in the population.