Population genetic theory predicts that selectively driven changes of allele frequency for both beneficial and deleterious mutants reduce polymorphism at tightly linked sites. All else being equal, these reductions in polymorphism are expected to be greater when recombination rates are lower. Therefore, the empirical observation of a positive correlation between recombination rates and amounts of DNA polymorphism across the Drosophila melanogaster genome can be explained by two very different types of natural selection. Here, we evaluate alternative models of effects of selection on linked sites by comparison of X-linked and autosomal variation. We present polymorphism data from 40 genes distributed across chromosome arms X and 3R of Drosophila simulans, a sibling species of D. melanogaster. We find significantly less silent polymorphism in D. simulans on the X chromosome than on 3R, but no difference between arms for silent divergence between species. This pattern is incompatible with predictions from theoretical studies on the effect of negative selection on linked sites. We propose that some form of positive selection having greater effects on sex chromosomes than on autosomes is the better explanation for the D. simulans data.