Hybrids between species are often sterile or inviable because the long-diverged genomes of their parents cause developmental problems when they come together in a single individual. According to the Dobzhansky-Muller (DM) model, the number of genes involved in these "intrinsic postzygotic incompatibilities" should increase faster than linearly with the divergence time between species. This straightforward prediction of the DM model has remained contentious owing to a lack of explicit tests. Examining two pairs of Drosophila species, we show that the number of genes involved in postzygotic isolation increases at least as fast as the square of the number of substitutions (an index of divergence time) between species. This observation verifies a key prediction of the DM model.