In birds and frogs, species pairs retain the capacity to produce viable hybrids for tens of millions of years, an order of magnitude longer than mammals. What accounts for these differences in relative rates of preand postzygotic isolation? We propose that reproductive mode is a critically important but previously overlooked factor in the speciation process. Viviparity creates a post-fertilization arena for genomic conflicts absent in egg-laying species. With viviparity, conflict can arise between: mothers and embryos; sibling embryos in the womb, and maternal and paternal genomes within individual embryos. Such intra-and intergenomic conflicts result in perpetual antagonistic coevolution, thereby accelerating interpopulation postzygotic isolation. In addition, by generating intrapopulation genetic incompatibility, viviparity-driven conflict favors polyandry and limits the potential for precopulatory divergence. Mammalian diversification is characterized by rapid evolution of incompatible feto-maternal interactions, asymmetrical postzygotic isolation, disproportionate effects of genomically-imprinted genes, and ``F 2 hybrid enhancement.'' The viviparity-driven conflict hypothesis provides a parsimonious explanation for these patterns in mammalian evolution. BioEssays 22:938ยฑ946, 2000.