conflict liypothesis

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 pre-and postzygotic isolation? We propose that reproduc-tive mode is a critically important but previously over-looked 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 post-zygotic isolation. In addition, by generating intrapopula-tion genetic incompatibility, viviparity-driven conflict favors polyandry and limits the potential for precopula-tory divergence. lUlammalian diversification is character-ized by rapid evolution of incompatible feto-maternal interactions, asymmetrical postzygotic isolation, dis-proportionate effects of genomically-imprinted genes, and "F2 hybrid enhancement. " The viviparity-driven conflict hypothesis provides a parsimonious explana-tion for these patterns in mammalian evolution. BioEs