Animals reproduce by egg-laying (oviparity) or live birth (viviparity) and in many species viviparity involves transfer of nutrients via a placenta. We propose and test the idea that the origins and evolution of viviparity and placentation have been driven by maternal-fetal conflict over resources. Such maternal-fetal conflict arises because a mother and her fetus are related genetically by only one-half, such that fetuses are subject to natural selection to take more resources from the mother than she is selected to provide. David Haig showed in 1993 that parent-offspring conflict can explain many puzzling features of human pregnancy, especially aspects of the placenta. Here we extend Haig's ideas in two ways, (1) by showing that embryos and fetuses of many viviparous taxa actively manipulate their interaction with the mother, and (2) by providing evidence that parent-offspring conflict is responsible for the extreme diversity among mammals in the form, function, and molecular genetics of the placenta. This work has important implications for human health and reproduction, as miscarriages are frequently caused by disruptions of placental development, placentation provides useful models for understanding immunology and carcinogenesis, parent-offspring conflict apparently can favor the evolution of genes that promote cancer, and adult health increasingly appears to be 'programmed' during fetal development.
For more information about this article and other articles in the May issue, see The American Naturalist online: www.journals.uchicago.edu/AN