The placenta is a complex organ that is a composite of many adaptations that must be well integrated for proper function. David Reznick and Michael Ghedotti will study the evolution of the placenta in the fish family Poeciliidae. They will develop a "family tree", based on DNA sequence and morphology, to determine if the placenta evolved independently on different branches and to define groups of species that do or do not have the placenta. Experiments on 30 species of those fish that are closely related and have different forms of maternal provisioning will test the predictions of a recent theory for the evolution of the placenta. Working together, the biologists will provide insights to refine the theory, while theoreticians will provide insights to suggest new lines of experimental inquiry.
In the Origin of Species, Darwin included a chapter entitled "Difficulties on Theory", considering the most serious challenges to the theory of evolution by natural selection. One was the origin of complex organs: it seemed improbable that evolution could integrate many traits at once. He proposed that complexity could evolve by small steps if each represented an adaptation. These fish provide an opportunity to test Darwin's proposition. The research will also offer multidisciplinary training for students, be relevant to several fields of biology, and present a case study in evolution.
