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Functional Ecology 2011 25 757 768 doi 10 1111 j 1365 2435 2011 01842 x Why do placentas evolve An evaluation of the life history facilitation hypothesis in the fish genus Poeciliopsis Marcelo N Pires Ronald D Bassar Kevin E McBride John U Regus Theodore Garland Jr and David N Reznick Department of Biology University of California Riverside California 900 University Avenue Riverside CA 92521 USA Summary 1 The Northern Clade of the sh genus Poeciliopsis includes six closely related species three of which lack placentas and three that have placentas but vary in the extent of post fertilization provisioning 2 We used this diversity to evaluate a series of hypotheses proposed in earlier publications concerning why the placenta has evolved All hypotheses share the attribute of arguing that the placenta evolved to enhance the evolution of some other life history trait such as to reduce the age at maturation or to increase offspring size We refer to these hypotheses collectively as life history facilitation hypotheses 3 A general way to evaluate the plausibility of such proposals is to ask whether the evolution of the placenta is predictably associated with the evolution of other components of the life history 4 We evaluated such associations in two ways First we performed a multivariate analysis of life history data for sh collected and preserved in nature This analysis included 16 populations across all six species Secondly we performed a more complete quanti cation of the life histories of the laboratory descendents from ve populations representing four species then performed a similar multivariate analysis The laboratory study added information about the timing of reproduction age at maturity frequency of reproduction 5 Both analyses yielded similar results which were that the evolution of increased placentation is correlated with the evolution of a smaller size at rst reproduction the production of fewer and smaller offspring per litter but an increase in the number

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