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Biologists link sexual selection and placenta formation

ity of modes of reproduction in the Poeciliidae family. Further, it uses this diversity to perform a formal statistical test of an evolutionary theory called the "viviparity driven conflict hypothesis."

Reznick explained that a research paper in 2000 originally proposed this hypothesis, arguing that there must be a relationship between how animals reproduce and how important sexual selection is in choosing mates. Specifically, when animals evolve placentas, the paper's authors predicted a shift away from choosing who to mate with towards mating with multiple mates, then choosing which fertilized egg to nourish through to the end of development.

"The question is why this change?" Reznick said. "Females of non-placental species fully provision eggs before they are fertilized. If they are to choose a mate, then the choice must be made on the basis of the mate's appearance or behavior. Females of placental species make most of their investment in offspring after the egg is fertilized. This is also a time when the father's genome is active and contributing to the development of the baby. They thus have the ability to choose fathers on the basis of the performance of the developing baby."

All that glitters (in fish tanks) is non-placental

The Poeciliidae family includes guppies, platys, swordtails, and mollies that are frequently on sale in pet stores.

"The reason they are in pet stores is that all of these species are non-placental," Reznick said. "They have males with gaudy colors or elaborate display structures, like the tail of a swordtail or the enlarged fins of sailfin mollies. Gaudy males, it turns out, are more profitable."

Reznick noted that the work his lab is engaged in could not be done on mammals.

"All placental mammals inherited their placenta from a single common ancestor that lived more than 100 million years ago," he explained. "Whatever was happening then has long since been

Contact: Iqbal Pittalwala
University of California - Riverside

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