Poeciliidae
Scientists argue about why Poeciliid fish developed to give their young eggs food chemicals inside their bodies.  There are two groups of ideas, adaptive hypotheses and conflict hypotheses. Adaptive ideas, including the locomotor hypothesis, Trexler-DeAngelis Model (reproductive allotment), and life-history facilitation, say that the fish's placenta evolved to help another advantage to evolve. The conflict hypothesis says the placenta is a nonadaptive byproduct of genetic tug-of-war between the mother and the eggs.