Stabilizing selective force in a population of humans

Dolphin embryos have both forelimbs, which will become the flippers, and hind limbs, which will not develop. Explain how this supports the conclusion that dolphins are related to humans.
Propose an explanation for why the circumference of newborn babies’ heads would act as a stabilizing selective force in a population of humans that does not have access to modern medical care.
Provide an example of analogous traits in two organisms and explain how/why those traits evolved in such distantly related organisms.