A fertilized egg develops into an embryo. Sometimes embryos develop directly into adults. However, many species have an intermediate form between the embryo and the adult which is known as a larva. Often the larva bear little resemblance to the adult. Accordingly, for the adult to form, the larva must undergo significant anatomical and physiological changes. Those changes are referred to as metamorphosis.

The selective advantage of metamorphosis is relatively clear in some cases. Consider the sea urchin, a sessile (non-swimming) marine invertebrate related to the starfish. Sea urchins would have little opportunity to disperse if it were not that they have a larval form, the pluteus, which is ciliated and which moves rapidly with sea currents. The pluteus larva bears essentially no resemblance to the adult form. Pluteus larvae permit dissemination of sea urchins which in turn prevents overcrowding of adults and allows for colonization of new...