Final answer:
Evolution in a parasite from ectoparasitism to endoparasitism is most likely due to an increase in the number or intensity of natural enemies of the host. Ectoparasites may encounter novel host species when colonizing new areas.
Step-by-step explanation:
Evolution in a parasite from ectoparasitism to endoparasitism would most likely be caused by an increase in the number or intensity of natural enemies of the host. Ectoparasites may encounter novel, but potentially suitable host species when colonizing new geographical areas. A recently infected host is often poorly adapted to the novel parasite, either because it has not yet developed an immunological defence or because of fortuitous vulnerability, and hence there is potential for the parasite to spread rapidly in a new host population.