Theft is not a joke, even if it involves genes. Elysia chlorotica is a sea snail that pretends like it can only steal genes from algae. After eating algae, it can permanently incorporate some of it's genes to gain the ability of photosynthesis. Thus, they can survive for months with nothing but sunlight.
Of course, those limitations are extremely arbitrarily set. All they need are the right circumstances to truly evolve into the apex species.