Pets

Pets evolved to be cuter and cuter. How? The ones that weren't cute—that weren't able to command our resources, to enslave us into taking care of them—they died! It's natural selection in action: the cute ones bred with each other until we reached the point we're at today: infected with the pet virus.

Obviously, this is a bit tongue-in-cheek: nobody's worried about being enslaved by pets.

The Extended Phenotype, which I'm currently reading, contains an entire chapter on similar topics (Arms Races and Manipulation). Here's one of the main points.

… it is natural to assume that even if a manipulator gets away with it temporarily, it is only a matter of evolutionary time before the lineage of manipulated organisms comes up with a counter-adaptation. In other words, we tend to assume that manipulation only works because of the ‘time-lag’ constraint on perfection. In this chapter I shall point out that, on the contrary, there are conditions under which we should expect manipulators to succeed consistently and for indefinite lengths of evolutionary time.

One of these conditions, the life/dinner principle, is the one that applies to pets.

This ‘rare-enemy effect’ is an important example of an asymmetry in selection pressures acting on the two sides of an arms race. Although we gave no formal models, Krebs and I considered one such asymmetry qualitatively under the catchphrase heading of the ‘life/dinner principle’, named after a fable of Aesop which was called to our attention by M. Slatkin: ‘The rabbit runs faster than the fox, because the rabbit is running for his life while the fox is only running for his dinner.’ The general point here is that for an animal on one side of the arms race the penalty of failure is more severe than for an animal on the other side of the arms race.