I'm reminded of the "helplessness" phenomenon shown by a rat when beaten by another rat. The loser often goes limp and elevates his throat, the victor holds the pose and relaxes the attack. I suspect Wilson or Dawkins would (and perhaps has) cite this as an adaptive mechanism that allows the weaker to survive and reproduce.
Playing dead, or a rabbit's freeze can pay off if a predators visual systems are gated by motion, especially rapid motion. If this relatively immediate response were to be prolonged by limbic systems then we may have a foundation for hopelessness (an "exaptation"?)
I wonder if there's a clinical difference between depression due to social rejection and that arising hopelessness? And, I wonder if there's a different response to various classes of antidepressants. Certainly, the SSRI's have effectiveness for lessening rejection feelings. I wonder if antidepressants with more of an adrenergic component would be effective for the hopelessness cases? Does anyone know of any data?
Also, someone could have a lot of fun combining these possibilities with hypotheses basing depression on responses to seasonal slow downs such as sleep changes (hibernation), higher fat intake, and motivational inertia.