Psychological Effects of Status Rank Changes

"that depression allows losers to adapt peacefully to a loss of status, and thus it improves the survival of the overall group. "

It's survival of genes that matter, though, not groups (except to the extent that this promotes survival of genes).

Take two individuals who lose status: A gets depressed and accepts inferior status, while B never accepts it and keeps trying to get back high status. In hierarchical societies, A will have greater genetic success than B because he won't get killed by the new dominant person / coalition.

Hunter-gatherer societies aren't hierarchical, so depression will only show up in agricultural and industrial populations. Sure enough, the variants of the serotonin transporter that predispose to depression are at higher frequencies in Europe and East Asia compared to sub-Saharan Africa.

A caveat: any individual plays many games (work, marriage, parenthood/motherhood, etc) at any time, and a serious loss in an important game can send him/her through depression, compelling him/her to new losses (any game), and so forth.

There is not survival of the group, natural selection works on each specimen, not on groups. That is a misconception.

that depression allows losers to adapt peacefully

I know you didn't mean it this way, but the first time through I read your use of "losers" as derisive. Maybe I was thinking too much of this classic Onion article.