While Nesse1 enumerates the possible ways in which low mood and/or depression may be adaptive for an individual, another possibility is that depression, with its known increase in morbidity and mortality, may be maladaptive to the individual, but adaptive to the species. Cyranowski et al2 point out the increase in depression and sensitivity to loss of relationships in females during childbearing years.
Fawcett RG. Is Depression Adaptive for the Human Species?. Arch Gen Psychiatry. 2001;58(11):1086. doi: