Depression in an evolutionary context.
Sadness and low levels of depression are adaptive since they lead the individual to try and make up a loss. By contrast, severe or clinical depression is not adaptive, but can be thought of as sadness having become malignant.
Wolpert
Lewis
L
D003468:Cultural Evolution; D003863:Depression / Q000145:classification / Q000209:etiology / Q000523:psychology*; D004644:Emotions; D006801:Humans; D013315:Stress, Psychological