I think the evolutionary psychologists make a good point that deserves to be listened to regarding depression, if you consider the kind of blues or depression that strikes the majority of people from time to time in their life span. In my limited perusal over the years regarding depression, I'm not sure how well scientists have been able to tease out the different kinds of depression, but I'd bet that it wouldn't be unlikely for the types of depression to have different etiologies. Currently a major depressive episode can be diagnosed as 1)melancholic, 2)atypical, 3)catatonic, 4)postpartum, 5)SAD. Furthermore, the following can also be in existence:
Personally, I think 1,2,4,5 of the previous MDD could be explained by adaptive processes, but that catatonic depression has entered into the biological realm. I think unstopped or uncured depression that goes on for months>years also probably causes some sort of brain lesions, since it appears that the cycle of depression can worsen (and as morgan pointed out). I see no reason why the beginning stages of depression cannot be beneficial in an evolutionary sense, and I've seen examples of these types of depression over the years in people I know, and where a soul searching change could be a lot more effective than taking the easy way out with a pill.dysthymia, adjustment disorder with depressed mood or bipolar disorder. Dysthymia is a chronic, milder mood disturbance in which a person reports a low mood almost daily over a span of at least two years. The symptoms are not as severe as those for major depression, although people with dysthymia are vulnerable to secondary episodes of major depression (sometimes referred to as double depression). Adjustment disorder with depressed mood is a mood disturbance appearing as a psychological response to an identifiable event or stressor, in which the resulting emotional or behavioral symptoms are significant but do not meet the criteria for a major depressive episode. Bipolar disorder, also known as manic-depressive disorder, is a condition in which depressive phases alternate with periods of mania or hypomania.
The ones who cannot spontaneously ruminate and 'get over' their depression in a reasonable amount of time must also have some sort of brain morphology that is not very conducive to mental well-being, that most of us possess; a deficit of some sort in protecting against normal psychological trauma or negative experiences. In which case, they might only be helped by medications. Just as some people with immuno compromised immune systems need help with medications from time to time.