I think you might be combining Darwinism with Dawkinsism here. Natural selection isn't necessarily preservational of individual genes (nor of genotypes). It ju

I think you might be combining Darwinism with Dawkinsism here. Natural selection isn't necessarily preservational of individual genes (nor of genotypes). It just argues that the survivors are more genetically fit for survival. This aligns with GP's comment. The Selfish Gene theory argues that behaviours among the group tend toward preservation of the genotype. Family over strangers, generally. I do take issue with GP's "evolutionary purpose" -- these two words cannot be combined in a meaningful way. But, interpreted as "evolutionary contribution", it could make sense. All that said, I also don't think that GP's idea floats, nor does it hold water. Genetic predisposition to depression is very poorly understood, but seems (to me) to be just ordinary phenotypical noise in the signal. Quasi-random variation which is to be expected in any sufficiently complicated system.