Depression is a heterogeneous collection of phenotypes sharing partially overlapping genes, neurobiology, and symptoms. This chapter applies an evolutionary perspective to the distinct etiologies and functions of three reliably identified depressive phenotypes: sickness behavior, starvation depression, and melancholia (i.e., depression with melancholic features). Infection and food shortage are evolutionarily ancient problems, and so sickness behavior and starvation depression probably evolved first. Melancholia probably evolved more recently and shows signs of an evolutionary design for a cognitive function. More specifically, evidence suggests that melancholia is an adaptation for promoting analytical reasoning, and probably evolved as an adaptive response to complex problems involving resource management or conflicts with close social partners. These depressive phenotypes, although distinct, are functionally similar, which explains the overlapping genetics, neurobiology, and symptomatology. In all three, depressive symptoms help the body coordinate the reallocation of limited energy resources in response to persistent threats.