I think the answer is evolution.
Depression that repeatedly occurs with the onset of autumn and winter is to seasonal affective disorder as chronic, low -grade depression is to dysthymic disorder. Seasonal affective disorder is a type of depression that is related to changes in seasons such that it begins and ends at about the same times every year. Dysthymic disorder is a smoldering mood disturbance characterized by a long duration as well as transient periods of normal blood.
A subspecies is a taxonomy category that ranks below species, usually permanent geographical isolated race. <span />
