Various illnesses are caused by promoter genes. These are genes that serve no purpose but they specialize in promoting themselves, so they spread through a population even though they serve no evolutionary fitness goal (often they bring illness). A good book on that topic:
Genes in Conflict: The Biology of Selfish Genetic Elements
However, referring to the original article of this thread, it is also possible that the same genes that give us depression also give us something positive, such as creativity.
Genes in Conflict: The Biology of Selfish Genetic Elements
by Austin Burt and Robert Trivers
https://www.amazon.com/gp/product/0674027221/
However, referring to the original article of this thread, it is also possible that the same genes that give us depression also give us something positive, such as creativity.