Earlier quoted context omitted.
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 by Austin Burt and Robert Trivers https://www.amazon.com/gp/product/0674027221/ However…
This feels like a just-so story. There are perfectly plausible reasons why the conditions we regard as mental illness are adaptations that help evolutionary fitness: - Depression puts one into a low-energy, contemplative state that enables reflection, healing and formulation of reformed ways of thinking and being ("dark night of the soul"); - Bipiolar is a pattern of swinging between high-energy bursts of inspiration…
Promoter genes are also part of evolutionary theory. You seem to think evolutionary theory came to an end in 1882. But a lot has happened since then.