Am biologist and keeping genes that are "advantageous" is the most common trap that people fall into about evolution. Evolution isint actually about keep advantageous traits the real idea is about differences in reproductive success, not about evolution systematically preserving only traits that are beneficial to the individual.

In fact Darwin's theory of evolution is kind of outdated as well. We now understand genetic drift and things like the neutral theory of molecular evolution from moto kimura and the nearly neutral theory of evolution from tomoko ohta may give some sort of explanation to polygenic nature of adhd.