So what's the actual proposition being asserted by "fitnessism"?

• "People and/or other animals actually act so as to maximize genetic fitness"? That isn't true; see, e.g., the first link in DeVliegendeHollander's comment.
  • Further, I don't see how to get from there to any sort of ethical theory. There's no valid inference from "people do X" to "people should do X". You might as well look at gravitation and conclude that virtue consists of proximity to large massive objects.
• "'Acting so as to maximize genetic fitness' is a principle that approximates actual people's and cultures' ethical systems, but unlike them has some kind of scientific underpinning"? That also seems to me to be very untrue (the problem being the first half more than the second); see, e.g., the rest of DVH's comment.
• "We should act so as to maximize genetic fitness"? Well, if you define "should" according to "fitnessism" then this is true but vacuous; if you define it according to most other ethical systems it's very false (here I'm just repeating my previous claim); if you define it to mean "According to the actual, real, One True Morality" then it's question-begging.

It looks to me as if the path to "fitnessism" goes like this: (1) Contemplate evolution. (2) Conclude that people (and other animals) act so as to maximize their genetic fitness (this is error #1, conflating adaptation execution with fitness maximization). (3) Conclude that "right" means something like "tending to maximize genetic fitness" (this is error #2, conflating ought with is).

Perhaps I'm missing something important; Survival Machine, would you care to set me straight?