You are repeating a common misconception revolving about the individual organism. Selection pressure rewards successful genetic reproduction. What actually ha

You are repeating a common misconception revolving about the individual organism. Selection pressure rewards successful genetic reproduction. What actually happens at the scale of the organism is mostly irrelevant except how it affects reproduction. A genetic predisposition towards chopping legs off would be selected for, if chopping legs off resulted in more children than otherwise. Chopping legs off after having no more children (e.g. after menopause or vasectomy) has no direct genetic outcome (instead indirectly via improved/worstened reproduction of children or relatives). To be clearer: reproduction is the key (survival matters for reproduction and upbringing). That's a good summary, although the details are sublimely trickier. https://en.wikipedia.org/wiki/Gene-centered_view_of_evolution https://en.wikipedia.org/wiki/Gene-centered_view_of_evolutio...