Few points.
In general I think competition for resources will drive population down in time. I think this is superseding. I think the discussion of the mentally handicap being such a burden to the race and a hindrance to human evolution as a whole is in the realm of fantasy.
Kind of a flimsy flashy notion, in my opinion.
On the topic of human evolution, I think a neurological and physiological perspective should be the basis in attempting to find a focal point. My perspective in general is that human evolution, all life in general, is spurned on and continuated by a singular force. Let's call it "nature". Your conscious self is in a symbiotic relationship. Without nature, life could not exist. If life is the thing, nature is the process. Nature is what makes you wake up in the morning. It's what makes an ant care. Nature the founder of all life. YOUR PARTNER. You get the point.
The real question is this, why did nature gamble with consciousness. An establishment clearly both beneficial and dangerous to nature and life in general as the world around us is testament.
It is very clear that human "ego" and it's proclivity to act as a distancing force from one's own nature may be where the battlefront ultimately lies. The false limitations we impose upon ourselves. The habits that become the framework of our narrative. Your narrative, your identity, is pliable. That is an important point. One should not hold much stock in it.
Let's talk about the physics of evolution. For change to take place, processes must occur (simply put). If we relegate our conversation to the brain, (which I believe we should) we can furthermore say that in order for the processes necessary for changes consistent with an "evolutionary arc" to occur, energy is required. If an individual is conscious of one's relation to nature and the world, and is actively attempting to align with nature's intent, i.e. evolution; an arc of constant change would be the best course for maximizing evolutionary potential (i.e. the process of evolution.) I think a healthy diet, exercise, rest, proper stimuli and perspective are more or less the requirements for evolution. Perspective and proper stimuli intertwine with philosophy, but for this example let's assume both are developing. It is easy to see the importance of nutrition for the physiological processes required for evolution. It's common overlooked sense. Diet, exercise, and proper rest are underrated.
The development of personal self takes a level of seclusion, of independence, of disassociation from the collective conscious. Great minds of the past spent months on single problems and much time alone in general whilst "evolving". The eb and flow of obtaining information, receiving feedback, reflecting and refining, the process of development.
I think it is crucial to keep a neurological perspective foremost. The narratives we tell ourselves are just that. All interactions throughout life have a neurological foci. Physiology precedes psychology. Our conscious selves are merely a point of reference. A spoke on a wheel. A part of a whole.