Evolution works by having two organisms, one male and one female, survive to sexual maturity then procreate together. The logic being, if the organisms are smart and strong enough to survive that long, there is something beneficial about their genes that helped them survive. Therefore, their genes are worth passing on to the next generation.
The strongest genes of both the male and female are combined and given to their offspring so it has the best of both parents and will be even better genetically suited for survival. If not, then the offspring will die off and another member of that species will out-compete them to survive and reproduce. It is not necessarily “survival of the fittest,” but survival of the fit enough. Evolution is designed so that sufficiently fit members of a species (and the most fit species versus other species) survive in order for that species to continually improve upon its genetic code, further promoting survival and reproduction.
If an organism is deficient physically and/or mentally, it will not survive long enough to reproduce while competing against more fit members of its species (and other species). This means that highly fit species physically who were deficient mentally would not survive. Think of a super-strong/fast athlete who isn’t smart enough to realize he can’t survive jumping off a high cliff. Likewise, a highly fit species mentally who is deficient physically would not survive. Think of a genius like Stephen Hawking incapacitated due to a disease like ALS. At least that’s the way it would go in nature.
Humans are outliers in that we have evolved past evolution. Through our intelligence and consciousness, we have become sapient. We can dictate our own evolution outside of natural selection. When a child is born with deficient genes—some sickness or deformity, either physically or cognitively—other members of the species help them survive. They can then reproduce themselves and pass on their previously deficient genes.
If humans weakened by genetic deficiencies were living in the wild, pre-civilization, like any other species (and like humans used to), then they would never survive on their own and reproduce. It’s not just that ordinary sick and disabled people can now survive due to modern medicine, but rare geniuses like Stephen Hawking can survive and thrive.
Imagine if Hawking had to fend for his own in the wild as a nomadic hunter-gatherer. He’d never survive without medical intervention and assistance from other humans (to eat, sleep, move, and speak). Evolution by natural selection would have killed him off as soon as he got the disease that disabled him from walking. But since humans have overtaken evolution, he survived to contribute some of the most important science and knowledge about the universe—to greater enhance the survival of all of humanity. And there are countless cases similar to Hawking’s. A genetic deficiency in one area often leads to a proficiency in another area. Due to technology and civilization, many natural “disabilities” are no longer barriers to survival and reproduction, and even more of those barriers will be lifted in the future.