A hundred years ago or so, most of us disabled people wouldn't have had the lives we do today. I know we complain about welfare reform (and we're right to do so) but back then, we'd have been in workhouses. And that's if we were alive at all. Natural selection - evolution - might have had a word to say about our continued existence.
natural selection, a characteristic (let's say being taller) becomes more or less common in a population depending on whether it makes the individuals with it more reproductively successful or not. That is, if being taller means they have more offspring, the genes that gave rise to that characteristic are passed on to a larger group, and become more common. If they have fewer offspring, they don't.
All very interesting (for sad science geeks like me, anyway). But how does it relate to disability?
Many disabilities are caused, or at least influenced, by genes carried by the individuals concerned. Those genes are as controlled by the rules of natural selection as any others. If the gene for a disability means more offspring, it should become more common, along with the disability. If it means fewer offspring, it should become less common, and ultimately be eliminated from the population. "Fewer offspring" could arise through the individual dying before reproductive age.
However (predictably) it's not always quite as simple as that. Sickle cell anaemia is a genetic blood disorder where the red blood cells take on a sickle shape. This decreases the cells' flexibility and increases the risk of various complications. It occurs most commonly in people (or their descendants) from tropical or sub-tropical regions where malaria is or was common.
Kin selection is a theory explaining why individuals behave in ways that favour their relatives rather than themselves, even at a cost to their own survival and/or reproduction. The classic example is a beehive, where asexual drones work for the benefit of the all-powerful queen bee. The kin selection theory states that they do this in order to help pass on the queen's genes, which are so close to their own.
genomes too. As families support each other, they make it more likely that their genes, including "defective" ones, will be passed on.
And, of course, disabled people make many contributions to the humanity-hive. In paid or voluntary work; as children, siblings, lovers, spouses, and parents; as employers (or the reason people are employed); and as friends, giving emotional support. I'm sure you can think of many more.
The genes that code for disabilities may not always be helpful. But I don't think evolution will be eliminating them just yet. Factors like carrier status being beneficial, and kin selection, mean they are neutral at worst in their effect.