... By saying that disabilities are what leads to evolution you're not making up your own theory, you're copying exactly what Darwin said
Darwin's Theory of Evolution says that individuals are born different and those that are better suited (or adapted) to their environment/life out-live those which are less suited and in turn reproduce with one another, passing on the 'better' genes to children. So yes, disabilities can lead to evolution if having a specific disability gives you an advantage over others. A good example (and one which was used often during school) is an animal species which lives in a place where the main food source is the foliage of trees - animals born with a disability eg. an abnormally (or even slightly) long neck have an advantage over the other animals of its species as it can reach the foliage much more easily giving it a greater chance at living than those which are short. As the long necked animals breed they create more long-necked animals until those left with shorter necks eventually die out and you're left with a
giraffe 
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