It's a little bit more complicated than that, see, what this article is suggesting is that men who did better in punch-ups survived and had more kids, thus giving them the 'good at punch up' genes. In other words, the theory behind the evolution of the jaw is that some men already had wider jaws, and these men survived to have children.
Now, cutting off people's arms isn't going to lead to any evolution directly, but if everyone invented everything to be used one-handed, then people who were born without left arms would not have any difficulties. They'd have children, and they would be likely to not have left arms... eventually (although extremely unlikely) conferring an adaptation. It's not that cutting off people's arms is going to make their children adapt, it's that the other people who were born without arms would be more likely to have children without arms, and face no particular problems.

I don't know if that helps. Sure hope it does.

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