The environment we have created for ourselves is capable of sustaining most forms of life - certainly very-slightly-mutated human beings anyway. It might not have been designed for them, but then, what environment has ever been designed for a new form of life? It either adapts to the environment it is given or it dies.
That said I know what you mean. I guess it's just that the variations are so small, and happen so slowly, that it won't be easily visible as a mutation at an individual level. But then suddenly in a few thousand years we'll all be a different shape or something.