The nylon bacteria is a great example (linked earlier). It digests only nylon, a compound that didn't exist 100 years ago:
http://www.nmsr.org/nylon.htm
And this link at the bottom of that page, leading to "Examples of Beneficial Mutations and Natural Selection"
http://www.gate.net/~rwms/EvoMutations.html
And then, there are lifeforms in extreme environments:
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