How have humans helped cotton to evolve?

First, I think you have to take a look at the word
"evolve."  What we have done to cotton has been of benefit to us, certainly, but I am
not convinced that it counts as true evolution. Cotton was doing just fine before we
came along.

The answer to your question is that humans have
helped cotton to "evolve" in the same way they've helped most everything
to:

1. Scientists and farmers selectively
   breed crops in the same way that they do animals.  An example is Tanguis
   cotton, which was created in Peru to survive a particularly nasty fungus.
    Agriculturalists take one plant and breed it with another related one with the hopes
   that the best qualities of both will mix.


2. Scientists and
   farmers genetically alter the cotton to make it more profitable
   (again, I am not sure it counts as evolution, but in the sense it is more likely to
   survive perhaps it does.) An example of this would be cotton that has certain bacteria
   genes spliced in that make the plant unattractive to some insect
   pests.

In short, whether or not it counts as
evolution, humans have changed cotton to make it yeild more, grow in more places, and be
less susceptible to both diseases and insects.