In Chapter 7, Diamond talks about a number of ways in which plants were changed by human selection.  People picked fruits or plants with certain attributes, which eventually made those plants evolve to have more of those attributes.  For example, people picked the biggest apples and the biggest corn and so eventually apples and corn became bigger.  Some other examples of those attributes include:

• Taste -- they picked ones that tasted good.
• Amount of flesh on the fruit.  More is better.
• Size of seeds in the fruit.  Smaller is better.
• Long fibers for things like cotton that will be used as thread or rope.
• Amount of oil in the seed.  More is better.

There are a few other things that would have been less obvious, but evolved nonetheless.

• You want seeds that don't have a dispersal mechanism.  If pea pods burst and the seeds fall out, they're no good.  So people picked things that didn't have that kind of mechanism.
• Seeds that germinate all at once.  Farming would have selected for seeds that all germinate at once.
• Plants that could pollinate themselves and still be pretty much the same as they had been.

These last three could not be seen and selected for (especially the last two).  But as the plant came to be cultivated for years and years, those traits were unconsciously selected for.