[anthonyb]

The several varieties of bananas that you'll see in the store are most definitely caused by human intervention (actively trying to cross slightly different plants to produce a new hybrid) and propagation. Normally hybrids die out in the wild.

Also, once you have a sterile hybrid, there's not much in the way of selection pressure. And since bananas are clones, there's nothing for the selection pressure to act on.

[codeflo]

Not quite true, if I understand you correctly. Clones are a form of asexual reproduction, not that different in principle from what is observed in bacteria. So plant clones do undergo mutation and natural selection.

[anthonyb]

I don't know about bacteria, but a banana variety is all essentially the same plant, propagated from a central stock. Have a look at the controls around planting and growing your own bananas, it's all red tape, permits and extremely fascist:

http://www.backyardbananas.com.au/faq.html

http://www.daff.qld.gov.au/plants/fruit-and-vegetables/fruit...

And for very good reason: every banana of a variety is susceptible to (eg.) a virus outbreak - there won't be any pockets of genetic variability like you would get with potatoes or citrus. If anything that affects bananas becomes widespread - no more bananas.