Just to give you this. You can be for 100% sure that nature's selective system works. Only the strong will survive, and only the strong will breed and lay eggs......ofcourse there are some weaker variants of 'flies' for example, but since they are weaker, they enter the 'circle of life' and they become pray for birds or just die faster cause of our environment - which straight away looses all inbreeds cause they do not have the power.
Also take for fact that every single fly you see is different and has it's own uniqueness. When people move around and bring flies from another continent you could have introduced a new TYPE of flies which (perhaps bringing a disease with them) could reign over other flies for being more effective / better survival skills and perhaps when mixing in with local flies providing a new breed of flies with different skills and techniques.
Diversity in nature is the key to it's existence. With diversity you have different kinds / and breeds. When one / more diseases would spread the lands, there will be (not in all cases) groups of animals / insects whatever that will survive the disease for being imuum yet another group will die for belonging to another group vulnarable to the disease. So diversity is natures way of surviving against diseases and other forms of disasters.
For example with humans. Aids is a disease spreading around rapidly. Yet there ARE people who are imuum to the disease. They are capable of living with the disease but not being affected by it because of a GENE in their DNA. And so we carry many other diseases (remember all of these shots you got when you where a baby) that are floating around in our bodies yet we are imuum to them.
During the middle ages there was a strong selection going on with some strong diseases where only a few where imuum to.
So this brings it back to genetically modified crops / plants and animals.......either by cloning or some other way. We create an *in our eyes* strong plant / animal, without any diversity......they are all the same.
So let's say we export and grow only rice plants from one kind in China - cause it's a strong plant, grows fast etc. etc. That means it would also only take ONE disease to destroy the WHOLE crop.
Diseases mutate and develop themselves as well and are fast adapters of the environment.
So ultimately, by cloning we put ourselves in the dangerous position of loosing EVERYTHING because of one error somewhere....meaning a disease would be able to attack a certain area.
The way diversity is setup is that it will sacrifice a group (weak) but allows a stronger group (imuum) to survive and continues on.
on edit:
do a search on terminator seeds by monsanto
http://www.ethicalinvesting.com/monsanto/terminator.shtmlthere;s much more about this, which explains everything much better then I did here