If you thought evolution was slow and gradual, think again. Humans may even be helping it along, as our activities force species to adapt or die.This is not different from the way "real" evolution works. It is a very good illustration of natural evolution. What happens is that when an environment wipes out a part of a population, what remains becomes the new population.
Every weekend angler knows to throw back very small and tiny fishes (the 'tiddlers'). Likewise, commercial fishermen use large-meshed nets to spare smaller fish. Both are working on the principle that by reducing their haul this way, they can keep fish populations vigorous and healthy. But they could be making a terrible mistake. It is becoming increasingly clear that such well-meaning strategies may actually have the opposite effect to what the fishermen intend.
What they and most of the rest of us have overlooked is evolution — not the familiar glacier-slow process found in textbooks, which takes millennia to work its wonders, but a quick evolutionary change that can occur in a matter of years or decades.
The strategy 'leave the smaller fish' is a selection, and it can influence and alter the natural selection of evolution. By leaving the smaller fish, fishermen may be shifting the rules of natural selection, reshaping fish species as they go. If humans only catch the big fish, then the fish population as a whole evolves to smaller and smaller fishes. The result is that the average size of fish shrinks dramatically. The fish species which are small enough survive. The others perish.
That's the way bacteria "develop" a restance to drugs. By killing off the bacteria that are vulnerable and leaving only the small percentage that somehow manage to resist the drug, we breed a new population that consists primarily of resistant bacteria — because that's all there is left.