The best example of a quick change in the environment and a species ability to adapt concerns the the color of the Gypsy Moths in England. When the industrial revolution occurred, coal and other industrial factories spewed out massive amounts of air pollutants, so much so that even during the day the skies were as dark as night. The original color of the gypsy moths was a light gray; such a color blended in with the trees in their environment, and acted as camouflage against predators. With the change in the environment the camouflage adaptation no longer functioned because the tree trunks were darker colored, due to the built up chemicals from the heightened air pollution.
The gypsy moths colored dark gray, which had once been at a severe disadvantage and were normally quickly eaten by predators, now survived and bred, while their lighter counterparts, which had hitherto thrived were eaten and almost eradicated. As a result the gypsy moth, through the spreading of the genes responsible for a darker skin pigment, was able to gradually--but extremely fast in evolutionary measurements--change it's coloring to a dark gray-black, to match the surface of the trees covered in pollution.
Such a thing could be feasible in the extremely reduced human population--some kind of physical or scent-masking camouflage.
