A doctor treats a patient's bacterial throat infection with an antibiotic. At first, the medicine works well and destroys almost all of the bacteria.

Biologists studying finches on an isolated island observed a sudden shift in the environment.

Male guppies can have bright, colorful spots or drab (dull) coloration.

Snowshoe hares have a genetic trait that causes their fur to turn white in the winter to camouflage with the snow, hiding them from predators.

A population of native plants exhibits natural variations in three physical traits that serve different survival functions:

Historically, most female African elephants grew tusks, but a natural genetic variation resulted in a small number being born "tuskless."