Final answer:
The competitive exclusion principle states that two species cannot occupy the same niche in a habitat. Competition between two species for the same resources leads to natural selection favoring traits that reduce reliance on the shared resource and reduce competition. An example of this principle is shown with two species of protozoa, where one outcompetes the other for food.
Step-by-step explanation:
The competitive exclusion principle states that two species cannot occupy the same niche in a habitat. Different species cannot coexist in a community if they are competing for all the same resources. This principle works because if there is competition between two species for the same resources, natural selection will favor traits that lessen reliance on the shared resource, thus reducing competition. If either species is unable to evolve to reduce competition, then the species that most efficiently exploits the resource will drive the other species to extinction.
An example of the competitive exclusion principle is shown with two species of protozoa, Paramecium aurelia and Paramecium caudatum. When grown individually, they both thrive. But when they are placed together in the same habitat, P. aurelia outcompetes P. caudatum for food, leading to the latter's eventual extinction.