In a bird population, there is disruptive selection for habitat: one group adapts to the treetops, while another adapts to the lower branches and ground. The two groups rarely interbreed, but when they do the hybrid offspring do not live long because they have a mixture of both kinds of adaptations and are not adapted to either habitat.
A small group from a large mainland population colonizes a remote island.
Due to meiotic error, a diploid plant capable of self-fertilization produces a self-fertilizing tetraploid offspring.
Over a period of several million years, a deerlike species evolves from being very small and feeding on grasses and small shrubs to being much larger and feeding on the lower branches of trees.
A river that has long divided two populations of mice is diverted by an earthquake, and the two mouse populations come into contact and breed together. The hybrid offspring, however, are sterile.