How can one species evolve from two species?

Answer 1

One species can evolve from two species if the two bypass the barriers between them. These barriers can be physical. For example, mountains or if they are similar in type. A plant can not breed with a human, there are too many genetic differences. If their offspring can interbreed or have offspring of their own they can form a new species. Horses and donkeys are close in genetic type but their offspring are mules and they are infertile and can not have offspring of their own.

Answer 2

As noted above, a new species may emerge from a hybridisation between two closely related existing species. This is not seen very often in the more complex animals, but happens frequently in plants and may occasionally occur in insects or even birds.