The theory of evolution by natural selection was Darwin's (and Wallace's) explanation for biodiversity (that is, Why are there so many species?).

Just how can evolutionary theory explain biodiversity? If a species evolves into a new species, doesn't that mean the old species disappears? If people evolved from apes, then why are there still apes around?

The type of speciation in this example is called allopatric (meaning "different place") speciation. In our illustration, evolution is caused by local adaptation through natural selection. However, allopatric speciation can have other causes, including genetic drift and non-random mating. Sympatric ("same place") speciation is also possible and, depending on its cause, can replace the existing species or create additional species.

Kocher TD. 2004. Adaptive evolution and explosive speciation: the cichlid fish model. Nature Reviews Genetics 5: 288-298.

Gould, SJ. 1977. Bushes and Ladders in Human Evolution. Ch. 6 in Ever since Darwin: Reflections in Natural History. Norton. New York.