Two main theoretical kinds of species concept have been suggested:

1. Species exist because of interbreeding: the biological and recognition species concepts are theories of this kind and can be called reproductive species concepts.

2. Species exist because selection maintains variation in the forms we recognize as species: this is called the ecological species concept.

It has also been suggested that species exist because of genetic constraints on variability: living things exist in discrete clusters because the mutations that would produce intermediates cannot arise. However, no one has seriously developed this theory, and it is only mentioned here for completeness.