Two organisms are of the same species if they can interbreed and produce a
breedable result. Compatible pollen, basically.

A cultivar is a breed of plant. A breed is a finer distinction than a
species, and it implies an intentional effort on the part of the grower
to elicit certain traits.

-- spud_demon -at- thundermaker.net
The above may not (yet) represent the opinions of my employer.