(I'm going to warn you ahead of time that this doesn't have a cut and dry answer, and I'm touching on very few of the big complex arguments and am doing none of them justice. So please excuse me as I butcher these arguments and examples. Feel free to ask any questions. I will try to clarify further.)
(Also, I'm going to do a lot of linking to Wikipedia. Bear with me please, none of the links are really necessary outside of providing easy routes for further research on your behalf.)
Species: A Definition
"The major subdivision of genus or subgenus, regarded as the basic category of biological classification, composed of related individuals that resemble one another, are able to breed amongst themselves, but are not able to breed with members of another species."
When Species Mingle
|The Beautifully Illustrated Mountain of Mice|