A simple way to define a 'species' is any group of creatures that can produce viable, fertile offspring. So dogs are breeds - they're part of the same species

A simple way to define a 'species' is any group of creatures that can produce viable, fertile offspring. So dogs are breeds - they're part of the same species - because if you mate a great dane with a chihuahua, you'll get a fertile dog (I believe; if you plan to experiment, I recommend the chihuahua is male and great dane is female). If you're lucky, that dog will have desirable characteristics that may one day end up being the parent of a new breed. But horses and donkeys are not breeds - they're different species - because if you mate them you get a mule which is not fertile (a hybrid, therefore). In the end, this definition is a bit too simplistic because it's not necessarily always so clear-cut. Hence the article here :)