Scientific American published a tongue-in-cheek piece advocating a reclassification of dog breeds into different species of dog. The author has a point: If a species is defined as a reproductively isolated population, then surely some breeds are reproductively isolated from others (the mastiff and the Chihuahua are mentioned).
Are this Great Dane and Chihuahua mix members of different species?
Following this line of reasoning, dogs would be most accurately described as a "ring species," in which there is a continuum of gradually varying — and potentially interbreeding — forms with two "ends" incapable of interbreeding. The mastiff and the Chihuahua are at the ends. But a German shepherd and a Labrador retriever, on the other hand, could certainly populate the animal shelters with hybridized mutts. And surely that Chihuahua could have some success, so to speak, with a Yorkshire terrier.
Some dog breeds are not capable of reproducing at all, at least not without technological intervention. French bulldog females usually must be artificially inseminated because males cannot mount effectively, and the puppies often must be delivered by Caesarian section. I am not sure how such creatures would fit into the classical species definition. It was not designed for populations that can't reproduce at all!
The impetus behind the proposed dog reclassification was to demonstrate that, in fact, speciation has been observed. Biblical literalist creationists often claim that science has never observed the splitting of one species into two different species.
Alas, if you know Creationists, you know this would not work. First, speciation has been observed already, and Creationists have no problem denying it. (See the Talk Origins information on observed speciation, an Internet classic.) Second, when a Creationist talks of a "species," he does not mean a reproductively isolated population. He means a "kind," sometimes called a "baramin," a concept exclusive to literal Creationism (i.e. not found in science). The Creationist would argue that the various canine breeds, along with wolves and wild dogs, comprise the dog "kind," and that while there might be "microevolution" within the kind, no dog would ever evolve into a new"kind." (Presumably, divine intervention would prevent "microevolution" from going too far.)
The proposal to call dog breeds different species was not made seriously. But it's good to think about the species concept once in a while.
Image credit: Ellen Levy Finch, licensed under the GFDL.
Dog breeds as different species, and observing evolution