Seeing as how in the wild, they can't naturally reproduce because of the differences in their size, can they be considered to be different species of dogs?

That's a very interesting question Joon.  I have heard  of different sized dogs, larger and much smaller dogs, having puppies, but never a Chihuahuas and Great Danes.  I suppose that it is going to be very unlikely that the two breeds would ever reproduce naturally together, or have puppies through artificial methods.  However, I have heard tales of brave (or stupid) male Chihuahuas trying their hardest to mate with a Rottweilers, Alsatians, and similar sized dogs.  So, although it is not likely that the two breeds would manage to reproduce, sometimes they do try.  Over time it is plausible to imagine that these breeds may become two species.

I think the key might be in your question here. You may be right that the size is a barrier to mating, but we can't really say what would happen "in the wild" since these are domestic animals.

The species concept is a bit tricky in many ways (e.g., there are quite a few things we call "species" that do naturally interbreed), but is also aimed at classifying natural biodiversity rather than domestic animals. Hypothestically, if all domestic dogs were suddenly released and became wild (or feral I guess) then I'd expect the very big differences between artificially selected breeds to get less with time (i.e. after multiple generations) as all the breeds began to cross. Sure a pure-bred Chihuahua and a great dane may never mate, but maybe both could reproduce with basset hound so the genes would still get mixed up in the wider "mongrel" dog gene pool.

... and this example highlight a big problem with the "biological species concept" that, oddly, I've not seen discussed before.  If you use the BSC, then two populations that can interbreed are considered to belong to the same species. but two species that can't interbreed are considered to be two separate species.  Now consider three populations: one each of Chihuahuas, one of Great Danes and one of Basset Hounds.  C and BH can interbreed, so they are the same species; GD and BH can interbreed so they are the same species; but C and GD can not interbreed to they are different species!  That makes no sense; and to me, that means that the biological species concept is fundamentally flawed.

Taxonomy and the concept of discrete separate species is very much a human construct and often in the real world the lines between different species isn't always very clear.  Many more factors are used to determine the species of organism than just their capacity to breed with one another.   Morphology, geographical location, genome makeup, fertility of offspring and so forth are all taken into account.  For instance, the wild tomatoes are regarded as a separate taxonomic species to the cultivated tomato (Solanum lycopersicum), even though their genomes are very similar and they are quite capable of being bred together, they are morphologically and geographically quite distinct (a brief summary of wild tomato species can be found at …  In the case of dogs the accepted wisdom is that Chihuahuas and Great Danes are merely different breeds of the same species.  While they may not be able to physically mate due to size differences a female Chihuahua could certainly be artificially inseminated with Great Dane.

Last edited by Daniel Buchan (23rd Apr 2007 15:47:36)

I agree with Daniel that "species" is a very human construct. Also to follow up Mike's post, the Biological Species Concept is not only flawed but its also just one of several flawed attempts to come up with a rule for what should/shouldn't be called a species.

Although not necessarily the source of all reliable knowledge, I think the wikipedia entry for "species" is quite good on all this stuff.