Dachshunds, Saint Bernards, Greyhounds,  the world of domesticated dog breeds is clearly a world of great variety.

If you didn't know that a hairless Chihuahua and an Irish wolf hound were both descended from the same basic species, you might think you were dealing with apples and oranges. Domesticated cats, however, tend to show much less variety. A Siamese and a tabby are both shaped like, well, cats. Why can't house cats be bred into giant or miniature varieties the same way dogs are?

The answer is in their genes. Most wild animal species have what scientists call buffering systems in their genes. These buffering systems can prevent the odd, unusual gene from showing up in the animal's appearance. For example, an individual of a wild dog species, like a wolf, might have the gene that gives a dachshund its short legs, but the wolf's genetic buffering system would keep this gene in check. The net result of such buffering systems is to keep individuals of a wild species more or less the same.

Over the years, dog breeders have managed to breed this buffering system out of domestic dogs. This opens the door to all kinds of unusual genes, and selective breeding can reinforce dramatically different traits.

However, domestic cats have a stronger buffering system, and it's still in place. This is why different cat breeds tend to be more or less the same size and shape.

By the way, humans also have a genetic buffering system. That's why, whatever our race, we are a lot more similar than we are different.