A Saint Bernard and a Chihuahua are of the same species but look very different. It is possible to breed the two and get a puppy that might look like something else entirely but still be a dog. Why might dogs be of interest to an anthropologist?

Because of the breeds

Explanation:

Like in humans, the dogs belong to the same scientific group, but through the passing of time, there has been some changes in their DNA, slightly differences that make the size, the hair, and another traits to be different.

In humans all the "breeds" suffer the same changes, that is the reason that spite we are homo sapiens, there are different groups of humans as the oriental, Caucasian, etc.