Canis familiaris can be broken down into essentially two main groups: One of pure breeds and mixed breeds—about 400 types—that make up most American pets and another, much larger group of free-roaming and breeding populations known simply as "village dogs." Despite the wide variation in outward physical appearance, pure breeds are not genetically very diverse, because they come from such small, controlled gene pools. (Remember, as crazy as it seems, a Chihuahua and a St. Bernard are still the same species.) Most modern breeds, in fact, are only about 200 years old (though artificial selection by humans on dogs has been occurring for thousands of years). The majority of dogs, along with their genetic diversity, fall in the group of "village dogs," and have much older lineages than the pure breeds. It makes them particularly important when studying dog evolution. Adam Boyko, of Cornell University, and his colleagues factored this group of dogs heavily into their study.