Species isn't the final word on differences between living things. Variation exists among different populations of the same species. Subspecies, race, and breed are used to describe the different types of variation:

  • Subspecies: A group within a particular species that shares genetic characteristics with other group members but that it doesn't share with members of the larger species. Subspecies may interbreed quite freely or may be partially reproductively isolated — that is, they can interbreed but don't do it as well, or produce offspring as viable, as when they mate within their own subspecies group. Subspecies can range from ever-so-slightly-different groups within a species to groups that are on the verge of speciation. For example, the cobra and the pine snake are completely different species, but pine snakes are divided into subspecies, such as the black pine snake, Florida pine snake, and Louisiana pine snake.
  • Race: Used most often to describe variation within the human species. Human races are differentiated primarily by skin color, but even though the genes responsible for skin color are noticeable, the actual genetic differences among races are slight. In fact, skin color doesn't accurately reflect the genetic differences among humans. Two people of African descent could easily be more genetically different from each other than a person of European descent may be from a person of Asian descent. Bottom line: Races have slight differences, and these differences are nowhere near the level they'd have to be to decrease gene exchange.
  • Breed: Domestic animals (such as dogs and cows) whose characteristics are artificially selected and maintained by humans through animal husbandry are divided into breeds. The goal of selective breeding is to create animals that differ from their wild counterparts and possess relatively predictable traits. Take dogs, for example. Humans have been breeding dogs for only a relatively short period, and over that time, starting with wolves, we've managed to produce everything from Chihuahuas to Great Danes. All breeds of dogs are the same species. They can all interbreed, although admittedly, interbreeding is easier for some pairs than for others.
    Note: Standard convention gives species names to products of animal husbandry. That doesn't mean, however, that dogs and wolves are different species (in fact they share the same species name: canis domesticus). Dogs and wolves can still interbreed, even though a "happily ever after" probably wouldn't be in the cards for the Big Bad Wolf and your Pomeranian.