By identifying specific traits — such as size, coat color, and demeanor — and allowing only those animals to mate, we've created at least 167 different "breeds," or groups of dogs with unique physical and mental characteristics. Still, they're all part of the same species.
The Science of Dogs blog put together a side-by-side comparison of several popular breeds from the 1915 book " Dogs of All Nations" by Walter Esplin Mason, showing what they look like today.