While they look vastly different, both breeds belong to the species *Canis lupus familiaris*, the domesticated dog. The differences in size and appearance are due to selective breeding by humans over centuries.
Here's a simplified explanation:
* Species: A group of organisms that can interbreed and produce fertile offspring.
* Breed: A group of animals within a species that share certain physical characteristics due to selective breeding.
Even though a Great Dane and a Chihuahua might seem like they belong to different planets, they are both dogs, and their differences are simply variations within the same species.