[Vaphell]

> my understanding of what classifies something as being a part of the same species is the fact that they can make children that are viable to have children themselves

things are a bit more complicated than that, because having fertile offspring is not a transitive property. Ring species: population A can mate with B, B with C, C with D, and D with E, but A and E cannot mate, even though they are part of the same continuous chain.

Ensatina eschscholtzii salamanders in California exhibit this non-transitive behavior. Populations at the ends of the coastal ranges can interbreed with their neighboring populations, but where they meet in the south, the "ends" of the ring do not interbreed.