[plain talk]

OK. Tell me about ring species. Maybe that will convince me.

[JeffAtlanta]

Ring Species

Ring species show the process of speciation in action. In ring species, the species is distributed more or less in a line, such as around the base of a mountain range. Each population is able to breed with its neighboring population, but the populations at the two ends are not able to interbreed. (In a true ring species, those two end populations are adjacent to each other, completing the ring.)

link

[From many - one.]

Here's one link.

http://www.actionbioscience.org/evolution/irwin.html

googling on "ring species" nets many more articles.

Since wolves can breed with some dogs speciation is not complete. That's why I asked you about fertility rates.
Lions and tigers, horses and zebras also have some interbreeding potential.

The definition of a species is a population that, in nature, does not breed with another.

[JeffAtlanta]

Here is an even better explanation (complete with graphics) of ring species. Wikipedia link