[Ichneumon]

[How about "kinds" where variety A can interbreed with B, and B can interbreed with C, but A cannot interbreed with C?]

Can you give me an example? I'm learning while I lurk!

Here's one of the more striking examples, although there are many more mundane cases as well:

Technical papers:

Ring species as bridges between microevolution and speciation (the Greenish Warbler material starts about halfway in)

SONG VARIATION IN AN AVIAN RING SPECIES

[shuckmaster]

Great example! Thank you for the info.