Ring species are a weird and fascinating corner case of that definition: https://en.wikipedia.org/wiki/Ring_species

Before the invention of radio this was a property of language as well. Famously you could walk from Paris to Rome and every village could speak comfortably and naturally with its neighbors, yet by the time you got to Rome they spoke a language essentially incomprehensible to Parisians.

30 yrs ago, I rode my bike from East Berlin to Amsterdam, hopping from tiny village to tiny village. I kept out of cities so I could sleep in the forest at night and it seemed as if the the local dialect drifted town by town from deutsch to dutch

And heading North: https://en.wikipedia.org/wiki/Frisian_languages

That’s a good example! I remember being on a train where a woman was reading a book in Dutch to her kids and my then 5 year old son (a German speaker) walked over to sit behind her and listen.

If I'm reading right, the two groups can only breed with each other. That is, white-striped males and tan-striped females can breed, as can white-striped females and tan-striped males. (And the offspring are about half white-striped and half tan-striped.)

So it's not speciation, any more than sexes themselves are speciation.

> white-striped males and tan-striped females can breed, as can white-striped females and tan-striped males

This alone doesn't contradict the two-species theory. It could be that white-M + tan-F is species #1, and white-F + tan-M is species #2. Sure, it sounds weird, but not nearly as weird as the idea of 4 sexes, IMO.

> (And the offspring are about half white-striped and half tan-striped.)

Now, this is the important part. As I understand it, the offspring of "species #1" could be "species #2" or the other way round. This indicates that #1 and #2 are the same species after all.

They do breed with each other: a-males mate with b-females, and vice versa, where a/b is the new sex gene.

Just wait until you learn about the sex life of hexaploid species... an offspring needs parents of at least 3 different sexes.

a good book which discusses species with over a dozen sexes read Matt Ridley The Red Queen

What species ist that?

If the tan ones could only mate with the tan ones, that would work. As it is, though, no. A white male and tan female can have a tan chick that can go on to mate with a white bird (indeed must, if it mates at all). That’s not speciation.

I thought so at first, but no... The two different "species" only mate with each other.