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Comment by dschiptsov | Hacker News Reader

The classic operational definition of different species, as far as I remember, is that cross-breading between species is not possible.

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If only it was! But then we discovered "ring species" (https://en.wikipedia.org/wiki/Ring_species) which threw a monkey wrench into what was once a simple definition. Basically, there are populations of animals A, B and C, where A and B can inter-breed, B and C can inter-breed, but A and C can't. So our definition of species no longer satisfies the transitive property, and it all falls apart. As far as I know, this issue hasn't yet been satisfactorily answered in the case of ring species.

A species is just a human construct to make it easier to understand biological diversity. In the real world living things are never black and white.

...except Zebras?

Even zebras are not black and white - more a dark brown and cream ;)

Depends on the camera.

We discovered setups where A, B, and C exist at the same time in different places. That's interesting, but it's not a new challenge. The very simplest version of evolution also involves A, B, and C, existing at different times in the same place. The difference with a ring species is that you can't dodge the complexity by ignoring extinct things. But the complexity was always there.

> then we discovered "ring species" (https://en.wikipedia.org/wiki/Ring_species) which threw a monkey wrench into what was once a simple definition

Well, ok, we documented some ring species. That had no effect on the biological species concept, though. It never satisfied the transitive property.

Two individuals may be capable of breeding and reliably producing fertile offspring but still be considered different species if they would never mate "naturally". This may be due to geography, different mating seasons, different songs or appearances, etc.

So, are there different species of people?)

Not if you consider alcohol.

It's a very blurred line.

It's really just a social construct.

My understanding was that they might be able to crossbreed, but offspring are not fertile, e.g. hybrids such as mules and ligers.

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