[bluebarbet]

Thought experiment. Three populations, A, B, and C, divided geographically along a line. Individuals from group A can breed successfully with those from B but not with those from C. Individuals from group C can breed successfully with those from B but not A.

How many species are there? This is why the term "species" can never be entirely objective. I remember the eureka moment I had when I finally understood this (admittedly somewhat simple) point.

[naasking]

It can even be more subtle, it's entirely possible that some rare members of A can C breed, and some members of C and B would not be able to breed. The "fertility" relation can only be decided between two individuals, not groups. Group-level fertility is a statistical average of individual fertility.

That said, I don't think that means that "species" is entirely subjective or meaningless.

[afpx]

Even within one species, not every individual can successfully breed with every other.

[lukan]

Is that based on a real example or hypothetical?

[usrnm]

https://en.wikipedia.org/wiki/Ring_species