[BMCDA]

Exactly my point. And therefore 481.28nm or 491.1nm are just as good as 490.5nm to represent the border between green and blue. And the same applies to two successive species. But because we do these kind of categorizations people expect that it is done because there is a sudden and visible change (leap) but there is no such thing. (IMO it would be less confusing if we deal with probabilities in such cases)

So given time we should expect a chimp to give birth to a human or vice versa?

Just what I was trying to say.
(Though I'm convinced you don't hold that opinion)

[AndrewC]

Exactly my point. And therefore 481.28nm or 491.1nm are just as good as 490.5nm to represent the border between green and blue. And the same applies to two successive species.

Good, then we agree. But the usefulness of that division might be as useful as the frequency division between ultraviolet and infrared.