[vdaniuk]

Statistically, it is very likely that at any given point in time there will be only one species with the trait that humans consider significant.

[boldfield]

I'm about to commit an HN faux pax here, but, citation? Off the top of my head I can think of traits which do not fit your assertion: vision in the 390-700nm wavelengths, hair/fur, eye placement (think predator v. prey), etc, etc.

[dragonwriter]

I think GP used significant in a sense more like "newsworthy", in that, ceteris paribus, the perceived significance of a trait is inversely related to the frequency with which it has been seen in the past.

[vdaniuk]

Yes, thanks, that's exactly what I meant.