General Discussion

These taxonomic designators are actually wholly arbitrary - oh, they're fine for current use. But when we go backwards or forwards in evolution they start to fall apart.

Sticking with the chicken... if we follow a chicken's maternal ancestry ALL the way back, from one hen to the next, we're going to notice something - at no point in that chain of organisms, is the individual we're looking at particularly different from its immediate ancestor or immediate progeny.

That is, there is no cutoff point where "not-chicken' stops, and "chicken" begins. We will never find a point where the progeny is a chicken, but the parent isn't. We can leapfrog several generations in either direction and probably find a "not-chicken" on the maternal line, but again, we will find no discernible point where that whatever-it-is organism "stops' and the next in line "begins."

This is part of the reason creationists go on about "micro-evolution' as if it disproves speciation - they can't wrap their heads around ht notion that there is no clear speciation "cutoff point" - Every organism born is going to resemble its parents vastly more than it will be different from them. And so will it offspring, and so on down the line. so it's easy enough to look around the modern day, see a crocodile and a duck and say "they're totally different' - it's a little different when you go back to the middle of the Triassic and find out that crocodiles and ducks share a common archosaur ancestor - one that happens to not be a crocodile or a duck (and is itself not very distinguished from its ancestors or descendants...)