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Re: Some thoughts on AVES (& gaps)
Sorry Demetrios, but Nick is right. We can never really hope to
discover "the" fossil species that gave rise to any particular clade. We'll
be lucky to even narrow it down to where we can identify a particular sister
group at genus level. The gaps are there (and always will be, due to
extinction and a sparse fossil record), so we might as well get some
advantage out of that "lack of information".
What makes the mammal transition so useful is that it is NOT a single
(population or species level) change. It is a series of character
transformations that followed in relatively rapid succession (at least fast
in geological time). This gap is huge compared to the little gaps which we
neotologists deal with at species level.
Punctuated equilibrium is not always marked by just a single isolated
"spike" of change. A quick series of such "punctuation" events can
sometimes give us a sizeable gap that will probably not even be partially
bridged for many decades (if ever). That's why the character-based
definition of Mammalia is still so widely used. It's not really just a
single character, but a series of character transformations which happened
in a geologically short amount of time.
There might not be anything quite that clear-cut for an apomorphy
definition of Aves, but I think we should look for the best one we can find,
and hopefully it will be less arbitrary than just anchoring it on
Archaeopteryx (which was a good tradition, but one whose usefulness is
NJPharris first wrote:
> Fortunately, I don't have to. We only have to classify the specimens we
actually discover. For this reason, abstract apomorphy-based definitions
like the one I gave do not bother me overly much.
Again, not only are you relying on a lack of knowledge (which we all labor
to fill), but are setting up a system which inherently has limitations.
One of these limitations is the fact that you can only work with fossil
taxa. Yeah, the biases in the fossil record are great for this type of
character-based classification system, but it doesn't work for extant
organisms. Evolution is still happening, and we'll need to deal with
speciation in the future. But contemporaneous speciation won't happen with
those gaps that are inseparable from the fossil record; the
overnight-character-acquisition system cannot apply today.
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