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Re: Defending grades (Was: Re: Archaeopteryx (rant))
Torfinn Ørmen <email@example.com> wrote:
The reason we still use the Linnean system is that it is a very good tool
for information storage.
Agreed. However, ranks such as Phylum (or Division, for the botanically
minded) and Class are misleading in evolutionary discussion. Sure, they can
be used as markers or signposts. But too much evolutionary debate has been
bogged down in sterile arguments about whether this-or-that group should be
a phylum or a class or an order.
Even worse are the useless debates over the placement of those taxa close to
the base of these "Classes". This is a perennial issue with "Class Aves".
Who's in and who's out? Do _Rahonavis_ and _Caudipteryx_ and _Microraptor_
get promoted to the Aves (*sound of trumpets*) - after all, they have
feathers! Or are one or more of these feathered taxa forced to languish in
Dealing with extant taxa (especially higher levels) analyses are now often
almost exlusively based on DNA because it is more reliable regarding
(Excuse me while I dust the dust of my clothes after falling off my chair.)
As a microbiologist who works with DNA almost very day I can tell you that
in response to the assertion that "Analyses based on DNA are more reliable
regarding ancestry" I would say "I WISH!".
Unfortunately, DNA analyses are not the universal panacea to resolving
phylogenies. Even mitochondrial analyses generate very different
phylogenies based on whether you use (a) whole mitochondrial genome
sequences; (b) all mitochondrial genes; (c) all nucleotides of
protein-coding genes; (d) all nucleotides of protein-coding genes with the
third base of each codon removed; (e) amino acids of protein-coding genes;
(f) transfer tRNA sequences. I only mention this because mitochondrial
sequences are often regarded as the gold standard in eukaryote phylogenies.
This is not a personal criticism of Torfinn Ørmen - I'm just trying to draw
attention to the fact that phylogenies based on DNA (or other molecular
data) have their own diffuculties. Again, I wish DNA analyses were the
answer to every evolutionary question regarding extant taxa. (Dear God, I
Working with extant anatomy you would e.g. never get the whales dumped in
the middle of Artiodactyla.
In this case, the fossil evidence indicates that Cetacea probably arose from
basal Artiodactyla. (Check out taxa such as _Artiocetus_, _Rodhocetus_ and
_Ambulocetus_). DNA will never replace paleontology.
BTW, this is another example where the anatomy of a group of animals have
changed so much that it is defensible to classify it outside the parent
taxon instead of squeezing it inside, e.g. as a sistertaxon to
First of all, as I said above, I think the molecular phylogenies that regard
cetaceans as the sister taxon to hippos are probably not quite correct. In
other words, I am not a fan of Whippomorpha (I like the name, but not the
Second of all, I don't see the problem of including the Cetacea as a clade
*withinin* the Artiodactyla. Similarly, I don't see the fuss in putting
Aves inside the Theropoda. After all, that's where evolution put them. Who
am I to argue?
Daniel Grossberg wrote:
>Classification is intended to ease our understanding of a species'
>One can note that aves evolved from reptiles, while still maintaining
>a separate class.
(Reaching for my crate of tomatoes...)
Why bother putting birds in a separate class? To do so actually *impedes*
an objective understanding of a species' origins. IMHO.
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