Hybrids and genera

Andrew Brower browera at science.oregonstate.edu
Thu Nov 7 19:24:44 EST 2002

I think Jeff and I basically agree, but I want to clarify (or at least comment some more)
on a couple of things:

> Also, taxonomy could never fully "reflect phylogeny," ...
> First of all, everyone agrees that the 'true phylogeny' of life is unknowable
> (until that time machine gets working).  However, if we can make reasonable
> estimates of that phylogeny, why should we not include that information in
> taxonomy?  Most folks, scientist or not, use taxonomy as a proxy for relatedness.
>  Think back to grade school - would you have ever thought birds and dinosaurs
> were closely related?  No, because the taxonomy didn't reflect the relatedness.
>  With very strong support for the bird+crocodilian clade, taxonomy should
> reflect this, so that folks that are only exposed to High School Biology
> texts, and not PNAS, could glean this knowledge.

What is "relatedness"?  I 'm pretty sure I'm related to my parents, but am I related to a
chimpanzee, or a dog, or an oak tree?  Once we get past the interbreeding criterion that
started this whole discussion, the idea of "relatednesss" becomes an inference based on
similarities and differences in morphology, DNA, etc.  Birds and crocodiles are diapsids
(two holes in the skull, or something like that, which is a uniquely derived feature in
that clade).  The characters that put crocodiles with snakes and lizards are either
features that unite a more general group, such as tetrapods, or convergently derived
features that are outweighed by other character evidence.  Birds have a bunch of other
uniquely derived features, such as feathers, that make them seem very different, but do
not provide any evidence of grouping except to say that various birds go together.
Phenetics, cladistics, maximum likelihood and other character-based methods of inferring
"phylogenies" all use character x taxon matrices, not any information about evolution per
se, to make their trees.  That is why systematic data provides such a powerful source of
evidence in support of the theory of evolution.  Darwin used it that way - to him the
theory of evolution was a material, mechanistic explanation for empirical patterns
observed in nature.

> The abandonment of phylogenetic information in taxonomy was attempted by
> pheneticists decades ago, and as we all know, Cladistics came out on top
> of that battle.  Although the times may be a' changin', given the arguments
> between the current schools of thought (another VERY messy situation).  I'm
> not making accusations about motives (the last thing I'd do was call someone
> from the New York School at pheneticist), but I think we should keep in mind
> the past debates about evolution and taxonomy.

What is "phylogenetic information"?  A cladist and a pheneticist start with exactly the
same data matrix, and use alternative algorithms to group taxa.  The differences are in
the treatment of complementary character states (cladistics counts character state
transformations, phenetics counts the number of states shared).  Phenetics is internally
contradictory, that's all.  It has nothing to do with some extra knowledge that the
cladist has access to - the pheneticists were pretty smart guys and wouldn't have
handicapped themselves that way.

> Taxonomy (at least at the species level) is only naming a point on the river(s).
>  For most points on the river (or most points in time), there are two rivers,
> or there is one river.  Attempting to apply species names over long periods
> of time (in which we may encounter such a coalescence) gets us into trouble.

But if you're a paleontologist or somebody like Willi Hennig who wanted a logically
consistent basis for his taxonomic scheme, you have to deal with the time/internode
problem.  Not to mention the rassenkreis problem.


Andy B.


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