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Re: Evolutionary taxonomy



At 09:31 AM 3/12/97 -0600, Rob Meyerson wrote:
>The current brushing of cladistic techniques gives me an opportunity to
pose a question I have had for a while now.  In a hypothetical lineage,
Species A evolves into Species B, which subsequently splits into Species C
and D.  A cladigram might look something like this:
>
>          A              B      C     D
>           \              \       \   /
>             \              \       V
>               \              \   /
>                 \              V
>                   \           /
>                     \       /
>                       \   /
>                         V
>
>How would one be able to go from this grouping of species, into a sense of
the evolutionary history of the group?   Could one reconstruct the lineage
of the group from this diagram?

You have asked an excellent questions, and phrased it perfectly!  The events
we are interested have occurred with no human witnesses: they must be
inferred and reconstructed from available evidence.

The direct evidence we have is the distribution of character traits among
the known taxa.  From this we can reconstruct the most parsimonious
distribution of derived character states.  For example, we could recognize
that Ardipithecus ramidus, Australopithecus anamensis, Homo habilis, and
Homo ergaster have a certain distribution of characters, uniting the latter
three together to the exclusion of Ardipithecus, and uniting the two Homo
species together to the exclusion of everything else.  We represent this
distribution in the form of a cladogram.

To transform a cladogram to a "phylogeny" (some perfer the term
"phylogram"), we can map the position of these taxa on time.  Furthermore,
using criteria which I have to repost to the net every six months or so
(check the archives), we can recognize POTENTIAL ancestors.  We cannot
resolve whether any given fossil is a member of the population directly
ancestral to the descendant group (although we can find evidence to reject
such a position).  If, in this case, we find no data to reject A. ramidus as
ancestral to all later hominines, and A. anamensis as ancestral to all later
forms, and H. habilis as ancestral to H. ergaster (as time and geography and
character states suggest), then we can reconstruct a phylogram showing this
relationship.

However, phylograms are a step or two removed from the primary phylogenetic
data, and thus are that much more questionable than cladograms.

A step further from a phylogram is a "scenario": an hypothesis suggesting
the selective forces or other causes which led to a particular sequence of
ancestor-descendant relationships.  This is even further removed from the
primary data, although this sort of information is often of more interest to
the general public.

(Note that this sequence is in constrast to the methodology employed by
certain paleornithologists, who assume you must develop a scenario first,
then find the position of taxa in time, and then try and find characters
which support such a relationship).

>Believe me, I am not trying to reflame the Clade Wars.

(There's no need to.  The wars have been over for over a decade.  Sure,
there are holdouts here and there, but there were also Japanese soldiers on
isolated islands fighting WWII long after 1945...)

>I simply wish to understand the dynamics of how Cladistics work.

A wise decision.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661