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Re: Re: Transitional Fossils
>No, no. That's not what I'm arguing. Of course species evolve into other
>species. But _in cladograms_, the taxa being classified are _always_ at the
>leaf nodes. The transitional species, which reside precisely on the branches
>of the cladogram--not at the leaf nodes--are never explicitly classified:
>they are hypothetical. Cladistic analysis cannot discover them. In this
>sense, all cladograms are "wrong." What should strike us as odd, therefore,
>is that, even when faced with cladograms and their analyses, you still
>correctly believe that transitional forms existed (though you are not
>necessarily correct in asserting that ALL fossils are transitional).
This is a common misunderstanding about ancestors & cladistics. Yes, all
taxa show up on the same line of the cladogram, but this is just a matter of
iconography. Ancestors can indeed be recognized in cladistics, thusly:
An ancestral taxon will always be the sister taxon to its descendants.
An ancestral taxon will share some synapomorphies of
the descendant clade, but not necessarily all of them.
An ancestral taxon will probably not have any autapomorphies of its own
(matter of debate).
An ancestral taxon will appear stratigraphically earlier than its
descendants, and (perferably) within the geographic range of the descendants.
For example, Ardipithecus ramidus shares the basal synapmorphies of the
hominids and of hominines (i.e., it shares derived features with Homo,
Paranthropus, and Australopithecus, but not Pan and Gorilla). It occurs
earlier than the other hominines. It is a very, very strong candidate for
an ancestor to all later members of the human line.
Another example used to be Archaeopteryx (okay, Dinogeorge might not agree
with this, but bear with me). It shares derived features with later birds
which are lacking in nonavian theropods. It occurs earlier than all other
birds known prior to 1994, and so was a good candidate for an ancestral
position within the bird line. However, if Confuciusornis lived at the same
time as it, and shares more derived characters with modern birds, than the
known specimens of Archie are NOT members of the population which gave rise
to modern birds. Archaeopteryx lithographica may represent the ancestor of the
Confusciusornis+later birds morphologically, and may indeed be unchanged
leftovers of the ancesral population, but the stratigraphic relationships
would show that the German protobirds were not directly ancestral to birds.
In general, since the weight of modern biology shows that speciation occurs
primarily through isolation of small demes from the ancestral population,
and since the chances of any individual organism being preserved as a fossil
are very slim, it should be expected that finding a member of the ancestral
population to any later lineage will be slim. Nevertheless, as in the case
of Ardipithecus, it does seem to happen sometimes.
Thomas R. Holtz, Jr.
Dept. of Geology
University of Maryland
College Park, MD 20742