Sheldon & gradually evolving trilobites
- Some comments on Sheldon's article, now that I've read it. This was the
article claiming gradual evolution occurred in certain trilobite lineages,
in opposition to PE. My problem with the paper was the claim of gradual
elaboration, not the claim of gradualism in general, but this is not an issue
for Sheldon, who is evidently comfortable with evolutionary reversion:
trilobites adding segments for a while, then reducing the number for a
while, whatever. In my own thinking, evolutionary reduction in number of
skeletal segments is a one-way trip.
The article (Nature 330: #6148) is the cover story, and comes with an
overview piece by John Maynard Smith discussing the significance of
Sheldon's more statistical presentation. Smith begins with a confusing
discussion of randomness, punctuationism, adaptation, and species
selection, then segues into a defense of Sheldon's interpretations, giving
the reader a clue that this is shaky ground.
In the Sheldon article I learn that the end-members of his claimed evolutionary
lineages had been previously assigned to different species and even genera,
but that he has overridden these determinations. He chafes at the application
of binomial taxonomy to fossils and the requirement that specimens "from
different horizons" be amassed in order to make proper species descriptions.
Basically he seems to believe one should have a free hand in constructing
phylogenies. He sees intermediate morphology and plugs it into his series
in the appropriate place, regardless of trends. He allows for "trend reversals"
which to me rather weakens the concept of the trend. For me the trend is solid.
Parts are lost, not gained. Apparent elaborations in number of skeletal parts
are erroneous interpretations.
He refers to partially developed ribs (counted as 0.5 ribs) as "incipient",
rather than 'vestigial', clearly favoring an elaborative trend in which a
partially developed element is on its way to fully developed status.
"No genus, locally at least, appears to have been represented by more than
one species at any particular time." This is odd, but highly convenient if the
object is to show changes in a species rather than variants within a genus.
Anyway, the paper shows how poor the evidence is for gradual elaboration
in the number of skeletal segments. But again, Sheldon is merely concerned
with gradualism vs. PE here, and does not address my issues head-on.