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Re: Hominid fossils FAQ file
Kathleen Hunt (jespah@u.washington.edu)
5 Jan 1995 02:01:02 GMT
In article <3e7tu5$qb8@charm.magnus.acs.ohio-state.edu>,
Brian D Harper <bharper@magnus.acs.ohio-state.edu> wrote:
>>[I discuss Gingerich, 1980]
> [then Brian quotes a paper that says:]
> But a
> detailed examination of the _entire mammal fauna_ (monographed by Bown,
> 1979, and Gingerich, 1989) shows that most of the rest of the species
> do not change gradually through time. Also, studies on specific
> lineages in restricted areas cannot account for the possibility that
> a gradual transition may actually reflect the migration of a clinally
> varying population across a region through time.
Well, first, yes, I fully agree that not all the species vary gradually.
I never said (and didn't mean to imply) that *all* species in the fossil
record vary gradually; I only said that *some* do. I believe Gingerich's
breakdown of the species he studied was: of 38 species that had a good
enough fossil record for their origins to be studied, 24 apparently arose
through gradual evolution and 14 appeared suddenly. That's not "most"
species appearing suddenly, but I haven't read the more recent papers
cited above, so maybe more cases of sudden-appearance have now turned up.
Anyway, he attributes those 14 to immigration from a different site where
rapid evolution occurred (punctuated equilibrium). I've seen other
similar comparative studies that also tend to come up with half or so of
the species arising through gradual evolution, and half appearing
suddenly. Another example: Macfadden studied the origins of 16 hipparion
equids and found 6 appeared suddenly, 5 appeared through anagenetic
transformation of an ancestor, and 5 appeared through cladogenesis
(splitting off of a lineage from a parent lineage that is itself not
changed). In contrast, the bovids (cows, antelopes, etc.) of Africa have
apparently arisen more often from punc-eq than from gradual evolution, as
one study (I forget the author...sorry...) found numerous cases of sudden
appearance and only one case of gradual change. Anyway, all these studies
always seem to find at least some cases of gradual change, mixed in with
various cases of sudden appearance.
Also note that often the "sudden appearances" quoted in the literature
are *not* recorded by a good fossil record, but are simply due to a total
lack of fossils from the time in question. Calling a lack of fossils a
"sudden appearance" is pretty questionable, IMO. For example the three
modern elephant genera are often said to have appeared suddenly, but in
fact there is a 500,000 year gap there with no elephant remains.
The point, again, is not that *all* species arise gradually, just that
*some* have been observed to do so. The punctuated-equilibrium literature
is very concerned with which mode occurs more frequently, punctuated
equilibrium or gradualiam. So punc-eq papers make a big point out of any
perceived *relative* scarcity of gradual-evolution cases (relative to
#cases of sudden appearance, that is). This is somehow often misread by
creationists as being evidence for an *absolute lack* of gradual
evolution. I think the balance of evidence shows now that both punc-eq
and gradual evolution occur. Though there are likely to be continued
skirmishes about which one is the dominant mode, it's clear that gradual
evolution occurs AT LEAST SOMETIMES. That's all. That was my point:
There ARE solid cases of gradual evolution occurring at least sometimes.
As for the migration of clinally varying populations...well...I can see
that being a possible explanation for Gingerich's earlier work on
_Pelycodus_ and other simple lineages that changed in a single direction,
anagenetically, over time. For instance, one lineage getting larger and
larger. But I don't think that can explain the much more complex
lineages that were worked out later on, such as the multiply-branching
_Plesiadapid_ lineage; you'd have to have some very strange populations
marching through that were getting both larger and smaller
simultaneously, with some individuals staying the same. Likewise, some of
the other papers I mentioned are hard to explain with a migration model.
Finally, I wouldn't necessarily expect the speciations that one worker
finds in one site to be found in another site, even a nearby site. Isn't
that one of the working models for punctuated equilibrium -- that
speciations may occur in small populations of limited extent? For
instance, in Krishtalka & Stucky's artiodactyl study, they found
Artiodactyls A & B only in one certain area -- presumably the area where
those speciations took place.
That said, though, it's certainly interesting to find out more about other
sites and there may indeed be some discrepancies that I don't know about
in Gingerich's work. He did do most of his work on molar teeth only, so
any studies that involve other characters will undoubtedly refine and
maybe change his conclusions. I don't expect we can expect much
multiple-site work from a single person, though, because of the whopping
amount of work involved in studying just one site closely. I understand
it took Gingerich about ten years and over 1500 specimens just to result
in the data presented in the 1980 paper. (I was just reading about
studies of early mammals in Texas, where the research involved thorough
processing of every grain and every fish tooth of twenty *tons* of
sediment to find just eleven mammal teeth...ouch...) I like punc-eq myself
and think it's a good, workable model for many and perhaps most cases of
speciation, but I also think Gingerich's work (and the many other papers
on gradual evolution that are out now) show pretty clearly that the fossil
record *has* recorded many instances of clear gradual evolution, with
abundant transitional forms.
Kathleen
--
If we increase the size of the penguin until it is the same height as
the man and then compare the relative brain size, we now find that the
penguin's brain is still smaller. But, and this is the point, it is
larger than it *was*. (Monty Python)
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