Darwin-L Message Log 7:66 (March 1994)

Academic Discussion on the History and Theory of the Historical Sciences

This is one message from the Archives of Darwin-L (1993–1997), a professional discussion group on the history and theory of the historical sciences.

Note: Additional publications on evolution and the historical sciences by the Darwin-L list owner are available on SSRN.

<7:66>From DARWIN@iris.uncg.edu  Fri Mar 18 23:32:37 1994

Date: Sat, 19 Mar 1994 00:32:33 -0500 (EST)
From: DARWIN@iris.uncg.edu
Subject: Hominid evolution and "species" (II)
To: darwin-l@ukanaix.cc.ukans.edu
Organization: University of NC at Greensboro

Ken Jacobs proposed that one of the sources of confusion in the debates
about recent hominid evolution was a tendency toward what I call "group
thinking."  Kent Holsinger took exception to Ken's interpretation, but I
want to suggest that both Ken and Kent are really pointing to the same
problem, though they are arriving at it from different directions.  Kent

  The question, is whether _H. erectus_ and _H. sapiens_ are related
  anagenetically or cladogenetically.  For those not familiar with the
  terms, let me explain. Imagine the following tree of relationships:

           A    B      C
            \  /      /
             \/      /
              D     /
               \   /
                \ /

  Species A and B share a more recent common ancestor with one another (D)
  than either shares with C (E).  Anagenesis is evolutionary change that
  happens *along* the branches, i.e., from E to C, E to D, D to A, or D to B.
  Cladogeneis is the process that leads to splitting of lineages, i.e., the
  process that takes the single lineage leading to E and splits it into two,
  one leading to A and B, the other to C.  Similarly, cladogenesis occurs at
  D producing two lineages.

  The multiregional hypothesis suggests that _H. erectus_ and _H. sapiens_
  are related anagenetically.  The single origin hypothesis suggests that
  they are related cladogenetically.  As I said in my early reply to Bayla
  Singer (though not in these words), I know of no case other than the origin
  of human beings where evolutionary biologists have postulated (in the last
  twenty years at least) an anagenetic relationship between two widely
  distributed species.

Kent's explanation here is exactly correct.  The problem as I see it,
though, is that at the population level the notions of cladogenesis and
anagenesis are somewhat ill-defined.  In other words, when we resolve a tree
like the one shown above down to a greater level of detail, it is not
entirely clear which populational events represent "branching" and which do
not.  It might be the case that some populations split off for a time
(cladogenesis), and then partially merge back in to other populations; there
might be a more or less complete cladogenetic event (a split) but there
might still be some occasional gene flow between the resulting lineages.
What anthropologists refer to as "_Homo erectus_" and "_Homo sapiens_" may
be related cladogenetically or anagenetically, but it may also be that the
historical interconnections between these two "species" is something of a
combination between the anagenesis and cladogenesis, especially at certain
time depths.

The new issue of _Systematic Biology_ that just arrived this week has a
very nice case study that illustrates these problems; folks interested in
the issue of species delimitation in the hominid case might find it very
interesting and helpful:

  Patton, J. L., & M. F. Smith.  1994.  Paraphyly, polyphyly, and the nature
  of species boundaries in pocket gophers (genus _Thomomys_).  _Systematic
  Biology_, 43(3):11-26.

From the abstract: "These molecular perspectives [described in the paper]
give somewhat conflicting views of polyphyly, paraphyly, and monophyly at
the population and species level due, in part, to probable differences in
times to monophyly, differential lineage sorting, retention of ancestral
polymorphisms, and/or episodes of asymmetrical introgressive hybridization.
As a consequence, strict adherence to any species concept in the objective
recognition of evolutionary units within this complex is difficult at
best."  (Sounds like _Homo_, yes?)

I have also written a theoretical piece that addresses many of the same
questions, and it might be of interest to the anthropologists who
are concerned with the conceptual roots of these problems as well:

  O'Hara, R. J.  1993.  Systematic generalization, historical fate, and
  the species problem.  _Systematic Biology_, 42(3):231-246.

This paper contains some nice diagrams (I like to think ;-) that might be
useful for people trying to picture the complexity of the tree when viewed
up close in the vicinity of a branch point.

Not wanting to leave our other historical scientists out of this discussion,
I hope that any brave linguists who have been following this thread can see
that what is being talked about here is the problem of when does one
language become two languages, how do we decide whether to populations of
speakers are speaking two different languages or two dialects of the same
language, are the similarities between two languages the result of borrowing
(introgression) or independent origin, etc.  Systematists have traditionally
worried a lot more about these questions, I think, with respect to species
than linguists ever have with respect to languages.  That is probably
because the original assumption of our field was that "species" were by
definition independent creations, and did not share ancestors in common.
The existence of "doubtful species" (as Darwin called them) was and is one
of the main pieces of evidence against independent creation and for the fact
of evolutionary history.

Bob O'Hara, Darwin-L list owner

Robert J. O'Hara (darwin@iris.uncg.edu)
Center for Critical Inquiry and Department of Biology
100 Foust Building, University of North Carolina at Greensboro
Greensboro, North Carolina 27412 U.S.A.

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