Message 2000-09-0003: Re: Nathan Wilson's question

Wed, 27 Sep 2000 11:18:12 -0600

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Date: Wed, 27 Sep 2000 11:18:12 -0600
From: christopher brochu <>
Subject: Re: Nathan Wilson's question

This brings up some problems I had with Gould's recent essay in Natural
History, in which he discusses the historical reasons for survival of the
Linnean system beyond the recognition of evolution.  Toward the end, he
states (citing a recent review paper in Science) that evolution in the
basalmost organisms may not be expressable as a simple hierarchy.

I got the strong impression that Gould was confusing lineage reticulation
(which is what Phil is talking about below) with lateral gene transfer.  Am
I correct in supposing that lateral gene transfer does not represent a
deviation from a hierarchical pattern of phylogeny?  The difference between
a gene tree and a species tree has been appreciated for many years - am I
correct in supposing that gene history may not be hierarchical, even if
taxon history is?


>Since no one else has responded to Nathan Wilson's question, I will
>make a stab at it.  I discussed this with Kevin over lunch today, so
>the ideas are partially his (I'll leave it to him to correct me if I
>misrepresent any of them).
>Nathan is concerned about the effect of interclade genetic transfer
>on clade nomenclature.  I don't think this is a problem if one
>accepts that a species may belong to two non-nested clades.  For
>example, if clades A and B are non-nested, and members of species 1
>(in clade A) and species 2 (in clade B) hybridize to produce an
>offspring that gives rise to species 3, species 3 is a member of both
>clade A and clade B.  This must be the case because species 3 is
>simultaneously a descendant of both the immediate common ancestor of
>clade A and the immediate common ancestor of clade B.
>Similarly, if genetic material is transferred by some other means
>from species 1 to species 2, then one might argue that species 2 (but
>not species 1) is a member of both clades A and B.  It is debatable
>whether it is worth treating species 2 nomenclaturally as belonging
>to two non-nested clades if only a small percentage of its genetic
>material came from clade A, but the principle is the same.
>There is nothing in the PhyloCode that prohibits naming a clade, some
>species of which belong to another non-nested clade, so I don't think
>that the issue that Nathan raised requires that the wording of clade
>definitions be modified, as he suggests.  However, the taxonomic
>issue of species belonging to non-nested clades is an interesting one
>that I don't think has been addressed in the literature.  It is a
>significant departure from the traditional rank-based system, in
>which a species may belong to only one genus, one family etc.  (I am
>comparing ranks to clades here by way of analogy, not suggesting that
>they are the same thing.)
>Philip D. Cantino
>Professor and Chair
>Department of Environmental and Plant Biology
>Ohio University
>Athens, OH 45701-2979
>Phone: (740) 593-1128; 593-1126
>Fax: (740) 593-1130

Christopher A. Brochu
Department of Geology
Field Museum
1400 S. Lake Shore Drive
Chicago IL 60605

312-665-7633 voice
312-665-7641 fax


Feedback to <> is welcome!