Reply: Woese's problematic ideas
magnus.liden at SYSTBOT.GU.SE
Mon Oct 20 14:24:08 CDT 1997
In my role as teacher of evolution and systematics in biology courses at
the university, it is natural that I take interest in the discussion of the
origin of life. As an amateur in these matters, I am dependent on what data
and interpretations that happen to catch my eyes during my weekly browsing
of the scientific literature that appears in our library, and yes, I have
been convinced by the Woesian band wagon (i.e. its factual content, though
not always its expression). I imagine to have been so on the basis of
evaluations of hard data but I will happily change my position if data are
presented that support alternative scenarios. Such data are, however,
missing from your letter, and from the articles you refer to. Further, you
will have to convincingly explain data that (to us not in the bacterian
field, at least) are consistent with eubacterian monophyly only. More
generally, to some of your lines of reasoning I have objections that partly
apply to Woese (or at least some of his followers) as well.
Establishing the sistergroup relations between the major domains of life
DOES NOT establish the way life originated, nor does it point to any
specified group as "primitive".
It is an extremely common misconception that a sister clade to a large and
specious clade is "primitive" (often labeled "THE basal clade", ignoring
that both clades are equally basal [yes! I know that many of you are very
well aware of this, and I also understand that this label is a very
convenient reference to a graphic representation, but it is a seductive
label, and you could find numerous examples of how people are gravely
mislead by it, just by scanning a volume of TAXON or any other taxonomic
journal, not to mention more popular works]).
Thus, even if one could establish that cyanobacteria is the sistergroup to
all other life (this is what Kenman claims), the cladistic position alone
does not indicate "primitiveness" of the group, nor that the characters of
cyanobacteria are more ancient than those of its (hypothetical)
sister-clade. What it would suggest is for example that absence of type I
introns was primitive, as it would occur in both of the basal branches in
this hypothetical phylogeny.
Back to facts: A strong argument for eubacterian monophyly is a molecular
tree of an early duplicated enzyme (some more knowledgeable TAXACOM-member
does certainly remember which one, and has an original reference - please
share it, I have lost it long ago) which provides an outgroup for all life.
In this tree the catalytic and non-catalytic versions of the enzyme both
give the same tree, showing eubacterial monophyly, and even suggesting
This single enzyme family thus presents two congruent trees of life, and to
me this is extremely convincing. I suppose more has been done since. Anyway
it is certainly false to claim that the case for monophyly of eubacteria
(at least of those that were included in the study, cyanobacteria,
chloroplasts and mitochondria among others) is based on circular reasoning.
Nothing could be less circular than a true outgroup. To my limited
imagination, a globally duplicated gene with recognizable and functional
sisters is the only possible means to solve the rooting of all life.
Explain this case in the light of your theory (or rid me of my ignorance of
a later reinterpretation of the data), and you will find me open to further
arguments (but fossils will not impress me).
Systematisk Botanik, Carl Skottsbergs gata 22B
S-413 19 G=D6TEBORG (GOTHENBURG), SWEDEN
Tel.nr.: xx46 (0)31 7732669
=46ax.nr.: xx46 (0)31 7732677
e-mail:magnus.liden at systbot.gu.se
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