What experimental -- as opposed to post hoc inferential -- evidence
is there for lateral gene transfer? I'm pretty familiar with the
literature on phylogenetic claims, i.e., sequence X is sitting where it
looks rather odd: postulate a lateral transfer. But what "real time"
(if you will) studies exist showing the acquisition of foreign genes by
a population or species?
Here's why this matters. I'll cite a nicely provocative paper by
Schwabe and Warr:
We believe that it is possible to draw up a list of basic
rules that underline existing molecular evolutionary models:
1. All theories are monophyletic, meaning that they all
start with the *Urgene* and the *Urzelle* which have given
rise to all proteins and all species, respectively.
2. Complexity evolves mainly through duplications and
mutations in structural and control genes.
3. Genes can mutate or remain stable, migrate laterally
from species to species, spread through a population by
mechanisms whose operation is not fully understand, evolve
coordinately, splice, stay silent, and exist as pseudogenes.
4. Ad hoc arguments can be invented (such as insect vectors
or viruses) that can transport a gene into places where no
monophyletic logic could otherwise explain its presence.
This liberal spread of rules, each of which can be observed
in use by scientists, does not just sound facetious but
also, in our opinion, robs monophyletic molecular evolution
of its vulnerability to disproof, and thereby of its
entitlement to the status of a scientific theory.
(C. Schwabe and G. Warr, _Perspectives in Biology and Medicine_ 27
[1984]: 465-485)
Actually, Schwabe and Warr tell only half the story, and not even
the scariest half.
It is standard practice in molecular phylogeny construction to determine
sequence alignment, the first analytical stage after the raw sequence
data are obtained, more or less "by eye" -- and then to assess the
reliability of that alignment by checking the phylogeny it produces. If
you get a wild phylogeny: well, must be the wrong alignment.
[A recent example of this sort of method, which should make anyone's hair
stand on end, is Christopher Wills's paper "Topiary Pruning and Weighting
Reinforce An African Origin for the Human Mitochondrial DNA Tree,"
_Evolution_ 50 (1996): 977-989. Wills "prunes" (i.e., manipulates) human
mtDNA sequence data iteratively until he gets the phylogenetic pattern he
wants, because the starting pattern shows a starburst phylogeny with the
primate outgroup located "very asymmetrically" from the root of the human
starburst. Basically Wills tidies up the data until he gets the phylogeny
he likes.]
Tim Ikeda and I have correspondended a bit about this. (Tim, are you
there?) But what I really want to know, re this post, is what
experimental literature exists on lateral transfer.
I'm guessing not much. If that's the case, then we don't really have
decent grounds for constraining hypotheses of lateral transfer.
Paul Nelson
University of Chicago