Molecular drive explained? (Reply to Dover)

Joe Felsenstein joe at GENETICS.WASHINGTON.EDU
Sun Dec 8 13:07:44 EST 1991



We may be starting to go in circles, and readers of this newsgroup may be
getting tired of this, so let me state my position just once more:

Processes such as gene conversion operating between members of families
on different (nonhomologous) chromosomes will make the pool of genetic
variation more similar in all the members of the family.  Genetic drift and
selection (and also biased gene conversion) will then bring about a pattern
of concerted evolution.

  It is rather similar to migration spreading genes between different
populations in a species, so that all populations tend to drift to fixation
for the same alleles in a concerted fashion.  In such a case we do not name
a new evolutionary force (such as "migrational drive").  We just recognize
that mutation, selection, and drift, in the presence of migration, can produce
goegraphically concerted evolution.  That is why I do not find the term
"molecular drive" useful.

  Certainly Dover's papers have led to increased attention to the role of
gene conversion in communicating genetic variation between members of gene
families dispersed on different chromosomes.  But the question is whether a
new term is needed; whether a new evolutionary force has been identified
separate from selection, mutation, drift, gene conversion, and unequal
crossing-over.  I can't see that it has.

There is one further point that needs comment.  Dover argues that a case like
the one I invoked in my first posting on the subject, a case in which all
conversion was imagined to be intra-chromosomal, was not a case of "molecular
drive":
 (18 Nov 1991)
  "There are no
  intellectual Brownie-points to be gained in describing a scenario in which
  molecular drive does not operate (Felsenstein's A1-A1-A1 evolving separately
  from A2-A2-A2) in order to say that molecular drive does not operate."
He emphasizes that interchromosomal gene conversion (including conversion to
homologous chromosomes) creates a force for change separate from selection and
drift:
 (18 Nov 1991)
  "To achieve independent family evolution (i.e.
  not relying in the first instance on whole chromosomal drift) I suggested that
  genomic mechanisms of gain-and-loss (whether biased or unbiased) need to
  operate between homologous (or where necessary non-homologous) chromosomes, so
  that in a sexual species, homogenisation and fixation occur gradually and
  concomitantly.  This is molecular drive."

   I don't see that, in the case of unbiased gene conversion, there is any
force separate from selection or drift that will fix the same allele
through a gene family.  If we have an infinite population (hence no drift)
and neutrality (hence no selection) and unbiased interchromosomal gene
conversion, it will not be true that "homogenisation and fixation occurs
gradually and concomitantly".  Instead what will happen is that the
allele frequencies will become the same in all copies of the gene family,
and, after that, unbiased gene conversion will not further change the gene
frequencies.  Biased gene conversion is a force changing gene frequencies, but
unbiased gene conversion simply equalizes them among members of a family.

   I doubt that either of us is going to convince the other, so I will
fall silent at this point unless I see a new argument, and let others
have the last word.  I will comment on Arlin Stoltzfus's recent arguments
in a separate posting.

-----
Joe Felsenstein, Dept. of Genetics, Univ. of Washington, Seattle, WA 98195
 Internet:         joe at genetics.washington.edu     (IP No. 128.95.12.41)
 Bitnet/EARN:      felsenst at uwavm




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