identical mutations and molecular systematics

Doug Yanega dyanega at MONO.ICB.UFMG.BR
Sun May 4 14:57:10 CDT 1997


Over on another internet mailing list, Mark Thogerson wrote:

>There are two ways I can imagine a beneficial recessive mutation resulting
>in incresing allele frequencies:
>(1) if the population is small and inbreeding is common
>(2) if the mutation rate toward the recessive allele is sufficiently high,
>given the population, so that the few (even 1 in 1000) homozygous
>recessives that do occur are significantly better adapted in some way.

This is not the first time I've seen a population genetics person suggest
hypothesis #2, as if it were an accepted general model, and as I consider
it now, I find the concept of independent but identical fitness-enhancing
mutations a little odd. Do people *literally* expect things like this to
occur often? After all, the implication is that all the individuals in the
population have a portion of their genome (obviously somewhat conserved
since it has to be virtually identical across the population) which is only
one or two simple nucleotide changes away from a functionally superior
alternative, so the same new beneficial allele can crop up repeatedly. This
seems like an unlikely set of circumstances to me, at least at first
glance.
        At any rate, if this were in fact how things operate in nature, and
if one extends the principle beyond the population, the expectation is that
fitness-enhancing molecular homoplasies will not be rare events. As I
recall, I once heard someone comment that one nice feature of molecular
data was that homoplasies in fitness-affecting portions of the genome were
only likely to arise if they involved tertiary structure of enzymes and
such (so that there was convergence on an ideal amino acid sequence), so it
would be uncommon for a homoplasy to exist and persist in multiple taxa
such that it truly appeared to be a synapomorphy - the idea also being that
neutral homoplasies would often revert or change, and thus rarely be
mistaken for synapomorphies in the final analysis. But if population
geneticists believe that beneficial mutations *often* have multiple
origins, has this been taken into consideration by molecular systematists,
and/or is it considered irrelevant? Or am I missing something and just
asleep at the switch today? ;-)

Cheers,

Doug Yanega    Depto. de Biologia Geral, Instituto de Ciencias Biologicas,
Univ. Fed. de Minas Gerais, Cx.P. 486, 30.161-970 Belo Horizonte, MG   BRAZIL
phone: 031-448-1223, fax: 031-44-5481  (from U.S., prefix 011-55)
                  http://www.icb.ufmg.br/~dyanega/
  "There are some enterprises in which a careful disorderliness
        is the true method" - Herman Melville, Moby Dick, Chap. 82




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