Darwin-L Message Log 2:75 (October 1993)

Academic Discussion on the History and Theory of the Historical Sciences

This is one message from the Archives of Darwin-L (1993–1997), a professional discussion group on the history and theory of the historical sciences.

Note: Additional publications on evolution and the historical sciences by the Darwin-L list owner are available on SSRN.

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<2:75>From mayerg@cs.uwp.edu  Tue Oct 12 13:42:42 1993

Date: Tue, 12 Oct 1993 13:33:18 -0500 (CDT)
From: Gregory Mayer <mayerg@cs.uwp.edu>
Subject: Re: manuscript polymorphism
To: darwin-l@ukanaix.cc.ukans.edu

	In recent postings, Jeff Wills has asked if there are any
biological phenomena equivalent to one of two (or more) variant readings
within a manuscript being more likely to be copied into a descendant
manuscript, and Margaret Winters has asked if there is an equivalent to
the end words of manuscripts being less likely to be changed by a scribe
than interior words.  The answer to both questions is yes, and thus the
parallels between manuscript transmission and genetic transmission are
furthered.
	The parallel to Wills' phenomenon in genetics is "meiotic drive".
Meiotic drive is when the two copies of a gene at a particular locus (that
word again) in a diploid individual are not passed on at random to that
individual's offspring.  (The equivalent could also happen in a triploid,
etc. individual.)  What this means is that instead of, on average, 50% of
the offspring receiving one copy, and 50% the other copy, one copy is
systematically over-represented (and thus the other is under-represented)
in the offspring.  Meiotic drive can also occur at the level of whole
chromosomes, and an example with sex chromosomes might make it clear.  A
male is heterozygous for the sex chromosomes, XY; on average, about half
his offspring will get the Y chromosome, and half will get the X.  If
there were a statistically consistent over-representation of the X
chromosome (so that, say, only daughters were produced among the
offspring), there would be meiotic drive in favor of the X chromosome.
It's called meiotic drive because the process during which the genetic
material of a parent is divided and packaged up for distribution to the
offspring (via sperm and egg) is called meiosis.
	Meiotic drive is a form of natural selection at the genic or
haploid level.  It is a very strong form of selection.  In general, a
chromsome or allele of a gene favored by meiotic drive will rapidly
increase in frequency until it is fixed (i.e. it is the only version of
the gene or chromosome around).  Because of their strong selective
advantage, meioticically driven alleles or chromosomes are expected to be
very rare; any place they occur, they should be rapidly fixed, and then
they are not recognizably driven.  They should generally only be
noticeable when some countervailing selection maintains them at some
intermediate frequency. One of the most well known cases of meiotic drive
are the t-alleles in house mice.  Some alleles at this locus are favored
by strong meiotic drive but are opposed by selection at the individual and
deme levels; the balance of selection leads to the maintenance of a
polymorphism.
	In the manuscript case, if particular readings are favored in
copying by scribes, then these favored readings should become the
"standard" reading fairly rapidly.  If the reasons for the copying
advantage are scribe-independent, then the same standard may emerge within
different branches of the same manuscript tradition, if the same multiple
readings are present in the branches (i.e. there could be parallel changes
in separate branches of the tradition).
	The parallel to Winters' example of different likelihoods of
change in different parts of a manuscript is variation in evolutionary
rates across different parts of the genome.  Some parts of the genome
evolve faster than others, e.g. the mitochondrial genome in general, the
major histocompatibility locus in the nuclear genome, and sites in the DNA
where new mutations would not alter a resulting protein wherever they
occur.  Others evolve more slowly, e.g. genes for histones and cytochrome.
A particular type of DNA sequence change (called transitions) is common
relative to the other possible type of change (called transversions).
It is not always clear whether this rate variation is due to higher
mutation rates (errors in copying), or selection (consistent differential
change in frequency among already existing variant copies).  The analogy
to manuscripts is more complex: while a scribe's slip of the pne is
clearly a copying error, the deliberate change of, say, a Castilian word
to a Catalan word, when it occurs for the first time, is perhaps not.  The
analogy between manuscripts and genetics, while fruitful, is not exact,
and here seems to be a point where it breaks down.  In genetics, the
machinery of replication is largely separate from the realm in which
differential survival and reproduction take place.  With manuscripts, the
scribe not only creates the variants, but decides their fate.  Whether a
consistent changing of words in the interior of a line is best analogized
to biased mutation or selection I do not know, but it is probably not
important as long as students of manuscripts understand what scribes
actually do.
	Regardless of whether we regard a particular scribe's act as error
or selection, the analogy to genetics, and variable evolutionary rates,
can still be useful.  Biologists routinely deal with variable evolutionary
rates in reconstructing evolutionary trees by _weighting_ different
changes, i.e. by considering certain changes more indicative of
relationship by descent than others.  There are a number of ways in which
this can be done, from the intuitive to the quite numerical, and many are
controversial.  A relatively non-controversial method is the differential
weighting of the two types of changes in DNA I mentioned above,
transitions and transversions.  Qualitatively (there are quantitative ways
of doing it), the method amounts to considering a shared transversion more
solid evidence of relationship, because such changes are rare and unlikely
to have happened twice.  A transition, because they occur more frequently,
is more likely to have occurred twice.  Thus, we might consider the
evidence of two shared transversions to outweigh th evidence of three
shared transitions.  Analogously, following Winters' example, two
manuscripts that share a variant reading in the rhymed words might be
considered to be more certainly related because scribes were reluctant to
make such changes, and thus the chance of two scribes adpoting the same
unlikely variant reading independently would be low.  Conversely,
introduction of dialectal variant readings within the interior of a line
might be less secure evidence of a relationship between the manuscripts,
because any scribe writing the same dialect might be likely to introduce
such changes, as they would not change the rhyme.

Gregory C. Mayer
mayerg@cs.uwp.edu

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