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Re: genes and species (long)

From:

David Ziolkowski

Reply-To:

David Ziolkowski

Date:

Wed, 21 Feb 2007 23:15:11 -0500

Paul and Gail bring up good thoughts about the need for
sampling more genes.  Given the bad wrap molecular
systematics sometimes receives for the wrong reasons though,
a few additional points are worth mentioning.

What is sometimes viewed as a weakness of molecular
systematics - that researchers have not sampled the genes that
control for the phenotypic traits that may delimit species
(morphology, song, etc.) - is actually the strength behind these
approaches.

When the goal is to elucidate factors promoting speciation,
examining genes underlying phenotypic traits is certainly
important.  Practitioners of modern molecular methods
however, typically have a goal quite removed from the
study of speciation.  They aspire to identify the pattern of
relatedness between populations (species) by measuring
their relative levels of genetic distinctiveness.

Here the 'relative' part is the key as, in this pursuit,
you need a molecular marker that is consistent enough
across species that it can be used as a common genetic
yardstick.  Genes underlying phenotypic traits don't suit this
purpose well since they are known to respond quickly to local
selection pressures (environmental and sexual).  This is
basically the same idea as why taxonomists do not use
direct measurements of song, bill length, etc. alone as
indicators of the history between species (phylogeny).

At the heart of the issue is that gene trees are not necessarily
the same as species trees.  This is because, say, when a
population splits off, some alleles (alternative forms of a gene)
are still more closely related (i.e., they descended from the same
ancestral allele) to alleles in the original population than to
alleles in their own population.  Given enough time though,
random allele loss (genetic drift) leads to a situation where only
those alleles that are more closely related to each other, than to
alleles from any foreign population, remain (the point of
reciprocal monophyly). Only at that time will All genes reveal the
same story - the story of a species tree.  That takes millions
of years after populations have been genetically separate though
(although, not necessarily millions of years after qualifying as a
"species" since genes affecting whether populations will
reproduce together may be the first to differ, or the last).

Few, if any, species have ever been found to be reciprocally
monophyletic so geneticists have taken to being judicious in
using only those genes that evidence suggests are likely to yield
true species trees in the interim. And that's where it gets more
interesting since variation in the amount of selection on
different genes determines the levels of genetic divergence that
they are informative at, and so on.  The following article
provides a great introduction to that, and other related topics.

Maclean, N, Collinson, M, and Newell, R."Taxonomy for birders:
a beginner's guide to DNA and species problems" October 2005. 
British Birds 98:512-537.

Cheers,  Dave

---------------------------
dziolkowski at usgs dot gov
PWRC, Laurel