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From: MBY134@sysh.surrey.ac.uk
Newsgroups: bionet.molbio.evolution
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Date: 24 Sep 90 16:50:48 GMT
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            SEQUENCE CONSERVATION WITHIN A SPECIES

Does anybody have any refs/data/views on the amount of overall
DNA sequence variation within species. A related question is
the amount of sequence divergence found between closely
related but distinct species. I am interested in how much
sequence variation is tolerated within a species before
species barriers intervene to restrict genetic exchange.

The reason I am interested in this is that there is
considerable pressure in bacterial taxonomy at the moment to
classify any group of bacteria with greater than 70% DNA
homology as belonging to the same species. I feel this would
throw together many closely related but distinct species -
including some I work with! The definition of a bacterial
species is controversial since species barriers can not be
empirically determined in prokaryotes. However, in sexual
species I have vague recollections that the sequence variation
found within species may be much less than this (true I
believe for Homo sapiens); also - in well studied groups,
pairs of closely related but distinct species may be found
(eg. amongst primates, mice or drosophila?) that have very
high levels of DNA homology.

Does anyone have any data to support or refute my prejudices?

Johnjoe McFadden
Department of Microbiology,
University of Surrey,