Re: Species [arbitrary to a degree]

["Tim Williams" <tijawi@hotmail.com>]

David Marjanovic wrote:

> BTW, I've learned today that the bacteriologists who can't use biospecies
> have taken the morphospecies to the logical extreme, _they have invented a
> specificometer and a genericometer_: Stems belong to the same species if
> they don't differ in G + C content by more than 5 mol%, and to the same
> genus if they don't differ by more than 10 mol%. This is immensely
> practical* because whether 2 bacteria belong to the same species or genus 
> has become testable.


As Jaime said; this "rule" is extremely arbitrary.  A change in a single 
gene can result in profound biochemical and pathogenic (disease-causing) 
changes between bacterial species.

Even if it was widely used, certain genera would be maintained in defiance 
of this "rule" simply for public health reasons.  It is helpful for highly 
dangerous strains/species of bacteria to have their own individual generic 
"label" separate from close relatives that are less pathogenic 
(disease-causing).

For example, the "genus" _Shigella_ is an enteropathogenic bacterium very 
closely related to _Escherichia coli_.  In fact, _Shigella_ may be regarded 
as a subset of _E. coli_ - essentially a collection of particularly 
dangerous strains within this species.  However, _Shigella_ is maintained as 
a separate genus because it's a damn nasty critter, to help avoid confusion 
with "true" _E. coli_.



im

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