Let me add some specific details to my original posting:
In Giardia lamblia, the degree of DNA sequence divergence
among isolates can be comparable to that seen between vertebrate
classes! Yet these organisms appear to be morphologically (and
medically) indistinguishable. This is also true of Naegleria
gruberi, Trypanosoma cruzi, etc. Clearly, one should not expect to
extrapolate from findings in one isolate to the 'species' as a whole.
A different sort of problem arises when organisms that are identical
by morphology prove to be polyphyletic upon molecular analysis.
This is something I have encountered myself. Does this warrant
redescription of the species based on the molecular data alone?
FYI, the description of new species based solely on molecular
data is already taking place in the Protista - Leishmania
colombiensis and Encephalitozoon hellem are two that come to mind.
I do not doubt the validity of these taxa but I feel that in the
absence of 'classical' criteria a greater degree of caution is
warranted before erecting new species. For instance, how can one
distinguish discontinuous from continuous molecular variation with
only one or a few examples of the new 'species' in hand?
C. Graham Clark,
Laboratory of Parasitic Diseases,
National Institutes of Health,
Bethesda, MD 20892
e-mail: cge at cu.nih.gov