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Re: Taxonomy (long)
In response to Dino George and Nicholas Longrich:
Yes indeed chain species are interesting. I believe there are such
occurences also in North American frogs along the east coast(?). Certainly
nature doesn't draw lines like we do, and with a species being a human
construct, we are attempting to delineate species and/or strains with a
useful purpose in mind. Taxonomy has to be useful, not a hindrance. What is
useful to one person may not be to the next.
To answer GO's questions regarding bacterial systematics, again one must
look at the usefulness of the present classifications. With disease causing
bacteria such as Salmonella, there are over 120 described strains! This
seemingly ludicrous situation, that of running a hundred or so serotyping
tests on an isolate, is exactly what many clinical labs do because the
doctors and epidemiologists like to have this information. Personally, I
think that's going a little too far. Now, consider Lyme disease. For years,
only one species of spirochete was the causative agent of the disease, now,
there are three different species which cause Lyme Disease. The doctors and
epidemiologists hate that, they only want to deal with one species.
Contradiction here between Lyme bacteria and Salmonella. Most bacterial
strain differences are antigenic, resolved at a surface protein level. Most
bacterial species differences are resolved at a nuclear gene level. Other
strain differences are resolved at a plasmid typing level, half way between.
Also, geographical delineations are used, but less frequently.
The practical upshot of all this is that differentiating bacteria depends on
what information you need. Ultimately, speciation depends on finding gene
sequences independent of observed variation. Many bacteria can exchange
genetic material at fast rates and exhibit proteins that were once thought
to be restricted to another species. Plasmids and phages are mechanisms of
genetic transfer. However, there are genes, such as 16S ribosomal RNA genes,
in bacteria which cannot be transferred , and as such may contain species
specific sequences. Usually, two or more genes must be used to convince the
Relating this back to dinosaur morphology, and finishing up this rather long
missive, the point I am trying to make is, when a subjective assessment must
be made to distinguish between variation and differentiation, sequence data
from the right gene or genes can resolve the issue. The key to this is
choosing the right gene. That's really the trick. Of the 200,000 or so genes
in a vertebrate, not all can provide this info. Indeed, only a handful have
been studied enough to be useful.
>>I say yes, you need to draw lines. Otherwise, how are you
supposed to talk about or think about anything? But we should also make
sure to remember that WE are drawing the lines,not nature, and that they
are arbitrary and subjective, not objective.
The more objective we can get, the better off we may be. Nature may present
apparent continuums, but somewhere in the genome are differences that can be
used to separate two organisms right down to the individual level, or right
up to a whole life level, past the three kingdoms(clades of life) on the
earth, and everywhere in between. There are gene sequences that never
change, at least in the three or so billion years so far life has existed,
and there are genes that change by the day in certain organisms.