[Taxacom] Usefulness vs. convenience (Protista)

[Kenneth Kinman]

 Hi Tony,
       As far as I know, most workers who still use Kingdom Protista are
referring to the traditional broad grouping (with both pigmented and
colourless forms).  Chloroplasts have independently invaded colourless
forms several times, so pigmented vs. colourless forms should not be
used to broadly classify protistan taxa at these highest levels.              
       Cavalier-Smith (1998) divides Protista into two separate Kingdoms
(Protozoa and Chromista), both of which contain colourless and pigmented
forms (which is not a problem per se).  However, some workers believe
Chromista is polyphyletic (which would be a huge problem).            
      My position has always been that whether Kingdom Chromista is
polyphyletic or not, it was still a bad idea, because it unnecessarily
makes his other Kingdom (Protozoa) even more paraphyletic either way.
If Chromista is holophyletic, it basically adds one more exgroup, but
why split Protista if Protozoa is still paraphyletic (there's no
significant advantage gained),  And more importantly, if Chromista is
actually polyphyletic, it is not only unnatural, but adds even more
exgroups to Protozoa.  It then really makes a huge mess of it.         
      Either way, it unnecessarily makes the classification more complex
and less stable.  As much as I respect Cavalier-Smith's work, this was
his one major mistake, and it seems to have cast a bit of a cloud over
much of the wonderful work he has done on the phylogeny of protists (and
prokaryotes as well).  I sincerely hope that he will stop trying to
divide Kingdom Protista into separate Kingdoms, and that his work will
then gain more support.  He certainly is not a strict cladist, so why he
wanted to divide Kingdom Protista into separate Kingdoms still puzzles
me.  Especially since he otherwise has incredible insight into how to
best subdivide the protists.  
         --------------Ken Kinman
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Tony Rees wrote:
Ken - I tend to agree (for what it's worth) - you wrote: 
<snip> 
Kingdom Protista (as 
part of the Five Kingdom classification) has long served as a convenient
paraphyletic group of all eukaryotes except Metazoa, Metaphyta, and
Fungi. Cavalier-Smith has done a fine job of gradually elucidating the
relationships within Kingdom Protista. Unfortunately, he also divided
this perfectly good taxon into separate Kingdoms that proved to be
either wrong or controversial (which has puzzled me since he does
recognize some paraphyletic taxa). </snip> 
Yes - now when someone talks about Protista, do they mean: 
- colourless unicellular organisms (without the pigmented ones i.e.
"microalgae" or phytoflagellates etc.) i.e. ~= protozoa 
- all unicellular organisms, colourless and pigmented i.e. Protista
sens. lat.,  pre C-S "Chromista" split 
- same less the heterokont algae and related colourless forms i.e
Protista less Chromista Mk 1 
- same less the heterokont algae and related colourless forms, also
rhizaria and alveolates i.e. Protista less enlarged Chromista (2010
version)... 
all very confusing (especially to casual readers) and also unstable
between versions and between different authors/usages... 
Regards -Tony 
 
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