chi9@quads.uchicago.edu (Lucius Chiaraviglio) (11/03/90)
In article <4578@husc6.harvard.edu> Ellington@Frodo.MGH.Harvard.EDU (Deaddog) writes: >In article <1990Oct30.030717.8923@midway.uchicago.edu> >chi9@quads.uchicago.edu (Lucius Chiaraviglio) writes: >> combination of slow evolution and deep phylogenetic divergence, >> which -- when the deeply-diverging organisms have a lot in common >> phenotypically, which they do -- implies greater resemblence to ancestral >> forms of life. > This is somewhat disturbing. > First, isn't the fact that only a few, deep phylogenetic divergences >have been found between the *known* Archaebacteria a function of the >relatively small number that have been catalogued? [. . .] While it is true that the number of Archaebacteria catalogued is small, and that several Archaebacterial kingdoms are probably yet to be discovered, a large number of Archaebacteria have been discovered. Find a review on Archaebacteria or some subgroup thereof which actually lists a bunch of sample species (a somewhat out-of-date sample: "Methanogens and the Diversity of Archaebacteria" in a 1987 issue of _Microbiological Reviews_ -- sorry, I don't remember the authors' names or the exact issue off hand, but if you want I could look it up when I get home). Thus, while the picture is not complete, it is definitely emerging. On the other hand, even though a much larger number of Eubacteria have been characterized, some recent discoveries (Thermotoga for example) imply that kingdoms remain to be discovered on this branch as well. > Second, while the Archaebacteria have some traits in common, their >phenotypic resemblances are immaterial to what a primitive life form looks >like; it is only those traits that are held in common by each of the >three ur-kingdoms (or at least two out of three) that give us a logically >valid picture of the common ancestor of modern life (what I like to call >the progenote, but Woese has disagreed). That doesn't disagree with what I said (except that it seems better to call the common ancestor a *protogenote* -- I think that's also what Woese said). > Also, what makes you think that >Archaebacteria are more phenotypically similar within their ur-kingdom >than eukaryotes or eubacteria; what traits are you referring to? I didn't say this. What I was getting at was that the features (thermophily is the main one that I was thinking of) which the last common ancestor of Archaebacteria has in common with the last common ancestor of Eubacteria are more likely to be features of the last common ancestor of the two lineages. What I forgot to say in my original article was that since recent molecular phylogeny (reference given below) indicates that Eukaryotes diverge from the Archaebacteria after their common ancestor diverges from the Eubacteria, this presumably applies to all three lineages. > Finally, the idea that the evolutionary rate of archaebacteria has been >slow Whoa -- I most certainly didn't say that. I was talking about slower- evolving and deep-diverging organisms WITHIN the Archaebacteria and the Eubacteria, and did not say, or want to imply, that one group is as a whole slower-evolving or has a sparser actual tree (as opposed to that which we have catalogued) than the other. (Actually, the Archaebacteria do fall into two major subdivisions, which Woese calls the Crenarchaeota and Euryarchaeota, whereas the Eubacteria have a more even branching pattern, and the Eukaryotes form a subtree with strong apical dominance.) > seems unsupported: aren't the ribosomal RNA sequences within the >archaebacterial lineage at least as divergent from each other as those >within the eubacterial or eukaryotic lineages? I didn't say they weren't. However, the Eukaryotes really do seem to be evolving at about twice the rate of the other two lineages -- see Carl Woese's article in one of the June (+ or - 1 month) issues of _Proceedings of the National Academy of Sciences_, and also the reference therein about how the root of the overall tree of life was determined. > Doesn't the wide variety >of metabolic adaptations seen for different archaebacteria (halophiles, >thermophiles, acidophiles, chemolithotrophs, chemoautotrophs) demonstrate >that the rate of evolution has been at least as quick as in the other >lineages? No, it doesn't (although by molecular phylogenetic criteria the Archaebacteria as a whole seem to be about as capable of rapid evolution as the Eubacteria -- reference given above). The Eubacteria have all of those phenotypes -- EXCEPT METHANOGENESIS -- plus even more if you further subdivide some of those that you listed above. In contrast, the known Eukaryotes, which have been evolving considerably more rapidly, have much less biochemical diversity than the other two groups. > I may be misreading your intent, but it seems as though you want to >classify archaebacteria as 'living fossils'--and there just ain't such a >thing. >A.E. Everything has some living fossil aspect to it, but those organisms which have evolved more slowly have more of it than others. However, I was NOT trying to classify the Archaebacteria as the prime living fossils. -- | Lucius Chiaraviglio | Internet: chi9@midway.uchicago.edu