JAHAYES@MIAMIU.BITNET (Josh Hayes) (11/04/90)
With respect to apparently primitive critters in vent communities and such. This is a bugaboo that I run into in my intro bio courses all the time; there is this idea that phylogenetically "primitive" organisms somehow stopped evolving when they arose...the idea being, I suppose, that sharks arose several hundred million years ago and just stopped changing. This is absurd, it is equivalent to saying that humans evolved from chimpanzees. Obviously not true, unless humans evolved yesterday....it is correct to say that humans and chimpanzees evolved from some common ancestor in the not terribly distant past, however. Sharks, archaebacteria, sponges, humans, that we see around us today, are all equally primitive and all equally highly-evolved. Mere semantics? Maybe. But I'm feeling grumpy today...humor me. Right. I'm done complaining. Carry on.... Josh Hayes, Zoology Department, Miami University, Oxford OH 45056 voice: 513-529-1679 fax: 513-529-6900 jahayes@miamiu.bitnet, or jahayes@miamiu.acs.muohio.edu "It is always wise to remember that it was the gods who put nipples on men, seeds in pomegranates, and priests in temples."
chi9@quads.uchicago.edu (Lucius Chiaraviglio) (11/06/90)
In article <90307.154236JAHAYES@MIAMIU.BITNET> JAHAYES@MIAMIU.BITNET (Josh Hayes) writes: >With respect to apparently primitive critters in vent communities >and such. This is a bugaboo that I run into in my intro bio courses >all the time; there is this idea that phylogenetically "primitive" >organisms somehow stopped evolving when they arose...[. . .] > [. . .] Sharks, archaebacteria, sponges, humans, >that we see around us today, are all equally primitive and all >equally highly-evolved. [. . .] This is partly true, but not 100% true. While it is highly unlikely that any extant organisms have STOPPED evolving, it has become fairly obvious from doing molecular phylogeny that not all organisms evolve at the same rate. The high thermophiles (so far all archaebacteria and eubacteria) have been evolving slowly; mycoplasmas evolve rapidly (thus, their lack of complexity is degenerate, not primitive); all of the known eukaryotes evolve rapidly; microsporidia and Giardia and its relatives evolve very rapidly; and mitochondria and chloroplasts evolve at superspeed. Many references on this are available -- the best ones that I can think of right off hand are Carl Woese's articles in a 1987 issue of _Microbiological Reviews_ and in a June (+ or - a month) 1990 issue of _Proceedings of the National Academy of Sciences_ and a reference (don't remember author's names off hand) in the latter article about how to root a tree using duplicated genes. I also have other references at home if you are interested. The moral of this is that the various organisms are NOT all equally primitive or equally highly-evolved. -- | Lucius Chiaraviglio | Internet: chi9@midway.uchicago.edu
Ellington@Frodo.MGH.Harvard.EDU (Deaddog) (11/07/90)
Well said! However, I think that the previous responses by Drs. Chiaraviglio and Yanega (like chemical companies, I find it simplest to assume everyone is a Dr.; no disrespect intended) were more subtle than "thermophilic archaebacteria stopped evolving." I believe the central issues are (1) Has one deeply-branched lineage of archaebacteria evolved MORE SLOWLY (i.e., adopted fewer metabolic or physiological changes over the course of time) than other organisms? (Obliquely, this applies to the bees as well; that is, can a lineage evolve slowly relative to other lineages.) (2) If so, does this imply that the ancient forebear to these organisms has characteristics similar to the modern versions? (I don't believe that (2) necessarily follows from (1), since both questions involve the rather complex question of what constitutes a 'trait' and whether all 'traits' are equal.) Nevertheless, I could not have summarized the argument against "frozen evolution" better than you did. I am currently struggling with the age-old (ha, ha) question of "mode/tempo of evolution" and whether thermophily is truly a 'trait.' If I produce anything remotely coherent, I will babble forth. Dept. Mol. Biol. Mass. General Hospital Non-woof
chi9@quads.uchicago.edu (Lucius Chiaraviglio) (11/08/90)
This thread does not seem to have picked up any chemistry, so I have directed followups to sci.bio. In article <4618@husc6.harvard.edu> Ellington@Frodo.MGH.Harvard.EDU (Deaddog) writes: >Well said! >However, I think that the previous responses by Drs. Chiaraviglio and ^^^^ >Yanega (like chemical companies, I find it simplest to assume everyone is >a Dr.; no disrespect intended) Unfortunately, not until a few years in the future. . . :-P > were more subtle than "thermophilic >archaebacteria stopped evolving." Yes, as I explained in a couple of previous posts, what I was trying to say was about slowly-evolving, deeply-diverging (with respect to main trunk(s) of closest primary lineage), thermophilic organisms. > I believe the central issues are >(1) Has one deeply-branched lineage of archaebacteria evolved MORE SLOWLY >(i.e., adopted fewer metabolic or physiological changes over the course of >time) than other organisms? (Obliquely, this applies to the bees as well; >that is, can a lineage evolve slowly relative to other lineages.) Yes. What Carl Woese calls the Crenarchaeota (in his recent _Proc. Natl. Acad. Sci._ article) fits this description, as does the "genus" Thermococcus (probably more appropriately defines a kingdom) in what Carl Woese calls the Euryarchaeota, which also includes Archaeoglobus, the three (or more) Methanogen kingdoms, and some non-methanogenic organisms derived from the Methanogens (Thermoplasma and the Extreme Halophilic Archaebacteria). Within the latter groups, which branch from the Euryarchaeote main sub-lineage, the more deeply-diverging kingdoms tend to be more slowly-evolving (except Thermoplasma, which isn't that much of a thermophile anyway) and contain members whose optimum and maximum growth temperatures are higher -- by a considerable margin (up to around 90`C maximum last time I looked for the Methanococcus kingdom as opposed to around 55`C to 65`C for the kingdom composed of the more highly-evolved Methanomicrobiales, for instance). The same pattern is also apparent within the Crenarchaeota, of which the fastest-evolving "genus," Sulfolobus, is also the one capable of growing at the lowest temperatures (however, being able to grow in and even use oxygen may also be a factor in its more rapid evolution, but this does not seem to be a factor among the Gram-positive Eubacteria, for which this possibility has been investigated). A similar but substantially muddier picture exists for the Eubacteria. Unfortunately, no such picture or refutation thereof exists for the Eukaryotes, because 1. The deepest-diverging members of the Eukaryotes are not very deep divergers when one takes into account the total amount of evolution the Eukaryotes have undergone since splitting off from the Archaebacteria 2. The deepest-diverging members of the Eukaryotes, the Microsporidia and Giardia and its relatives, have been evolving even more rapidly than the other Eukaryotes characterized by molecular phylogeny, and the next-deepest divergers, the Trypanosomes and their relatives, have also been logging evolutionary distance pretty rapidly. 3. No high thermophiles have been found among the Eukaryotes. The best Eukaryotic thermophiles only grow at up to about 60`C (give or take ~2`C), and these ones of these for which the molecular phylogeny has been done have been shown to have evolved from mesophilic groups. >(2) If so, does this imply that the ancient forebear to these organisms >has characteristics similar to the modern versions? (I don't believe that >(2) necessarily follows from (1), since both questions involve the rather >complex question of what constitutes a 'trait' and whether all 'traits' >are equal.) You are right that (1) does not prove (2). However, it lends considerable credence to (2), since it is unlikely that several slowly- evolving lineages which diverged from each other long ago will have undergone great phenotypic changes relative to each other. >Nevertheless, I could not have summarized the argument against "frozen >evolution" better than you did. [That refers to a poster following up one of my articles.] I never said anything about "frozen evolution." What I have talked about is rates of evolution which differ considerably, as outlined in a previous message. -- | Lucius Chiaraviglio | Internet: chi9@midway.uchicago.edu