Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!elroy.jpl.nasa.gov!sdd.hp.com!news.cs.indiana.edu!nstn.ns.ca!ac.dal.ca!arlin From: arlin@ac.dal.ca Newsgroups: sci.bio Subject: Re: Coelocanth and evolution:x Message-ID: <1991Jun7.041654.477@ac.dal.ca> Date: 7 Jun 91 07:16:54 GMT References: <17580003@hpfcdj.HP.COM> Organization: Dalhousie University, Halifax, Nova Scotia, Canada Lines: 85 In article <17580003@hpfcdj.HP.COM>, sharp@hpfcdj.HP.COM (Darrin Sharp) writes: > > Last night, on the "National Geographic Explorer", > there was a segment on the coelocanth (sp?). This > is a fish that was thought to be extinct, but live > specimens were rediscovered in the 1930's off the > S.E. coast of Africa. Since then, several are > caught each year. > > The show made mention of the fact that these fish > were unchanged for the last 400 million years. > > I know that outwardly, the live specimens very closely > resemble the 400 million year old fossils. But how > can this be? Is it common for organisms to not evolve > for 400 million years? How long has it been since sharks > and alligators/turtles/crocodiles evolved? Any other > species that haven't changed in this long? > > Darrin Sharp (sharp@hpfcla.fc.hp.com) Previous respondents have made rigid statements, and I hope they haven't discouraged you. As several people have pointed out, it is a bit odd to speak of a "living fossil," and it is quite true that all extant species are "modern" in the sense of living in the present and being the product of the last 4.6 billion years of evolution. However, the respondents misunderstand the word "primitive." Also, to insist that there is no such thing as a living fossil and that all organisms evolve at the molecular level is to miss the point, since it does absolutely nothing to explain why the coelocanth (if that is how one spells it-- I forget) and other primitive species have undergone less apparent morphological change than one might expect. "Primitive" may be a value judgement sometimes-- when used in reference to professional hockey, for example-- but in evolution it can have a precise meaning, when used carefully. Any two species will have had a common ancestor at some time in the past: the species that is more similar to the ancestor is more primitive, in the exact sense. Usually, it is misleading to talk about whole organisms-- much better to speak of component properties. The ancestor of humans and whales walked on land. The terrestrial habit of humans can thus be considered primitive in comparison to the whale's marine habit. One can think of E. coli (as opposed to H. sapiens) as primitive in that it is unicellular, however, many features of this bacterium are probably not primitive. Thus, it is easy to use the word "primitive" in an exact and objective way, provided that one actually knows something about the relevant ancestor (this is often difficult). The coelocanth looks alot like some 400 MY-old fossils, and that is why people can correctly call it primitive. To be precise, its gross morphology is primitive: note that 1) as one respondent pointed out, no one can tell about the details of external or internal morphology; and 2) as many respondents pointed out, this apparent morphological primitiveness does not imply molecular primitiveness. There is good reason to expect that coelocanth molecules have been diverging from the ancestral type just as do those of other organisms. The Port Jackson Shark is another morphologically primitive species: it has a morphology similar to that of some sharks from the Carboniferous period (ca. 300 MYA). When the hemoglobin proteins of this shark were sequenced over 15 years ago (Fisher, et al., Aust. J. Biol. Sci. 30: 487), it was made clear that molecular evolution in the shark is proceeding at a rate similar to that seen in other organisms, even though the gross morphology of the shark changes very slowly. A similar result has been obtained for the case of the opossum, another organism whose morphology has remained substantially unchanged for a long time, in this case for roughly 150 MY (see Stenzel, Nature 252: 62). So . . . many respondents have corrected the way you stated your question ("is it common for species to not evolve?") by pointing out, as I have, that every organism is expected to evolve at the molecular level, regardless of the rate of evolution at the morphological level. But this doesn't answer the intent of your question, does it? Its exciting to think of primitive organisms and to imagine that they are a window to an otherwise murky past: you want to know why the coelocanth appears to live the same way for 400 MY while other organisms diverge from their ancestors and quickly acquire new properties! There is presently no good answer to this question, as far as I know. One possibility is that a) these organisms have simply achieved a morphology that is well suited to their lifestyle *and* b) their niche has remained stable for long periods of time, so that there is no pressure to change. Meanwhile, other species diverge from the primitive type, as they a) continue to improve on their body plan and/or b) change to keep up with the times. There are other possible explanations. Arlin Stoltzfus, Department of Biochemistry, Dalhousie University Arlin@ac.dal.ca