Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!tdatirv!sarima From: sarima@tdatirv.UUCP (Stanley Friesen) Newsgroups: sci.bio Subject: Re: Coelocanth and evolution:x Message-ID: <34@tdatirv.UUCP> Date: 11 Jun 91 17:49:04 GMT References: <17580003@hpfcdj.HP.COM> <18@tdatirv.UUCP> <471.284d6041@mbcl.rutgers.edu> <29@tdatirv.UUCP> <472.28511bf9@mbcl.rutgers.edu> Reply-To: sarima@tdatirv.UUCP (Stanley Friesen) Organization: Teradata Corp., Irvine Lines: 77 In article <472.28511bf9@mbcl.rutgers.edu> kliman@mbcl.rutgers.edu writes: >In article <29@tdatirv.UUCP>, sarima@tdatirv.UUCP (Stanley Friesen) writes: >> Perhaps so. But there is no known living *species* that is more than a few >> million years old (certainly no more than 10 MY). ... >What is the definition of species here? Certainly bifurcations have occurred >in the phylogenies of all extant species. However, does this cause the >extinction of the ancestral species? Well, if you use the cladist's definition of species - YES :-| However, that is *not* the definition I was using. > I don't think we can determine if any >extant species can produce fertile offspring in a cross with some distant >ancestor (if we use that particular definition of species), so to say that no >species is more than 10 MY old is just an assumption. True, but that is not even sufficient evidence for living forms. Inability to cross-breed almost proves a species distinction, but ability to interbreed does *not* prove that two forms are the same species. The basic definition of species for living forms is that they do not *normally* interbreed successfully. This can be because they are too different in ecology, or mating habits for the adults to ever mate in the wild; or it could be that they are too different in lifestyle or morphology for the offspring to survive in the wild. With extinct forms you basicly have to *guess* whether this criterion holds. It is, of course, not as certain as with living forms, but it appears to work better than might be expected. (Of course its hard to tell!) > I'm not saying that >the coelocanth is morphologically identical to its ancestor 400 MY back, or >that it is even the same biological species. However, I stick to my >prediction that *some* species (not necessarily a chordate) has not changed >substantially for 400 MY. That is not to say that diversification through >offshoot lineages (e.g., by "founder effect") has not occurred. I do not believe this is likely. I suspect that any form would have changed enough in that amount of time that offspring would either never occur or fail to survive. The closest I have yet seen to this kind of constancy in known fossils is in the genus Lingula. (Which I had forgotten about in my previous postings). A case might be made here that living Lingula is close enough in morphology to one of the Paleozoic Lingula's to be considered the same species (but I stll doubt it - Lingula's have changed somewhat, and the differences seem to exceed the level typical for distinct species of brachiopods). [P.S. this is the practical criterion for fossil species - morphological differences exceeding typical within-species variation among living relatives]. >> Quite true. But none are so closely identical (even morphologically) as to >> be considered to be the same species. Why this is so is, perhaps, >> a problem worth studying, but it is nonetheless true (by observation). >What I'm saying is purely hypothetical. I'm inclined to agree that all extant >chordates probably differ from their ancestors from 400 MY. All I originally >wanted to convey was disagreement with a statement that I considered too >rigid. We simply don't know what the phylogenies of all living species look >like, and all "rules" are bound to be broken occasionally. Hmm, well, I wouldn't exactly call it a 'rule', more like a generalization from observation. The longest spans known for individual morpho-species are on the order of 10 million years, so on statistical grounds, I reject the likelyhood of a species so *far* out on the tail of the distribution. [Typical species last about 1 million years or so]. Certainly it is strictly speaking *possible* that a form remains so stable in all aspects of its biology as to remain viable with its ancestors for many milions of years, but it is *very* unlikely (P << .01). [In math the symbol '<<' means 'much less than']. -- --------------- uunet!tdatirv!sarima (Stanley Friesen)