honig@BONNIE.ICS.UCI.EDU ("David A. Honig") (06/29/90)
(Excuse the naivite; I'm not a biologist..) Classically, you specify the name of species (which is the leaf of the tree graphing speciation) starting with Kingdom, Phylum, etc. Now, why should all species be specifiable by just 7 levels? Is this just human oversimplification? Am I not understanding something? I understand that there are now Super-orders, classes, etc. But why is the less than a dozen branch points in the classification sufficient? Because there were a certain number of speciating episodes in common, due to climate, for all living things? (doubtful). It seems to me that there should be quite a range of species' 'depths' in the tree of life. Its kinda like this (apologies to the biologists! :-) : in a unix system, there are files (species) at just a few levels below root, and there are files twenty subdirectories below root. It would be highty suspicious if most files were found exactly 7 subdirectories below root. Please email (and publically post if you like) responses to me. Thanks much. Confused but interested, David
knox@whittaker.rice.edu (Robert G. Knox) (07/02/90)
In article <9006290838.aa23173@ICS.UCI.EDU> honig@BONNIE.ICS.UCI.EDU ("David A. Honig") writes: > >Classically, you specify the name of species (which is the leaf of >the tree graphing speciation) starting with Kingdom, Phylum, etc. >Now, why should all species be specifiable by just 7 levels? > >Is this just human oversimplification? Yes, mostly.... >. . . I understand that there >are now Super-orders, classes, etc. But why is the less than >a dozen branch points in the classification sufficient? >Because there were a certain number of speciating episodes in >common, due to climate, for all living things? (doubtful). > This is part of the answer. Major episodes of diversification may result in the origin of groups that we (later) recognize as being of the same rank; for example, most of the major metazoan phyla first appear just prior to or during a major radiation at the base of the Cambrian. A large number of body plans appear in these strata (esp. the Burgess Shale, as described in a recent book by Steven J. Gould). Most left no modern descendents, but those that later diversified define "major" phyla. In contrast to phyla, "kingdoms" appear to have little taxonomic meaning in present discussions. The Five Kingdom system promoted by R.H. Whitter was based on ecology--not ancestry and descent. However, the three multicellular kingdoms (Plantae, Animalia, and Fungi) can be cleaned up, so that they include all the living descendants of a single ancestor, by moving some traditional fungi, sponges and maybe cnidarians, and many multicellular algae into a catch-all Protoctista Kingdom (Protists). Then the protists include a much greater diversity of basic forms than the other 3 eukaryote kingdoms combined, and are only separated from the Moneran Kingdom by their grade of organization (having a "true" nucleus and not much else that's universal). Making it worse is the, now well-established, theory that the various eukaryote groups arose as symbioses of prokaryotes--not one but several different associations in different lineages of protists. >It seems to me that there should be quite a range of species' 'depths' >in the tree of life. > There are, easily illustrated by two examples: 1) Studies of divergence in ribosomal RNA (a highly conserved molecule) have shown more divergence within the bacterial genus _Bacillus_ than among the 3 big multicellular kingdoms above. 2) Most species on earth belong to only one class, Insecta. The perhaps 20 million members of this unwieldy group are mostly undescribed. [Any budding biologists out there who care to go down in history as naming more species than Linnaeus? Now is the chance, while there are still tropical rainforests to explore.] >It's kinda like this (apologies to the biologists! :-) : in a unix system, >there are files (species) at just a few levels below root, and there >are files twenty subdirectories below root. It would be highty suspicious >if most files were found exactly 7 subdirectories below root. > Just as tradition plays a role in UNIX (/usr, /tmp, /etc, seem to be pretty universal), it is much more important in biological classification. Systematicists might like the full classification to reflect the best current understanding, but the rest of biology *needs* a stable system for referring to groups and representing relationships. The need for stable names led to such legalistic documents as "The International Code of Botanical Nomenclature." On the other hand, cladists (or practitioners of "phylogenetic systematics") have little use for Linnaean groups or ranks. They focus on the points of separation between groups and summarize their results as dichotomous trees--defending their methods on philosophical grounds (see Ellliot Sober. 1988. Reconstructing the Past). Imagine trying to manually navigate through files on a large computer using a strictly binary directory structure! Even if group splitting is mostly simply seen as dichotomous branching, the higher level groups formed may not be very intuitive. Who wants to recognize a super-class of Chordate for every group of fish alive in the Devonian, just because one group gave rise to modern amphibians, reptiles, birds, and mammals? Cladistics dominates current research in systematics, but new taxonomic manuals continue to use the Linnaean hierarchy. No fully acceptable compromise has emerged. At best there is an uneasy truce. > >Confused but interested, . . . like most practicing biologists, I'd say.