Subject: Re: Scientific Names-we've got it all wrong (fwd) Date: Nov 29 15:32:56 1994 From: David Wright - wrightdb at PIGSTY.DENTAL.WASHINGTON.EDU
Regarding genera, families, and other higher taxa, it is true that the *ranks* "genus," "family" etc. are human constructs, but the taxa, the clades (monophyletic groups) that are labeled with these names are quite *real*. Clades exist independent of our ability to recognize them. It is the notion of *equivalent* levels ("categories" or "ranks") in the hierarchy of taxa that is artificial. As Bill Wimley alluded, the Linnaean categories that we still use today predate Darwin and Wallace by a hundred years (i.e., they are pre-evolutionary). The notion of equivalent ranks grossly distorts the way we view the evolution of biodiversity. Biologists for the most part pay lip service to the realization that rank is artificial, but nevertheless many comparative analyses at the "genus" or "family" or whatever "level" will be published this year. We use the nomenclatural system that was around when Mozart was a boy to describe biodiversity today, despite fundamental changes in how we view the genesis of that biodiversity.
Bill Wimley recommended that we shift away from the Linnaean system and move to one based on "quantitative genetic relatedness." I agree that we should dump Linnaeus, but these are really two different issues -- method of nomenclature vs. method of analysis of relationships. It is not necessary to move to a system based on genetic data in order to jettison Linnaean taxonomy. Moreover (but without wanting to completely open another can of worms...), it is not clear that a system based on (e.g.) genetic *distance* (e.g. DNA-DNA hybridization, UPGMA) even represents clades. DNA sequence data can be (and sometimes are) analyzed cladistically, though (but this is not genetic distance). Morphological characters can be (and nowadays usually are) analyzed cladistically, too. (And interestingly enough, cladistic analyses of the two kinds of datasets have the same ranges of degree of internal consistency, a yardstick of the "goodness" of the data.) My point here is that the method of phylogenetic analysis is more important than the kind of characters analyzed -- molecular vs. morphological. To get really robust cladograms we should look at both kinds of characters (and note that by including morphology you can include fossils and absolute time). Some people equate "Linnaean taxonomy" with "morphological systematics" but that is simply wrong. But regardless of the method of discovering clades it is desirable to *name* them once discovered so they can be conveniently referred to. Obviously it is possible to name clades without assigning them to the old Linnaean categories. A good paper that covers the Linnaean nomenclature issue (and why this taxonomy forces "splitting" and "lumping") is de Queiroz and Gauthier 1994. Trends in Ecol. & Evol 9:27-31.
On an issue Joe Morlan raised, it is often said that the species is the only "natural" category in the Linnaean hierarchy. It is true that species can be identified objectively, but which populations are recognized as species depends on which Species Concept is used to identify them. The Biological Species Concept (BSC) has held sway in evolutionary biology for about 50 years now. It is based on reproductive isolation. The Phylogenetic Species Concept (PSC) is younger. It is based on recognizing distinct *clades* (evolutionary "lineages"). Both of these species concepts are objective. Both are also arbitrary in that we must define the criteria used to identify which populations we call species. Which of these Species Concepts (or another, and there are others) you use depends on what properties you believe are most important. (For example, if you want a species concept that deals with asexual as well as sexual organisms you would choose the PSC over the BSC). Species as such are arbitrarily delimited and thus are, ironically enough, less "natural" than "higher" taxa (in the pure sense of clades, not categories). Species thus have more in common with the other Linnaean categories than might be apparent at first glance. A paper worth reading if you into pondering such issues is de Queiroz and Donohue. 1988. Cladistics 4:317-338.
A practical matter that is affected by the arbitrary nature of species definitions/concepts is the Endangered Species Act. Many of the populations currently regarded as "subspecies" using the BSC (including extrapolations such as "potential interbreeding") are simply "species" using the PSC. My understanding of the current ESA is that it protects populations, not just "full species." By doing so it simply (and wisely) sidesteps the issue of alternative species concepts.