dubois@uwmacc.UUCP (Paul DuBois) (05/16/85)
> [Lyle McElhaney] > Ah, but there is an agreed upon meaning of species. My eyes popped when I saw this. > To quote the paleontologist Roger J. Cuff[e]y: > "Low-rank taxa - the many species known to us - have a real existance > in nature, in that they consist of populations or morphologically similar, > actually and potentially interbreeding individuals which live during a > continuous segment of geologic time." > "On the other hand, high-rank taxa - those above species rank, from genera > up through phyla - do not have a real existance in nature in quite the same > sense that species do. Instead, higher taxa of various ranks are simply > the scientists' mental abstractions by which the many species comprising > the organic world are grouped according to the various degrees of over-all > morphological similarity displayed." What sort of species does Cuffey have in mind? Linnean, biological or evolutionary? Cuffey's definition [1p268] seems to be a mixture of the Linnean species (based primarily on morphological characters), and the biological species (based on reproductive characteristics) -- e.g., his definition is very close to one of Mayr's: "groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups." [2p19] Three problems arise immediately: (i) Cuffey includes both morphological and reproductive characteristics as criteria for species definition. But morphological similarity and reproductive compatibility are sometimes at odds with each other. The morphological variability of dogs belies the extreme interfertility of breeds. On the other hand, some (biological) species of Drosophila cannot cross, but are morphologically identical. (ii) The fact that both the Linnean and biological species classifications are subject to Gregg's paradox tends to invalidate the distinction made by Cuffey between the "real existence in nature" of species and the "mental abstraction" nature of higher-order taxa. (iii) Cuffey is a paleontologist. But how does he know which fossil was able to interbreed with which other fossil, particularly in light of (i)? Is Cuffey, then, talking about the evolutionary species? An evolutionary species is a lineage of ancestral descendant populations. This might account for his reference to the "continuous segment of geologic time." It might also explain why Cuffey makes his remark (below) about the desirability of making higher-rank classifications reflecting ancestor-descendant relationships. However, I think that this is unlikely for three reasons. (i) The lineage is missing in so many cases (i.e., it is the norm) that one can rarely know what the species consists of. The horse "phylogeny", for example, is so much of a mess that very few authors are willing to stick their neck out to get really specific about what descended from what. (ii) Even if the lineage were known, it would in most cases involve morphological change and cross reproductive barriers. What then becomes of Cuffey's criteria regarding morphology and reproduction? (iii) Assuming we descended from, e.g., bacteria (if not, substitute in the name of any proposed ancestor), one ends up with the problem of whether to call a bacteria Homo sapiens or to call a man some sort of Eucaryota. I confess that I'm confused as to exactly what Cuffey means. If Lyle can give some help here, it would be appreciated. Anyway, it should be clear that there is in fact no agreed-upon meaning of species. It is unreasonable simply to quote one person's definition and expect everyone to believe that all investigators agree with it. (Especially with me on the net! :-)) > "The practice has developed among modern taxonomists that higher rank > classifications, which are initially based upon observable degrees of > morphologic similarity between species, also should reflect evolutionary > ancestor-descendent relationships among those species as much as possible." Seen in phone booth in British Museum of Natural History: "for a good time, call Roger Cuffey at..." --- (As an aside: the original discussion to which Lyle is responding concerned a dispute regarding the (dis)agreement about finding "a" classification system. The variability of these "mental abstractions" (to which Cuffey makes reference) among various taxonomists and systematists tends to support the original point that the systems are highly subjective. (This is not a remark directed at the thrust of Lyle's posting. He seems concerned more with clarifying the status of the species.)) --- References [1] Roger J Cuffey, "Paleontologic Evidence and Organic Evolution", In _Science and Creationism_, Ashley Montagu ed, Oxford University Press, 1984, 255 - 281. [2] Ernst Mayr, _Animal Species and Evolution_, Belknap Press, Cambridge, 1963. Mayr seems to be quoting himself ("Speciation phenomena in birds", American Naturalist, 74, 1940, 249-278), but I couldn't find the quote in the article. -- | Paul DuBois {allegra,ihnp4,seismo}!uwvax!uwmacc!dubois --+-- | In the human instantiation, the image is the substance. |
ethan@utastro.UUCP (Ethan Vishniac) (05/20/85)
Actually, I'm not going to quote Paul's article at all. [My blow
against tree destruction! :-)]. I just have a few comments to make.
In practice it is usually possible to unamibiguously define separate
species that exist at the same time in the same general area. There
are many different criteria that scientists and nonscientists use and
with *very* few exceptions that all yield the same results.
Species separated geographically can frequently have an ambiguous status.
Species separated in time (e.g. homo sapiens and homo erectus) present
such problems that it becomes difficult to know where to draw lines. For
example one may note that discoverers of human fossils have tended to make
extreme distinctions between human fossils (probably because deviations
from the human norm strike us as more significant than deviations from
the wombat norm.)
I can't see how this presents evolution with any problem. Since creationists
believe in divinely ordained "kinds" these ambiguities do provide them with
considerable food for thought. However, I must admit that the conumdrum of
"kinds" and "species" plays absolutely no role in why I think creationism is
nonsense.
"Don't argue with a fool. Ethan Vishniac
Borrow his money." {charm,ut-sally,ut-ngp,noao}!utastro!ethan
Department of Astronomy
University of Texas