The Term 'Species'
Posted on: October 12, 2008 - 4:35am
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The Term 'Species'
...I hear two opposing arguments constantly:
One is that species is an outdated and more or less useless term anymore, the other is what is posited in the video above. Could someone elaborate on this a bit? If species is no longer a good definition, why is it still used in academia?
Quote:
"Natasha has just come up to the window from the courtyard and opened it wider so that the air may enter more freely into my room. I can see the bright green strip of grass beneath the wall, and the clear blue sky above the wall, and sunlight everywhere. Life is beautiful. Let the future generations cleanse it of all evil, oppression and violence, and enjoy it to the full."
- Leon Trotsky, Last Will & Testament
February 27, 1940
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Excellent video, covers all the crucial points pretty well.
The idea of species is not really useless, just a bit fuzzier that has been traditionally considered. There is of course a continuum in the degree of difference between any two lineages of organisms - we see examples of very close similarity where the two lineages can still interbreed, out thru successively greater degrees of differences which reduce the likelihood of successful interbreeding, all the way out to entirely distinct lineages.
The line where the possibility of a fruitful mating drops below some inevitably arbitrary threshhold becomes the point where we declare the two lineages separate species. As long as we keep in mind that it does no longer indicate an absolute distinction , but a measure of the degree to which any lineage has diverged from other closely related, it helps us organise our thoughts.
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I bitch a lot about species, but mainly because it is so often misused and misunderstood by creationists. If everybody used it only so far as it is useful, it would be fine.
"Species" is actually just one division in the linnaean taxonomy structure that you probably learned in gradeschool: Kingdom, Phylum, Class, Order, Family, Genus, Species. This structure is not perfect, but these days, it's pretty good. There are some organisms which are very rare and have never been subjected to thorough DNA analysis. However, there are generally pretty good methods for guessing where they belong. The International Commission on Zoological Nomenclature is the ruling body where there is a difference of opinion or a problem with older assignment. See there website: http://www.iczn.org/
Note that the ICZN was formed in 1895. Enough said.
In cladistic taxonomy, organisms are grouped evolutionarily based on genetic information. The three main kinds of classifications are polyphylectic, monophylectic, and paraphylectic. Birds and reptiles are descended from a common ancestor, and are considered a monophylectic group. Reptiles are a paraphylectic group because they all have a common ancestor but do not represent all of the descendants of that ancestor. (Birds, eg.) Polyphylectic groups share traits but not the most recent common ancestor. Polyphylectic groups are often representative of linnaean groupings.
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Great video. I bitch about the term species a great deal myself, largely going back to a talk by Richard Lewontin (Gould's partner), in which he said that he thought that species were "kinds". Just like the Bible.
It's that kind of intellectual laziness in otherwise brilliant persons that has kept the term at a nineteenth century level of usefulness. Darwin devotes the first two chapters of On the Origin of Species to the concept of variation, and calmly argues against the Platonic essentialism that Linnaeus worked from (and God cleverly anticipated, evidently) by citing an exhaustive list of examples. Boring, but effective. He ends with the conclusion that defining species is fraught with opinion. But Platonic essentialism never went away and the scientists of the twentieth century spent lots and lots of time trying to create a Biological Species Concept (capitalization intended) with a solid means of quantification.
It's the opinion of plenty of biologists that we aren't there yet. I'm of the opinion that "species" should be maintained to the extent that it remains useful and is going to undergo a huge reevaluation when genomic biology is cheap and fast enough to weigh in at the population level.
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It occurs to me that "species" or something like it will stick around for a long time simply because the basics of life are rather counterintuitive, even to many scientists. That is, we can clearly look at a tree frog and an octopus and see that they are different. They belong in two separate boxes. For that matter, it's kind of silly not to separate octopi and squid, even though they kind of look like each other, and they both swim around in salt water. Calling a tuna the same thing as a chichlid is absurd, even though they are obviously very similar in categorical ways.
No matter which nomenclature we use, we are still delineating between groups of organisms. The nature of life is such that no such strict boundaries exist as platonic realities.
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