More on Darwin
Aug. 19th, 2003 08:37 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
tarigwaemirAd Mundo Exteriore,
The Gaara skin, finally up, though I finished it last week. This one is dedicated to Dean, a.k.a. xseiferothx, because (1) he requested it, (2) he supplied pictures for it, (3) he nagged me to no end until I went ahead and finished it, and (4) he introduced me to the series and kept me supplied with episodes. O_O Scary boy.
A week or two ago, I said that there is no consistent scheme for classifying species, which is not entirely correct. The cladistic approach to taxonomy is extremely rigorous--note the word "extreme"--but leads easily into absurdities. I remember that Ernst Mayr had a long passage examining the pros and cons of cladistics in This is Biology, but I didn't understand most of it and yawned through the rest. >_< Eh, one of the books I should definitely read before the end of freshman year.
Nevertheless, it's easy to see how cladistics can quickly become quite ridiculous. The basic principle is that each taxonomic group is distinguished from the parent branch by one key differing characteristic. So vertebrates are distinguished from the rest of the animals by the presence of a backbone; mammals further distinguished from the rest of the vertebrates by possessing hair as a body covering; placentals determined by the, well, placenta connecting fetus to mother...the flaw is obvious: when there is more than one key characteristic in common throughout a group, how do you choose?
It becomes especially knotty if you imagine two groups A and B arranged in an overlapping Venn diagram. Label the overlap C. Members of group A all have trait A in common, members of group B all have trait B in common. Where do you put the members of group C?
Another issue with cladistics is implicit in a comment of Darwin's: "In genera having more than the average number of species in any country, the species of these genera have more than the average number of varieties. In large genera the species are apt to be closely, but unequally allied together, forming little clusters round certain species." He interprets this trend to mean that since there has already been more speciation in a larger genus than a smaller one, it is unsurprising to find that there is more ongoing speciation in the larger genus (as seen in the presence of more varieties within the species of the genus). Varieties, Darwin argues (very convincingly, I might add), are "incipient" species, or potential species in formation. But getting back to the original point, the statement brings up another important problem for cladistics: the relative "importance" of the key characteristic. After all, Darwin notes that the degree of variation among the species in larger genera is much less than in smaller genera. Undoubtedly, the subjective aspect is pretty important here. For species in larger genera, the criteria for division into another clade does not require the same degree of variation as it would for species in smaller genera. Slight contrasts become much more noticeable when you have a larger range of species, I suppose. Again, a matter of perspective. Thus, cladistics can never be as rigorous as it wants to be, which sort of undermines the whole theory behind the approach.
We would like to say that our classification system is as objective as possible, that it accurately describes the diversity of species. But ultimately, it's a matter of utility. When biology was still natural history, appearance and habitat was mostly what mattered. Once the naturalists started dissecting organisms and looking at skeletons, the differences in internal body structure became more important. Now, with the advent of genetics, it's all about the genome sequences. We now say that Archaea are just as different from Bacteria as they are from eukaryotic organisms, as counterintuitive as that sounds, because our priorities for classification have shifted from cell structure to ribosomal RNA. (Anyone notice the microscoping process here? It happened in physics too.)
Anyway, I'm not really arguing properly because Darwin is looking from the bottom up, so to speak, because he's touching upon the question of how to determine species in the first place, while cladistics is from the top down, how to split up the great kingdoms into smaller and smaller categories. Furthermore, large-scale taxonomy usually treats species as indivisible units (well, loosely speaking, if we ignore subspecies), like marbles to be tossed into the proper bins. It doesn't bother to angst over whether two marbles are actually one marble or even one and a half marbles, (for that matter, whether one marble is in the process of splitting into two or not). And cladistics, when it is useful, is best applied to large-scale taxonomy. A good example is the fourth floor of the AMNH.
I think the most deeply rooted problem is that taxonomy began with Linnaeus, who preceded Darwin and other evolutionists, and it still tends to operate under the assumption that species are now static. Although taxonomy does reflect evolutionary history (well, we think it does anyway, despite the subjective factor), it often forgets that species are still in the process of changing. (Look, I personified taxonomy! Urgh. >_<) I mean, whether you agree with the punctuated equilibrium theory or not, evolution still takes place in ultra slow motion, and it's easy to treat species as, well, predetermined elements. And let's not forget that transmutation is technically possible through radioactive decay.
Um, I think I should stop before I start mixing up my metaphors and analogies to the point of no return. I'm restating the obvious for the most part, but I did want to note down the Darwin quote before I forgot.
Speaking of quotes, how about this one: "[F]or instance, on a piece of ground three feet long and two feet wide, dug and cleared, and where there could be no choking from other plants, I marked all the seedlings of our native weeds as they came up, and out of the 357 no less than 295 were destroyed, chiefly by slugs and insects. If turf which has long been mown, and the case would be the same with turf closely browsed by quadrupeds, be let to grow, the more vigorous plants gradually kill the less vigorous, though fully grown plants: thus out of twenty species growing on a little plot of turf (three feet by four) nine species perished from the other species being allowed to grow up freely." (from "Struggle For Existence", in Origin)
Can you believe that Darwin actually counted and tagged all 357 plants?! And that he kept track of which ones died and which survived? What kind of patience (::cough:: obstinacy) does that kind of observation take? I'm so impressed. I knew Darwin was meticulous, but I didn't know he was such an excellent researcher. It's also a good precedent for modern ecological studies, neh? Perhaps this sort of study was a normal pursuit for a Victorian gentleman with a strong interest in science and natural history, but it amazes me nonetheless.
Part of the reason why my progress through Origin is so abominably slow is that I spend hours on a page, thinking along silly tangents in this fashion. Argh. There are fourteen chapters, and I've already read three. I should go up to chapter nine by the end of today if I want to finish it by tomorrow. ::begins reading spurt now::
...Tari
The Gaara skin, finally up, though I finished it last week. This one is dedicated to Dean, a.k.a. xseiferothx, because (1) he requested it, (2) he supplied pictures for it, (3) he nagged me to no end until I went ahead and finished it, and (4) he introduced me to the series and kept me supplied with episodes. O_O Scary boy.
A week or two ago, I said that there is no consistent scheme for classifying species, which is not entirely correct. The cladistic approach to taxonomy is extremely rigorous--note the word "extreme"--but leads easily into absurdities. I remember that Ernst Mayr had a long passage examining the pros and cons of cladistics in This is Biology, but I didn't understand most of it and yawned through the rest. >_< Eh, one of the books I should definitely read before the end of freshman year.
Nevertheless, it's easy to see how cladistics can quickly become quite ridiculous. The basic principle is that each taxonomic group is distinguished from the parent branch by one key differing characteristic. So vertebrates are distinguished from the rest of the animals by the presence of a backbone; mammals further distinguished from the rest of the vertebrates by possessing hair as a body covering; placentals determined by the, well, placenta connecting fetus to mother...the flaw is obvious: when there is more than one key characteristic in common throughout a group, how do you choose?
It becomes especially knotty if you imagine two groups A and B arranged in an overlapping Venn diagram. Label the overlap C. Members of group A all have trait A in common, members of group B all have trait B in common. Where do you put the members of group C?
Another issue with cladistics is implicit in a comment of Darwin's: "In genera having more than the average number of species in any country, the species of these genera have more than the average number of varieties. In large genera the species are apt to be closely, but unequally allied together, forming little clusters round certain species." He interprets this trend to mean that since there has already been more speciation in a larger genus than a smaller one, it is unsurprising to find that there is more ongoing speciation in the larger genus (as seen in the presence of more varieties within the species of the genus). Varieties, Darwin argues (very convincingly, I might add), are "incipient" species, or potential species in formation. But getting back to the original point, the statement brings up another important problem for cladistics: the relative "importance" of the key characteristic. After all, Darwin notes that the degree of variation among the species in larger genera is much less than in smaller genera. Undoubtedly, the subjective aspect is pretty important here. For species in larger genera, the criteria for division into another clade does not require the same degree of variation as it would for species in smaller genera. Slight contrasts become much more noticeable when you have a larger range of species, I suppose. Again, a matter of perspective. Thus, cladistics can never be as rigorous as it wants to be, which sort of undermines the whole theory behind the approach.
We would like to say that our classification system is as objective as possible, that it accurately describes the diversity of species. But ultimately, it's a matter of utility. When biology was still natural history, appearance and habitat was mostly what mattered. Once the naturalists started dissecting organisms and looking at skeletons, the differences in internal body structure became more important. Now, with the advent of genetics, it's all about the genome sequences. We now say that Archaea are just as different from Bacteria as they are from eukaryotic organisms, as counterintuitive as that sounds, because our priorities for classification have shifted from cell structure to ribosomal RNA. (Anyone notice the microscoping process here? It happened in physics too.)
Anyway, I'm not really arguing properly because Darwin is looking from the bottom up, so to speak, because he's touching upon the question of how to determine species in the first place, while cladistics is from the top down, how to split up the great kingdoms into smaller and smaller categories. Furthermore, large-scale taxonomy usually treats species as indivisible units (well, loosely speaking, if we ignore subspecies), like marbles to be tossed into the proper bins. It doesn't bother to angst over whether two marbles are actually one marble or even one and a half marbles, (for that matter, whether one marble is in the process of splitting into two or not). And cladistics, when it is useful, is best applied to large-scale taxonomy. A good example is the fourth floor of the AMNH.
I think the most deeply rooted problem is that taxonomy began with Linnaeus, who preceded Darwin and other evolutionists, and it still tends to operate under the assumption that species are now static. Although taxonomy does reflect evolutionary history (well, we think it does anyway, despite the subjective factor), it often forgets that species are still in the process of changing. (Look, I personified taxonomy! Urgh. >_<) I mean, whether you agree with the punctuated equilibrium theory or not, evolution still takes place in ultra slow motion, and it's easy to treat species as, well, predetermined elements. And let's not forget that transmutation is technically possible through radioactive decay.
Um, I think I should stop before I start mixing up my metaphors and analogies to the point of no return. I'm restating the obvious for the most part, but I did want to note down the Darwin quote before I forgot.
Speaking of quotes, how about this one: "[F]or instance, on a piece of ground three feet long and two feet wide, dug and cleared, and where there could be no choking from other plants, I marked all the seedlings of our native weeds as they came up, and out of the 357 no less than 295 were destroyed, chiefly by slugs and insects. If turf which has long been mown, and the case would be the same with turf closely browsed by quadrupeds, be let to grow, the more vigorous plants gradually kill the less vigorous, though fully grown plants: thus out of twenty species growing on a little plot of turf (three feet by four) nine species perished from the other species being allowed to grow up freely." (from "Struggle For Existence", in Origin)
Can you believe that Darwin actually counted and tagged all 357 plants?! And that he kept track of which ones died and which survived? What kind of patience (::cough:: obstinacy) does that kind of observation take? I'm so impressed. I knew Darwin was meticulous, but I didn't know he was such an excellent researcher. It's also a good precedent for modern ecological studies, neh? Perhaps this sort of study was a normal pursuit for a Victorian gentleman with a strong interest in science and natural history, but it amazes me nonetheless.
Part of the reason why my progress through Origin is so abominably slow is that I spend hours on a page, thinking along silly tangents in this fashion. Argh. There are fourteen chapters, and I've already read three. I should go up to chapter nine by the end of today if I want to finish it by tomorrow. ::begins reading spurt now::
...Tari
![[identity profile]](https://www.dreamwidth.org/img/silk/identity/openid.png){kind=small}
(no subject)
Date: 2003-08-19 07:32 pm (UTC)(no subject)
Date: 2003-08-21 04:52 pm (UTC)...Tari
(no subject)
Date: 2003-08-21 12:23 pm (UTC)Ha, for a moment I had this flash thought: has Hana been taken over by Prof. X? Because I thought that logo was a silver X instead of that infinity sign thingy. Ha.
Okay. I'll stop now.
O.o
(no subject)
Date: 2003-08-21 04:55 pm (UTC)...Tari