Wednesday, March 18, 2009

One helix to teach them all, and in the taxonomy bind them?

I originally saw this last summer in some free tourist guide, and neglected to write on it, but a little googling verified my memory. There is a game show on one of the channels now called "Are you smarter than a 5th grader", in which adults go up against 5th graders in a quiz show format, with the questions supposedly representative of that sample of elementary school. When I saw this particular item, my eyes rolled at first but then I pondered some more -- and realized that while I'd probably stick to my original position, it is a bit more nuanced than my first reaction.

Name 3 of the 5 kingdoms.

Okay, this was enough to generate an autonomic response. Back in high school we probably a good chunk of a class going over various Kingdom proposals. I don't have that textbook, but one of a similar strata would be my freshman bio textbook, Biological Science by Keeton & Gould, 4th Edition. K&G (p.1019) outlines eight different kingdom systems, ranging from 2 to 8 kingdoms.

Now, of course, one must ask what exactly is a kingdom? Ideally a kingdom would consist of a bunch of organisms with a common theme (which wouldn't be simply the lack of the all the themes of other kingdoms), all organisms with that theme would be in the kingdom, and no extant organism outside that kingdom would trace its ancestry to a member of that kingdom. At least, off the cuff, that is definition I would give.

So which one induced a reflex? It is the five kingdom system: Plant, Fungi, Protists, Animals & Monera, which it turns out is the one Keeton & Gould used for organizing their survey of the living world.

Now, it isn't an awful system, particularly back in the late '80s when I had it. Monera are all the single-celled thingies which lack a nucleus. Eukaryotes are what we know best, so they are subdivided into single celled (Protists), multi-cellular with cell walls & photosynthesis (Plants), multi-cellular, with cell walls but never photosynthetic (Fungi) and multi-cellular with no walls (Animals).

In that era, issues with these grouping were certainly recognized and taught. Yeasts clearly were related to Fungi, so they went there despite unicellularity. Some plants lack photosynthesis (e.g. dodder), but clearly this is a late loss and they belong in Plants. Protists is a handy way to lasso all sorts of traditional problems such as Euglena, which both photosynthesizes and moves.

But, what was just emerging when I was taught these things, but is now quite evident, is that the non-nucleated world is really two worlds, Eubacteria and Archea. While they both have many similarities (such as mostly circular chromosomes), they are very, very different in other fundamental cellular processes, such as RNA transcription. Plus, now we have DNA & RNA phylogenetic methods which show them to have diverged very long ago.

There are other issues DNA methods have illuminated. Protists are not an evolutionarily coherent group but are instead a mishmash of various lineages ("polyphyletic"). Eukaryotes as a whole don't fit a simple tree lineage, due to multiple endosymbiont captures resulting in organelles such as mitochondria and chloroplasts (and perhaps more).

Which asks the question: what should we be teaching 5th graders? My reflex reaction is that we shouldn't teach them things they'll need to unlearn later, and the Monera kingdom concept is just not a very good one in the light of molecular phylogenies. But, what my further pondering brought up is one goal of science education is to teach students to methods of science rather than just rote facts. Given a microscope or some photographs, it is pretty easy to teach a young student how to classify organisms into the 5 kingdom system. Trying to explain why archea and eubacteria should be in different groups isn't so easy. Okay, a lot of archea have pretty wierd lifestyles (insanely low pH, even more insanely high heavy metal content, boiling water, etc), but not all do. Just being strange to us isn't really a useful way to categorize.

On the other hand, perhaps at least the notion of molecular classification can be introduced early. Granted, it's an N of 1, but I've successfully shown that you can teach the concept to a 3rd grader. It's also something which can be easy to diagram out & count -- with (obviously!) only a subset of informative positions. And in the end, wouldn't that be the best science lesson of all -- that things which look superficially alike may have an underlying, nearly hidden great difference?

Of course, the hardest part of any change is getting change. It appears that a generation of science teachers have been taught the 5 kingdom system, and so will need to be updated. Numerous textbooks probably also encapsulate this archaic (but not archean! :-) concept. Probably the hardest to change will be those statewide curriculum standards or standardized tests which contain these phylogenetic fossils.

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