Monday, August 24, 2009

Cladding About

My last post in this short series about the classification of dinosaurs, I wrote about the Linnaean system of classification, which uses physical characteristics to arrange organisms into clans. It's the source of binomial nomenclature, the convention that gives us those familiar "scientific names" for organisms; a wolf is known as Canis lupus.

Now we'll deal with cladistics, a method of biological classification which uses evolutionary relationships to create a sort of family tree of life. It was developed in the middle of the last century by German entomologist Willi Hennig. He's the fellow there to the right (and if you know a good way to caption right- and left-justified images on blogger, fill me in).

Anyway, a graphic representation of this tree is called a cladogram. I'll use a cladogram of the dinosauria for an example; it comes from the useful, easily-navigated, longtime bastion of dinosaur knowledge on-line, Jeff Poling's omnipedia at The page for the cladogram is here; follow this link or click the image to see it full size.

Cladogram copyright Jeff Poling

What you see here is a tree- or bush-like shape. Each intersection, or node, between branches represents a shared ancestor for all branches that follow the split. The cladist is not interested in building a "top-down" system with large, grandly named categories containing a huge number of organisms. The major unit of classification is the clade. This means that every node signifies a clade, and that traditional terms such as "reptile" don't make as much sense as they used to. Since a clade is defined as "All organisms derived from the most recent common ancestor of organisms X and Y," descendants with radically different morphological traits cause a bit of a problem when "translating" traditional Linnaean groups to cladistics.

For instance, we might try to say that "reptiles" are the first reptile and all of its descendants on a cladogram. Assuming that birds are descendants of small Jurassic theropod dinosaurs, and that the original dinosaurs derived from Permian reptiles, birds are actually reptiles. The Linnaean method, using physical traits, clearly states that reptiles are scaly, egg-laying, cold-blooded tetrapods. Birds are bi-pedal, warm-blooded, egg-laying, and covered in feathers.

Dinosaurs represent an interesting puzzle, then. As does reconciling the fat and happy Linnaean system with lean, mean cladistics. But we'll save that for the next post.

As a side note, I really cannot say enough nice things about It is a great example of the way the internet worked before advertising became so prominent. Nothing flashy. Everything loads quickly. Well-written, clearly explained information. Accessible. Mega-props to Mr. Poling.

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