Friday, July 25, 2008

Vary. Vary. Good.

I just read Darren's post on the living and fossil species of Tursiops. (The skeleton above is that of Lagenorhynchus, stolen from here.) Was surprised by this:

The large, offshore dolphins are now called T. truncatus Montagu, 1821, and the smaller, nearshore ones are T. aduncus Ehrenberg, 1832. T. truncatus is longer-bodied (with 62-67 vertebrae, as opposed to 59-62 in T. aduncus) and has a proportionally shorter rostrum.

That's a range of 8 vertebral positions with the genus (whatever that is), and 5 just within T. truncatus. That's a lot. In most tetrapods, it's not unusual to find individuals that vary by one or two positions from their conspecifics. Humans usually have 7 cervicals, 12 thoracics, 5 lumbars, 5 sacrals, and four coccygeals (caudals), for a total of 33, but according to White and Folkens (2000) as many as 1 in 10 people may vary from that norm. Most of the people who do vary do so at the lumbo-sacral junction; for example, having 4 lumbars and 6 sacrals or vice versa. But that wouldn't change the total count 33. My guess is that of the people who actually have a different total number of vertebrae, the difference is in the coccyx. It's tempting to write that off as a consequence of our caudal vertebrae being vestigial, but I immediately wonder if caudal vertebrae are not inherently more variable in number (than other kinds of vertebrae) in other tetrapods. I mean, having a tail that is one vert longer or shorter is probably going entail a lot fewer changes in the rest of the body than having a different count in the neck, thorax, or sacrum.

Which brings me back to the dolphins. I am curious about how much of that 8-vertebra spread is accounted for by varying numbers of caudal vertebrae.

One question that might come to mind is how one would tell. In most tetrapods it's easy: start counting at the end of the sacrum, and keep going until you run out of vertebrae to count. But extant cetaceans--even those with pelvic elements, like most baleen whales--don't have sacral vertebrae. So figuring out where the tail starts is a bit of a pain. I read a paper on it once, which I don't have to hand and am not going to dig up, and the upshot is that even though the sacral ribs that define the bony sacrum are gone, you can still identify the base of the tail based on patterns of blood vessels and nerves. Which is fine when you're dissecting a dolphin (I'd love to but never have), but not much use when you're just looking at a skeleton. If it's a complete articulated skeleton, you could use the haemal arches to get you in the right neighborhood, but the haemal arches are probably variable themselves, and might not be present anyway.

If I was a more dedicated blogger I'd be citing more refs and getting your more answers and fewer guesses. But I've got other fish to fry.

I know that the argument from personal incredulity is weak, but I just can't believe that it's wrong all the time. And, despite my near-total ignorance of vertebral variation in most tetrapods and in cetaceans specifically, a 5-vertebra range of variation strikes me as too much for one species. And since Darren's post touches on variation, cryptic species, and the splitting of Tursiops anyway, it made me wonder if some of that range of variation in T. truncatus isn't actually parceled out among multiple species.

But that's not the actual thought that went through my head. The thought that went through my head was "How do you tell individual variation from biodiversity?"

And the answer, of course, is that you can't. Individual variation IS biodiversity. I've just been teaching my intro bio students about Darwin's views on variation and the origin of species. For Darwin it's all one big continuum, from individual variation to incipient varieties to varieties to subspecies to species to genera and so on up to the entire tree of life. People sometimes rag on Darwin for writing On the Origin of Species without actually addressing speciation. But I think that's because we think species are special (well, not all of us), and must come into existence through some process that is not just business-as-usual. But Darwin didn't think species were special. He worked out the logic of natural selection and showed that it worked. And he argued that organisms struggle most fiercely with those to which they are most similar, usually their closest relatives. Natural selection gives you a mechanism for change, and struggle among the similar gives you a mechanism for diversification (if the struggle is most intense against those to whom you are most similar, it has to be less intense elsewhere). And that's all you need (according to Darwin [according to my reading]). As things diversify they pick up different mate recognition signals or lose the ability to interbreed or do whatever it is that defines them as species to us, but those are effects of diversification that is already in motion, that starts with variation among individuals.

Not everyone agrees. For all the stuff they disagreed about, Ernst Mayr and Stephen Jay Gould were both convinced that species are special. And even if Darwin was right, that doesn't mean that speciation isn't interesting and worth studying. Only maybe we should call it lineage divergence rather than speciation so we can stop pretending that we know what a species is. But my point is that it is all connected, all the way up--from individual variation up to the whole tree of life--and back down again--because there is no tree of life, not as a Ding an sich (thank you, Great Books Discussion Group). There are only individuals. There is only variation.

White, T.D., and Folkens, P.A. 2000. Human Osteology, Second Edition. Academic Press, New York.

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Blogger Neil said...

Two random observations:

- Natural variability in vertebral counts has often been exploited by livestock breeders, e.g. American Landrace Swine. Mmmm...pork chops.

- Now I'm wondering about genus and species level variability in Ichthyosaur vertebral counts, given their propensity for multiplying the number over evolutionary time. Mmmm...ichthyosaur chops.

wait, another random thought:

There's some "Evo-devo", tagmatization, homeobox stuff going on here too right?

1:49 PM  
Blogger Dr. Vector said...

There's some "Evo-devo", tagmatization, homeobox stuff going on here too right?

Yeah, by definition. And that would be the ideal arena in which to try to answer the question of how vertebral counts come to be more or less variable. (Whether can be decided on the basis of bones, and why is one o' them tricky evolution-purpose-teleology things that people get all riled up about.)

12:47 AM  

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