And the answer is…
Ahem, I don’t really know. I was just reading a paper published in Journal of Evolutionary Biology*, and it appears that mosses were qualified by a bryophyte systematicist as early as in 1972 as evolutionary sphinxes. Well, at least we’re not left completely out of touch, since these plantee-evo-mythic beasts were then defined as ‘unmoving, unchanging sphinxes of the past‘. (As a funny remark, it is interesting to note that it’s truer than it appears, given mosses are rather sedentary organisms even if clones can spread a little bit through vegetative multiplication).
And that, beyond the results discussed in this study per se, is rather interesting. This is the first time I’ve ever read about evolutionary sphinxes, and it sounds as a brand new concept weighting deep in mystery (ghosh! the ghost of exciting science comes back!). But it isn’t new (Okay, everything is relative, but an expression coming out directly from the 70s cannot be considered brand new, scientifically speaking). On the other hand, anybody would say she’s somewhat heard about “living fossils” in a way or another. That’s because a living fossil (a species morphologically very close to known fossils, that doesn’t appear to have evolved much) made it through science and science popularization successfully, while evolutionary sphinx did not. And that’s amazing, because the two expressions seem rather close, conceptually speaking. Even more surprising, evolutionary sphinx has an immediate appeal to mystery and further enigma to solve, and matches more precisely the scientific wonders science has to offer (even if a living fossil also does, it does because it is successful in framing the wonder of the fact).
Why would it be so? I don’t know for sure, but I am enclined to think that speaking about living fossils, the coelacanth comes to mind first. Did you notice? It’s an animal. Maybe that’s the raw reason why living fossil entered common discourse through the big bright door, while the evolutionary sphinx, sticking to static and less beloved plants, left the race and got forgotten.
I think this is where any scientific debate about successful framing should begin first. The fate of a ‘frame’ appears to be strongly dependent on the ground it is first born, and its roots are of importance (hey, plants are fabulous for framing too!). Before trying hard to seed an unfertile ground of maladapted frames, one should wonder why most successful frames are simplistic (and even wrong) assertions first, feeding from a mineral -yep, fossilized- ideological agenda. Is it really something science can take upon? Not sure.
— Going back to mosses, the study that inspired this post has some interesting points. Mosses appear to have ‘genes’ evolving slower than other groups of plants (crudely, the rate of nucleotide substitution is significantly lower compared to algae or flowering plants, or even compared to ferns). This is the first study to document a correlation between a slower genome evolution and a slower morphological evolution, and in these terms, mosses indeed are evolutionary sphinxes. The reasons for this are unclear. Maybe the fact that mosses are living a relatively important part of their lives in an haploid stage has something to do with it, though non-synonymous substitutions (that is, the mutating substitutions that change something in proteins and are expected to be of selective importance) are not different from that of the other groups. Moreover, it is not clear how a difference in the indirect potential selective effects (like fixing beneficial mutations faster and driving associated sequences to fixation through hitch hiking effects) should translate into a slowed down genome evolution. Even more, the result seems to stand in for plastid and mitochondrial genomes. But then, the ploidy effect speculation is harder to analyse (I am not sure how far differences in organella numbers are documented and allow for further investigation of any copy number effect). Well, you guessed, this is something that deepens the homework left to do… But that’s science after all. Someday the sphinx will tell you!
* H. K. Stenoien (2008). Slow molecular evolution in 18S rDNA, rbcL and nad5 genes of mosses compared with higher plants. Journal of Evolutionary Biology, 21: 566-571.