Often, as I've looked at my embryonic zebrafish, I've noticed their prominent median fins. You can see them in this image, although it really doesn't do them justice—they're thin, membranous folds that make the tail paddle-shaped.
These midline fins are everywhere in fish—lampreys have them, sharks have them, teleosts have them, and we've got traces of them in the fossil record. Midline fins are more common and more primitive, yet usually its the paired fins, the pelvic and pectoral fins, that get all the attention, because they are cousins to our paired limbs…and of course, we completely lack any midline fins. A story is beginning to emerge, though, that shows that midline fin development and evolution is a wonderful example of a general principle: modularity and the reuse of hierarchies of genes.
Continue reading "Evolution of median fins" (on Pharyngula)