liquidspoon wrote:I just read a Wikipedia article about Echinoderms (a phylum of invertebrates that includes starfish and sea urchins). Apparently every species in the phylum has fivefold radial symmetry. Does anyone know where the five comes from, and why it would be preferred to other radial symmetries (3,4,6...). I would think that the lower number symmetries would be more likely to evolve - 3 should be more likely than 4 and so on. So 5 should have a really strong justification. Is there any fossil evidence of predecessor species with other radial symmetries?
The different symmetries arose early in the evolutionary process. I have no idea what is thought about why different symmetries got tried. But once established, your symmetry is not easily changed through the process of evolution, and so remains conserved across an entire lineage. Which is why everything in the entire phylum would have the same symmetry.
That said, there are other symmetries out there. http://en.wikipedia.org/wiki/Symmetry_(biology
) gives as examples that many jellyfish have a 4-fold symmetry, sea anemones and most corals have 6-fold symmetry, there is an order of corals (which is not really related to the 6-fold one) with 8-fold symmetry.
Interestingly 5-fold symmetry is very common in flowering plants (cut an apple crosswise for instance). I have no idea why.
Incidentally the idea that certain things (like symmetries) are evolutionarily conserved shows up in other apparently puzzling places.
For a famous example the vertebrate eye has a stupid arrangement with the nerves running in front of your photoreceptors, thereby blocking the light. And resulting in a blind spot where the nerves converge and dive down. This arrangement is not universal though - the octopus puts the nerves behind the eye and has no blind spot. Why, then, do we? Well in the primitive eye it is sheer chance where the nerves go and the eye is so inefficient that the placement of the nerves is not significant either way. But once established, that doesn't change.
For a far less famous example, Gould had an interesting essay on why the kiwi bird would produce such a large egg (15-20% of the mass of the adult bird) which I believe was based on http://www.jstor.org/pss/1307538
(unfortunately not a free article). The theory is that it is a result of allometric scaling. Allometric scaling is the principle that as a an organism scales up or down in size, each part scales up or down at a different rate. For instance in many species of insects a larger insect will have appendages that are larger relative to the body than a smaller insect, and this kind of relationship within a species tends to be conserved across species as well.
Anyways across kiwi species there is an allometric relationship between the size of the bird and the size of the egg where a smaller bird will have a larger egg relative to its mass than a larger bird. If you project that relationship out you will find that a 1.5 meter tall kiwi would have a normal sized egg. Therefore, the theory goes, the kiwi has a large egg because it is descended from a large ancestor that underwent evolutionary pressure to become small, and the allometric mechanics of becoming small left it with a large egg relative to body size. In support of this theory I note that the genetic evidence suggests that the kiwi is related to the New Zealand moa and the South American rhea (both large birds) and is in the same group as the African ostrich, and Australia's emu - all of which are large birds.
This example is important because it demonstrates that some puzzling characteristics may not have been evolutionarily selected for at all! They came about as a side product of some other trait that was selected for.