On Sat, Dec 13, 2008 at 3:30 PM, Olin Lathrop wrote: > M. Adam Davis wrote: >> Our eyes converge because they can > > You have absolutely no way of knowing that, and in fact it's highly > unlikely. So very many species, some quite distantly related, have pivoting > eyes. There clearly was significant selection pressure to maintain this > complex system. This in turn says that being able to swivel the eyes is a > competitive advantage. In other words, it's useful. > > So let's say it correctly: "Our eyes move because that has survival value". I don't see how your statement refutes mine, but there's nothing wrong with your statement either... so ... sure, that sounds good. >> the primary reason for eye >> movement to to avoid head movement. Convergence is simply "icing on >> the cake" - a free tool we can use. > > More wild speculation. You can not say why a feature evolved. At best you > can say that it appears to have certain advantages currently. These may be > rather different from those that caused the feature to originally evolve. > > It's likely that stereo vision and the resulting better 3D comprehension of > one's surroundings is a survival advantage. Allowing the eyes to converge > reduces the processing power required for 3D perception and applies the high > resolution area of the image sensor to the subject being observed. > > While there is no way to prove that's what caused the selection pressure for > this mechanisms, it is at least a plausible hypothesis. It is quite > unlikely this is merely a "free tool" that is "icing on the cake". If it > were, it would very unlikely have been selected for so many different times. I think you're putting the cart before the horse. I'm saying that the evolutionary pressure for movable eyes without having to move the head is likely greater than the evolutionary pressure for convergence. It appears to me that you are saying that convergence is a first class feature - in other words that the pressure to have convergence is greater, or perhaps equal to, the pressure to have eyes that move without moving the head. If that's the case, then show me one or more animals that exhibit convergence and stero vision that doesn't have eye movement beyond what's required for convergence. Otherwise, I'll sit by my unproven speculation and say that convergence is a additional feature we are able to use because we've already selected for eye movement that doesn't require head movement. But yes, I'm _wildly_ speculating based only on my limited experience, understanding, and thought experiments. Your argument, however, is unconvincing. Give me an animal with convergence and no additional eye movement and I'll revisit it. At the moment, however, every stereo vision animal I know that exhibits convergence also has eye movement beyond convergence, and it appears to me that this is more valuable (and would have a higher selection pressure) than convergence. -Adam -- Please rate and vote for my contest entry: http://mypic32.com/web/guest/profiles?profileID=50331 -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist