miciah, in his search for convergence and having failed at demonstrating how the pax6 gene helps his argument, suggested other features, and the one he seemed most interested in was echolocation, particularly in bats and dolphins. To start with, all that is convergent about these is that both use sound in order to detect the location and other properties of objects. The question then, is how similar are the methods that they actually use. the answer, is not very.
All microbats echolocate, the sound they make is a chirp of varying frequency from high to low(interestingly humans use this in very sophisticated radar banks, it's called frequency modulation). Dolphins however use clicks.
As bats get close to their target, the reflections will be louder and louder. to compensate for this, the hearing systems of the bat become less sensitive by reducing the tension on the tympanic membrane and adjustment of the angle at which the stapes meets the oval window. As Dolphins get closer to their target, they face the same problem, however in their case, they modulate the volume of their clicks.
Bats sense the returned sound through their ears,
Dolphins sense the returned sound through their lower jaw.
So already we can see that there are a large number of differences in these echolocation methods, so there is little claim for convergence, especially given that
many blind people can echolocate too.
so all we really have in the case of echolocation that is similar between bats and dolphins, is the modification of an already existing system (hearing) in order to be more fine tuned. The adaptations that both have undergone are completely different, particularly when you look at the auditory systems. I have already mentioned a couple of ways the bats fine tune their auditory systems though adjustment of membrane tensions and bone positions, but dolphins can't do this. The dolphin ear is a very small hole and basically irrelevant in dolphin hearing. Dolphins detect the sound through a fluid filled cavity in the jawbone, which transmits the sounds to the auditory bulla. this whole area is quite different from terrestrial mammals. The auditory bulla is a bone case that is isolated from the rest of the skull by fibrous tissue. So the actual hearing systems are very different between dolphins and bats also (interestingly, we see intermediates between the hearing systems of terrestrial and aquatic cetaceans as we examine the fossil record)
so where is the claim of convergance? that both use sound to find where they are?