Re: Predicting eyes

David Campbell (
Wed, 19 Nov 1997 17:06:09 -0400

>You wrote,
Actually, I wrote, not Don.
>"Bilateral symmetry is predictable from evolution-it makes sense for a mobile
>organism to specialize so as to have a front and a back end. Sense organs
>are most useful up front, to see/smell/taste/feel/hear/etc. where you are
>going, so having eyes symmetrically distributed on the front end makes sense.
> Two eyes rather than one medial eye allows better vision to
>each side, and if the eye is good enough, can allow binocular vision. Since
>we're usually going forwards, extra eyes in back have not been a particular
>advantage, and the pineal "eye" is reduced or lost in many vertebrate
>lineages. Additionally, there's the "other mammals have two eyes so we are
>likely to maintain this pattern" line of reasoning, which is less a priori.
> Directionality is probably important in the presence of more than one eye-we
>also have two ears, but one nose and one mouth-light and sound have more
>direction to them than smell or taste."
>I find it difficult to understand why bilateral symmetry, as you so ably
>outlined it, is predictable from evolution. The only argument you give in
>support of your assertion is that "it makes sense." I don't see how you can
>justify evolution with this argument? You offer no mechanism as to how
>evolution brought bilateral symmetry about. Natural selection?
> Non-selective evolution? Did it all occur by myriads of unguided
>happenstances? I would like to see you support your "makes sense" with some
>substantial arguments about just how evolution brings about bilateral
My wording was certainly quite vague.
Are you asking about the mutations involved in generating the
symmetry or the selection that would make the mutant survive? If so, I do
not know if anyone has studied the relevant genes adequately to answer
this; a good model system may be comparing the Venus' girdle ctenophore
(comb jelly) with more normal, radially symmetric forms, but flatworm
versus cnidarian development would be more important as that's probably
close to the actual point at which bilateral symmetry evolved as a major
animal body plan.
If an organism with more or less radial symmetry (typical of lower
animals, so presumably the starting point for evolving bilateral symmetry)
evolved bilateral symmetry. If a mobile radial organism mutated so as to
be a bit elongate, it would be more streamlined for travel in two
directions, though less suited for going sideways. Mutations improving the
ability to move forwards (as opposed to in all directions) would become
advantageous (it's easier to build strong back and forth moving limbs than
strong omnidirectional limbs, for example). If one end is usually
encountering things first, a concentration of sense organs at that end
would be be selected for once mutations generated this configuration.
>Apparently you have not given serious consideration to the alternative
>hypothesis that all examples of bilateral symmetry provide evidence of the
>work of an intelligent designer. As Moorad wrote, "It all smells like a
>Designer with a purpose to me."
Certainly God created all of them, but I do not believe that He would go to
such great lengths to make it appear as though the organisms had common
ancestry and had changed over time if He had not created them via
evolutionary processes. Evolution is a very effective method of design.
For certain processes, one of the best ways for humans to solve the problem
is to use an evolutionary approach. God, being omniscient, does not have
to experiment to find the best answer in such a way, but perhaps He decided
that evolutionary means were the best way. There seems to be little
serious consideration of the possibility that God can design through
evolution among intelligent design advocates.