From: Glenn Morton (glennmorton@entouch.net)
Date: Fri Jul 18 2003 - 23:49:22 EDT
David wrote:
>-----Original Message-----
>From: asa-owner@lists.calvin.edu [mailto:asa-owner@lists.calvin.edu]On
@lists.calvin. [mailto:asa-owner@lists.calvin.edu]On
>Behalf Of bivalve
>Sent: Friday, July 18, 2003 8:31 PM
>To: asa@calvin.edu
>Subject: Re: Cambrian Explosion
>
>
>Although the evolution of improved sense organs, including better
>eyes, probably played a role in the Cambrian radiation by
>providing novel options in the predator-prey arms/defense race, no
>one factor probably explains all of it.
>
>A few specific details:
>>the image-forming eye.<
>
>I am not sure that arthropod compound eyes exactly form images.
>However, they do provide visual acuity.
What I have seen is that they are best at detecting movement. They do see
images, just not as clearly as we see.
"The compound-eye principle works well enough for, say, a dragon fly zeroing
in on a moveing fly but, in order for a compund eye to see as much detail as
we see, it would need to be hugely bigger than the kind of simple camera eye
that we possess. Here is approximately why that is. Obviously, the more
ommatidia you have, all looking in slightly different directions, the more
fine detail you can see. A dragonfly may have 30,000 ommatidia and it is
pretty good at hawking insects on the wing. But in order to see as much
detail as we can see, you'd need millions of ommatidia. The only way to fit
in millions of ommatidia is to make them exceedingly tiny. And
unfortunately, there is a strict limit on how small an ommatidium can be.
It is the same limit as we met in talking about very small pinholes, and it
is called the diffraction limit. The consequence is that, in order to make a
compound eye see as precisely as the human camera eye, the compound eye
would have to be ludicrously large: twenty-four meters in diameter!" Richard
Dawkins, Climbing Mount Improbable, (New York: W. W. Norton, 1996), p. 180
"The moral is, if you want to see precise, detailed images of the world, use
a simple cmaera eye with a single, good lens, not a compound eye." Richard
Dawkins, Climbing Mount Improbable, (New York: W. W. Norton, 1996), p.
180-181
That a compound eye does see images is seen by the following:
"A camera eye forms an upside-down image. A compound eye's image is the righ
way up." Richard Dawkins, Climbing Mount Improbable, (New York: W. W.
Norton, 1996), p. 182
You can also see a compound eye image on page 186 of Dawkin's book.
>
>>The only light sensitivity might well have been light sensitive
>patchs which didn't form images.<
>
>Light sensitivity is quite likely. The relevant genes are very
>widespread. However, there is no definite evidence of very
>well-developed eyes before the Cambrian.
Which fits Parker's idea.
>
>> But these animals were probably too primitive to have good
>hearing so life mostly consisted of touch, taste, smell and the
>detection of light and dark only. They would get their food via
>accidentally bumping into it.<
>
>Smell can be quite effective at a distance.
>
>>And the evolutionary outcome? There are considerably fewer
>species active at night compared with the day.<
>
>I am rather doubtful about this claim. Most mammals are
>nocturnal, as are quite a lot of insects. Many marine species are
>also more active at night.
The claim isn't that there aren't any nocturnal animals, just fewer of them
than the day-jobbers. I would be interested in any contrary data to Parker's
claim.
>
>
>>But as one went deeper into the seas, thus into the dark, the
>diversity of species declined significantly.<
>
>This depends very strongly on the choice of taxon and the
>sampling. Deep sea diversity appears to be quite high for many groups.
Which ones? I would prefer specifics than generalities. If Parker's
suggestion is wrong, I would like data to put it in the grave, not
generalities.
>
>>Evolution was going forward at a snails pace in the dark.<
>
>Ignoring examples of rapid evolution in snails, the deep sea seems
>to provide a refuge for some archaic-looking groups, but there are
>also a variety of relatively young species there.
But are they recent imigrants to that area?
This archive was generated by hypermail 2.1.4 : Fri Jul 18 2003 - 23:49:34 EDT