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Eye Evolution
There is a great variety of living eyes on earth, ranging from the very simple to the extremely complex. What are the simplest living visual systems and can the design of these systems give us a new way of considering our own visual process?
Some simple multi-cellular creatures have certain cells that are more light senstive than others that transmit information to other cells. These cells, concentrated in an area called the 'eye spot' are either flat on the surface of the body, or set in an indented dimple. The curvature of the indented eye, called a 'cup eye' allows the living creature to determine the directionality of the light stimulus. The flatworm has a light sensitive cup eye with cells similar to the human retina.
In other more evolved visual systems, the indentation closes at the edges, creating something like the pinhole of a pinhole camera. The inside of this biological pinhole camera is filled with a protective clear gel-like substace. Like a pinhole camera, there is a balance between the perceived image and the size of the pinhole. The smaller the pinhole, the more detailed the perceived image, however, if the pinhole is too small, not enough light can enter to register an image. Creatures with pinhole eyes include the Nautilus and the marine snail.
Eyes with lenses allow the entry of more light while still focusing the image. Some simple eye lenses are forms of a clear gel substance, others from a mass or transparent cells.
For creatures living in environments that have various light intensities, a pupil regulating the light input is a great benefit to vision. The pupil of an eye doesn't have to be circular like the human eye. Look closely at the eye of a snake or even your cat and you will see pupils that look more like vertical slits than circles. Then look closely at the aperture of your camera and you will see an imperfect circle, a polygon made of many slats.
In The Origin of Species, Charles Darwin discusses the lack of vision in some mammals:
"In South America, a burrowing rodent, the tuco-tuco, or Ctenomys, is even more subterranean in its habits than the mole; and I was assured by a Spaniard, who had often caught them, that they were frequently blind; one which I kept alive was certainly in this condition, the cause, as appeared on dissection, having been inflammation of the nictitating membrane. As frequent inflammation of the eyes must be injurious to any animal, and as eyes are certainly not indispensable to animals with subterranean habits, a reduction in their size with the adhesion of the eyelids and growth of fur over them, might in such case be an advantage; and if so, natural selection would constantly aid the effects of disuse. "
The compound eye of the insect, a kind of eye found in fossilized remains from over 400 million years ago, is very different than the eyes previously discussed. In the compound eye, the eye spot area curved outward rather than inward. The insect eye detects motion more effectively than it can form an image with its hundreds of miniature eye like structures callled ommatidia.
Curiously, Walter J. Gehring of the University of Basel in Switzerland in 1993 and various other researchers since have discovered an insect gene called the 'eyeless' gene. Abnormal activity of this gene can cause a fly to sprout eyes on its wings or legs, creating grotesque creatures with as many as fouteen eyes placed all over their bodies. The eyeless gene is similar to a gene identified in humans and other mammals, and the eyeless gene from a mouse was transplanted in a fly and caused the fly to grow additional fly eyes.
"Darwin will rest easier thanks to flies with eyes on their wings" Elizabeth Finkel as broadcast on ABC's Ockham's Razor on the 12/11/95 http://www.wehi.edu.au/resources/vce_biol_science/articles/finkel3.html
Isabella Chow "The Fly" The Moment: Columbia's Science and Engineering Newspaper
http://www.columbia.edu/cu/moment/v0/040595/fly.html
Dawkins, R. 1994. The eye in a twinkling. Nature 368(April 21):690.
Goldsmith, T. 1990. Optimization, constraint, and history in the evolution of eyes. Quarterly Review of Biology 65(September):281.
Gould, S. 1994. Common pathways of illumination. Natural History 103(December):10.
Harris, W. 1997. Pax-6: Where to be conserved is not conservative. Proceedings of the National Academy of Sciences 94(March 18):2098.
Mestel, R. 1996. Secrets in fly's eye. Discover (July):106.
Nilsson, D. 1989. Vision optics and evolution. Bioscience 39(May):298.
Zuker, C. 1994. On the evolution of eyes: Would you like it simple or compound. Science 265(Aug. 5):742.
On the evolution of photoreceptors and eyes.
Evolutionary Biology 10 (1997), 207-263.
Don Lindsay http://www.cs.colorado.edu/~lindsay/creation/eye_stages.html
Dr. John Maiello Department of Biology Rutgers, The State University of New Jersey Campus at Newark course outline Fall 2003 http://tecn.rutgers.edu/genbio102/genbio102/4EvoEyes.html
Charles Darwin, The Origin of Species. Chapter 5 - Laws of Variation
Science News Online "Eye-opening Gene" by John Travis http://www.sciencenews.org/sn_arc97/5_10_97/bob1.htm
Climbing Mount Improbable, Richard Dawkins, W. H. Norton & Company, Inc.
last modified Sep 4, 2003 at 10:27
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