The Evolution of the Eye!!!

By Dan Zembrosky and Dr. Pepper, M.D.

Types of Eyes

1. The Positive Lens or Camera EyeFound in vertebrates and some invertebrates. This is the type of eye humans have. You can even poke it with your finger right now. Go ahead, gently. That’s right.

The Simple Eye (3 Main Kinds):

Types of Eyes

• The Simple Eye

2. The Concave Mirror Eye

Found in the clam Pecten and a few ostracod crustaceans. This produces bright but reasonably hazy picture.

3. The Pinhole Eye

Pit or Cup eyes are found mainly in mollusks and can only resolve location of objects.

The Compound Eye• Made up of ommatidia receptors, each of which

functions as a separate visual receptor.

• The Apposition EyeOmmatidia function

independently.

• The Superposition EyeOmmatidia cooperate to produce a brighter,

superimposed image on the retina.

The Two Kinds of Photoreceptors

All photoreceptors use a light sensitive pigment derived from vitamin A which is bound to an opsin. After being exposing the photopigment to light the opsin binds to a G-protein (common neurotransmitter). These similarities suggest a shared ancestry.

• Rhabdomeric Photoreceptors (in protosomes)

Found mainly in the compund eyes of arthropods.

• Ciliary Photoreceptors (in deuterosomes)

Common in vertebrates.

Rhabdomeric Photoreceptors

• Increase their surface area by throwing up their apical surfaces into numerous folds (think Bart Simpson’s hair). Many morphologies for this receptor exist.

Ciliary Photoreceptor

• Increases membrane surface area by modifying the cilium. The ciliary membrane is expanded and thrown into deep folds, so that the actual receptor region of the cell looks like a stack of discs.

Evidence of Common Receptor Ancestry

Rods and Cones seem to have evolved from common ciliary photoreceptor precursors, while retinal ganglion, amacrine and horizontal cells seem to have evolved from a rhabdomeric precursor.

How Could Eyes Evolve?

• Creationists often use the eye as a debate point to show that evolution is flawed, citing that the eye is too complex and perfect to have evolved. Darwin himself noted that “To suppose that the eye… could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree,” (The Origin of Species). He follows, however, with the assertion that eyes could likely evolve from light sensitive neurons. This seems to be the case.

The Evolution of the Eye

• Research by Dan-E Nilsson and Susanne Pelger indicates that it is in fact easier to estimate the number of generations necessary to evolve an eye than complex organs. This is because these changes can be viewed as quantitative local modifications to a pre-existing tissue.

• In order to determine the number of generations needed to evolve an eye, Nilsson simply made calculations outlining the plausible sequence of alterations leading from a light sensitive spot to a fully developed lens eye.

The Evolution of the Eye

• Nilsson assumed an organism with a light sensitive patch of cells resting on a dark pigmented background and placed in a selection for spatial recognition. The first method to create a spatial recognition is either for a depression to form in the center of the patch, or for the edges of the patch to constrict and raise. This cupping would allow for the vague correlation of light to position where the exposure of an area on the patch is dependent on its angle to the light source.

The Evolution of the Eye

• This cupping evolution should first favor the formation of a depression in the patch, than the constriction of an aperture via the raising and constriction of the surrounding pigment epithelium. This results in a sunken eye cup that resembles that of some mollusks.

The Evolution of the Eye

• This pinhole-like eye is not very good at resolving detail and creates a very dim image. Because of this, any change that improves clarity and illumination will be favored. The two routes of change for this would be the development of a lens, or the increase in the size of the eye. Increasing the size of the eye, however, presents physical problems and less acute vision than a lens would.

The Evolution of the Eye

• Nillson gauged number of 1% changes in structures in this diagram. The number of 1% steps comes out to 1829 necessary steps to progress from a light sensitive disk to a camera-eye.

• But how many generations will that take? Prepare for math on the next slide.

• I am so, so sorry.

The Evolution of the Eye

• R=h2iVm• m=mean increase/decrease in a feature.• h2=heritability.• i=intensity of selection.• V=coefficient of variation=ratio between standard deviation and mean in a

population• n=number of generations.

h2=0.5 (common heritability), i and V both = .01 (low values for conservative estimation), therefore…

R=.00005m, so small variation and weak selection produce a .005% change per generation. So…

1.00005n=80129540 so n=363992 generations.

What the Heck was that?

• So basically, it takes 363992 generations, roughly 364,000 years to evolve camera-type eyes given that reproduction occurs yearly and the brain of the animal can handle such visual processing.

• Things to note:• Nilsson’s simulation does not take into account more

specialized structures such as sclera and capillaries because they are not necessary for all types of camera eyes (gastropod mollusks lack these). This simulation also does not take into account the evolution of photoreceptors.

Are We Done Yet? NO!

• Sit down and shut up!

• Nilsson’s simulation demonstrated basic structural evolution, but what about genetic evidence? Did eyes evolve independently or is there one common ancestor for all eyes?

• Get ready for some crazy, messed up, stuff.

PAX-6 and Aniridia

• Prior to 1993 all evidence pointed to independent evolution of the eye. Then, while looking for transcription factors in fruit flies Walter Gehring and Rebecca Quiring discovered a gene nearly identical to the PAX-6 gene in mice and Aniridia in humans. All of which control the expression of eyes in a major way.

• Mutations in these analogues can truncate the development of eyes in mice and cause serious defects in the human eye.

• Could this be evidence against independent evolution of the eye? Asked Gehring. In order to find out he created…

Flytato: All the Eyes of a Potato, All the Deliciousness of a Fly

• By “turning on” this gene, dubbed “eyeless” in developing cells that do not normally express it, it caused the fly to develop EXTRA EYES IN ODD PLACES. AS DID THE ADDITION OF THE PAX-6 AND ANIDIRIA GENES. WTF!?!?!

Flytato: Continued

• Extra eyes ARE light sensitive, ARE NOT wired into the brain like normal eyes.

• Is this evidence for a single origin of the eye? MAYBE.• Ernst Mayr contests many eyeless organisms have

similar genes.• Mayr believes that this gene was originally part of a

group of genes that shape the nervous system. As different organisms evolved, its role shifted.

• PAX-6 also regulates expression of the nose in mice and the production of tentacles in naughty children squid.

Summary• There are two basic types of eyes, the Simple and Compound eyes.• Simple eyes include the Pinhole Eye, the Concave Mirror Eye and

the Positive Lens Eye.• Compound eyes are composed of multiple Ommatidia and have

Apposition types and Superposition types.• Two types of photoreceptors are believed to have evolved from a

proto-receptor, Rhabdomeric and Ciliary.• Nillson demonstrated how the structure of the eye could evolve from

a light sensitive region to a Camera Eye structure in less than half a million years.

• PAX-6, Aniridia and “eyeless” are relatively analogues genes that control the expression of eyes.

• Flytato: WTF?

Bibliography• Barnett, Adrian. Evolution for Creationists. 14 Mar. 1999.

<http://www.abarnett.demon.co.uk/atheism/evolution.html#EYES>.

• Evolution Library. 2001. <http://www.pbs.org/wgbh/evolution/library/01/1/l_011_01.html>.

• Fernald, Russel D. Karger Gazette. Stanford University.

<http://www.karger.com/gazette/64/fernald/art_1_0.htm>.

• Land, M F., and Dan E. Nilsson. Animal Eyes. 2nd ed. N.p.: Oxford UP, 2004.

• Meyers, Paul Z. Pharyngula. 09 Nov. 2004. University of Minnesota, Morris.

<http://pharyngula.org/index/weblog/comments/rhabdomeric_and_ciliary_eyes/>.

• Nillson, Dan E. "A Pessemistic Estimate of the Time Required for an Eye to Evolve." Biological

Sciences 256 (1994): 53-58.

• Semo, Ma'ayan. Ma'ayan's Vanity Website. 09 Feb. 1998.

<http://www.maayan.uk.com/evoeyes1.html>.

• Travis, John. Science News Online. 10 May 1997.

<http://www.sciencenews.org/pages/sn_arc97/5_10_97/bob1.htm>.