Eye Evolution
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[edit] Introduction
We can see just how straightforward such an evolutionary development would be by observing that there are many grades of vision in nature, from mere light-sensitivity to the fully-fledged "camera eye" of humans, and that it is possible to get from start to finish of this process by the series of small cumulative improvements that are produced by natural selection acting on random variation.
[edit] Phototaxis in prokaryotes
Phototaxis means moving towards the light; negative phototaxis, or photophobia, means moving away from the light. Many small organisms display both behaviors, being attracted to moderate light and fleeing from intense light.
Some prokaryotes, including both bacteria and archaea, exhibit phototactic behavior. Interestingly some phototactic bacteria and archaea use rhodopsins for light detection in phototaxis --- the same sort of proteins used for light detection in the rods of your own eyes[1][2].
As rhodopsins perform other functions in bacteria and archaea besides light detection, it is plausible --- we should say probable --- that opsins first originated for a purpose other than light detection, and was co-opted for that purpose later.
[edit] Light antennae in single-celled algae
Euglena (named from the Greek for "good eyeball") is a euglenoid alga: it is single-celled, and has both chloroplasts (so it can photosynthesise just like a plant) and a flagellum, making it mobile. It exhibits both phototaxy (being attracted to moderate light) and photophobia (retreating from strong light).
The mechanism it uses is simple. At the base of the emergent flagellum is a swelling which rejoices in various names: "the flagellar swelling", "the photoreceptor", "the paraxonemal body", and the "paraflagellar body". It is highly sensitive to light. Euglena also has a "stigma", a pigmented body which shields the photoreceptor from light in one direction.
(NOTE: it used to be believed that the stigma was the light detector in this arrangement. This is not true.[3] We mention this because many websites still haven't caught up with the facts. Because of this confusion, the stigma is still sometimes referred to as the "eyespot", a thoroughly misleading term.)
As Euglenia swims, it rotates on the axis of its direction of travel. The result is that if it is side-on to the light source, the intensity of the signal received from the photodetector will vary periodically.
Below is a video of some euglenoid algae swimming around; the flagella are too fine to make out at this resolution, but you should be able to see the spiral swimming pattern and the stigma --- the red patch near the front end of each alga.
Chlamydomonas, another alga, has a similar arrangement, with a stigma and photoreceptor, though in this case the photoreceptor chemicals are not attached directly to the flagella, but communicate with it by electrical impulses in a manner reminiscent of our own nervous system. Like Euglena, it rotates as it swims, about twice per second, and it is the variation in light intensity that gives it the clue as to its orientation.
On the molecular level, there is a key difference between Euglena and Chlamydomonas. The photoreceptive chemical in Euglena is a blue-light-activated adenylyl cyclase[4]. But Chlamydomonas uses a rhodopsin[5], like the phototactic bacteria and archaea we cited earlier, and as can be found in your own eyes and the eyes of invertebrates.
For more information on these interesting creatures, see K W Foster and R D Smyth, "Light Antennas in phototactic algae", Microbiol Rev. 1980 December; 44(4): 572–630 or Suneel Kateriya, Georg Nagel, Ernst Bamberg, Peter Hegemann: "Vision in Single-Celled Algae"; both the articles are free access.
[edit] Grades of vision in molluscs
Living molluscs are often used as a textbook example of grades of vision in nature.
- Simplest of all we have the mere eyespot, as sported by the limpet Patella: a flat collection of pigmented cells and nerves with nerve fibers leading to whatever Patella has resembling a brain. This allows sufficient vison for phototaxis.
- In the slit shell mollusc Pleurotomaria, the eyespot has been deformed into a cup. This allows the mollusc to tell in which direction light or shadow lies, according to which side of the cup it falls on.
- In Nautilus, the far end of the cup has nearly closed off, leaving a "pinhole" aperture. This has the effect of reducing the amount of light entering what we might now call an "eye", but it improves acuity of vision, since now the view of each photoreceptor is confined to light passing in a straight line that passes through the pinhole and terminates at the photoreceptor.
- The eye fills up with a thick fluid: we have not been able to find out whether this has happened in Nautilus, as our sources conflict on this issue.
- A transparent layer of skin grows over the pinhole opening. A primitive spherical lens develops in the fluid in the eye. We should emphasize that the acquisition of a lens is not an all-or nothing step: the refraction of light is caused by differences in density between the media that the light has to pass through, and density is a property capable of almost infinitely fine gradations. This stage is represented by the eye of Murex. The focus provided by the primitive lens is sufficient that the pupil can get bigger again, allowing more light into the eye.
- Finally, we have the lens-shaped lenses of octopods and squid such as Loligo. Note that the ability to change focus, either by moving the lens backwards and forwards, as cephalopods do, or by stretching and contracting it, as we do, is certainly advantageous; and any small ability to do so, or small improvement on this ability, must therefore be favored by natural selection.
Diagrams of these various kinds of eyes in molluscs are supplied here by the Encyclopædia Britannica Photographs of sections of the pinhole camera eye (in Haliosis) simple lens eye (in Helix) and squid eye can be found here.
Note once again the use of rhodopsin as the photosensitive chemical in molluscs [6][7].
[edit] Grades of vision in cnidarians
The molluscs, as we mentioned, are a textbook example; in fact, they have been done to death. As an alternative, we offer the grades of vision in cnidarians (jellyfish) as given in V J Martin, "Photoreceptors of cnidarians", Can. J. Zool. 80: 1703–1722 (2002). Note that "ocelli" are the sense-organs of cnidarians (when they have them):
- Many cnidarians are sensitive to light, yet they bear no distinct ocelli. Lentz and Barnett (1965) observed ciliated sensory cells in the outer epithelium of hydra and suggested that these cells are photosensitive. The precursor of the photoreceptor cells in cnidarians was probably a photosensitive ciliated ectodermal cell, similar to those described in hydra . Such ciliated photosensory cells possess greater information capacity if they are grouped with nonciliated pigment cells to form a primitive distinct ocellus. The ocelli of L. octona illustrate this design, as they are composed of a simple patch of ciliated photosensory cells intermingled with pigment cells. The photosensory cells expanded their apical, light-receptive surfaces with microvilli, and their basal ends were drawn out to form axons. Such simple eyespots would be useful for informing an animal about the distribution of light and dark in the surroundings. Over time the light-sensitive patch invaginated to form a cup-shaped structure, and the plasma membrane covering the cilium of the photoreceptor cell evaginated to form villous processes, thus increasing the surface area for photon detection. The pigment cells also formed microvillous processes that interdigitated with the villous processes of the sensory cells, both processes filling the ocellar cup. This design is seen in P. penicillatus. Through the formation of a pigmented cup, spatial resolution was introduced, as the angle through which the individual photoreceptor cells received light was reduced. Spatial differentiation of the villous processes of the pigment cells and photoreceptor cells occurred, as is seen in B. principis. In some animals, such as C. radiatum, primitive lenses derived from villous extensions of pigment cells formed in the ocellar cups. Finally, the ocelli of cubomedusae represent the most highly evolved eyes in the Cnidaria. In these ocelli, the opening to the eye cup constricted and a spherical, graded-index lens formed in the center of curvature of the retina, producing a camera-type eye. [8]
So we can see a gradation in nature from cnidarians which are merely light-sensitive, but have no specialized organs of light-detection right the way up to lensed camera-like eyes. It should not surprise you to learn that the chemicals used to detect light in all these cnidarians are opsins.[9][10][11]
[edit] Nillson and Pelger
It is possible to apply the laws of optics to (a set of data representing) an eye and calculate how good an image will be formed; hence we can see if there really is an unbroken path of improvement from an eyespot to a lensed eye. This exercise was undertaken by Nllson and Pelger, as reported in Nilsson and Susanne Pelger, "A Pessimistic Estimate of the Time Required for an Eye to Evolve", Proceedings: Biological Sciences, Vol. 256, No. 1345 (Apr. 22, 1994), pp. 53-58. Even with the most pessismistic of assumptions, they found that such a process could take place in "only a few hundred thousand years".
It is sometimes said that Nillson and Pelger "simulated the evolution of the eye". This is not really an accurate account of what they did. What they actually showed was that there was a pathway from a simple light-sensitive patch to a lensed eye involving only small changes in morphology each of which improved vision.
Now, this is certainly sufficient for us to conclude that a lensed eye can evolve from a light-sensitive patch. For if such a pathway exists, then natural selection must favor those mutations that drive a lineage along that path, so long as the benefits (of increased visual acuity) outweigh the costs (of building a more complex eye). To quote Darwin:
- Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be inherited, as is likewise certainly the case; and if such variations should be useful to any animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, should not be considered as subversive of the theory.
Now, Nillson and Pelger certainly showed that there were "numerous gradations ... each grade being useful to its possessor". However, they did not actually simulate a population undergoing random variation combined with selection for visual acuity, though their results are sufficient to tell us what would happen if you did.
[edit] Evolution of compound eyes
We have concentrated on discussion of the camera-like eye, often held up as the peak of eye evolution. However, there is an alternative, though lower peak --- the compound eye. We describe it as a peak, because there seems little way to improve it by any small adjustment; and lower, because it does not give the same degree of visual acuity: if humans were to have compound eyes, we'd need eyes the size of basketballs to see as well as we do with our lensed eyes. It does have advantages, though: it has a faster "frame speed" than our eyes, so that a bee watching television would see a set of still pictures where we are deceived into seeing continuous motion.
The principle behind the evolution of a compound eye is simple: where evolution towards the camera eye begins with invagination into a single pigmented cup, a compound eye invaginates into several pigmented cups. The more and smaller they are, the greater the visual acuity. Any convex curvature to the general aspect of the eye also improves vision, since then the different cups (ommatidia) point in different directions.
What we have described so far is the evolution of a very simple sort of "apposition eye". There are lots of different ways (nine at the lowest count) that increased complexity in compound eyes can then be achieved: some of them are discussed here.
You will be unsurprised to learn that once more opsins serve as the photoreceptive chemicals in compound eyes.[12] The eyes of vertebrates and arthropods also seem to share a genetic basis for the control of eye development. Fruit flies have a gene known as eyeless: as so often, the gene is named for the effect that breaking it has on the organism: with a functioning copy of the gene, fruit flies have eyes. In mice, a gene has been found called small eyes, or Sey, for short: again, it is named after what happens to the mice when the gene is broken. The two genes look very similar, and, remarkably, a working copy of the Sey gene will function as a substitute for a broken eyeless gene, causing fruit-flies to develop normal eyes. [13]
[edit] Creationist misconceptions
[edit] What use is half an eye?
The rhetorical question: "What use is half an eye?" is often asked by creationists. If they really suppose that evolution works by adding half an eye, and then the other half, then they are, of course, wrong.
If, however, this rhetorical flourish conceals any understanding of the actual theory of evolution, then the question has, we think, been answered by the article above: phototactic bacteria have a lot less than "half an eye", but it is still useful to them. The nearest thing in our evolutionary sequence to "half an eye" is, we suppose, the pinhole eye, and there are plenty of organisms which use them to see out of: the devout Creationist must therefore either claim that God screwed up, or admit that for some purposes this "half-eye" is useful enough.
[edit] Octopus and human eyes
One of the more bizarre creationist claims is that human eyes are more similar to octopus eyes than to chimp eyes: this claim may have originated with Kent Hovind and has certainly been employed by him[14].
This claim, if true, would certainly confute evolution, which could not explain such a strange instance of chimerism. However, it is untrue. Humans have normal vertebrate eyes, and the difference from molluscs is clear even on the most cursory examination. For example, in humans and chimps the optic nerves are on the outside of the retina's light-sensitive cones and rods, which means that they have to pass through the retina to get to the brain, causing a "blind spot"; the octopus eye has no such flaw. Human and chimp eyes both focus by changing the shape of their lenses: octopods move the lens backwards and forwards.[15]
This creationist claim seems to have resulted, like so many creationist claims, by playing a sort of game of Chinese Whispers with real science. The similarity of the octopus eye and the human eye has often been cited as an example of a place where we should expect, and find, convergent evolution, but they are certainly not identical, nor is there any reason why the similarities that we do find apply to humans more than any other vertebrate.
[edit] Berlinski versus Nilsson and Pelger
The paper of Nillson and Pelger described above has been criticized with remarkable laziness and inaccuracy by Intelligent Design proponent David Berlinski[16]: Nillson's response (and others') can be read here.
[edit] Darwin's imaginary "admission"
Creationists are fond of partially quoting a passage from Darwin which they construe as him "admitting" that the eye could not have evolved, when in fact he was saying the exact opposite. For a rebuttal of this deceitful gambit, see the article on Darwin on Eye Evolution in our rogues' gallery of Creationist Arguments: see also the Appendix at the end of this article.
[edit] Appendix: Darwin on eye evolution
- It is scarcely possible to avoid comparing the eye to a telescope. We know that this instrument has been perfected by the long-continued efforts of the highest human intellects; and we naturally infer that the eye has been formed by a somewhat analogous process. But may not this inference be presumptuous? Have we any right to assume that the Creator works by intellectual powers like those of man? If we must compare the eye to an optical instrument, we ought in imagination to take a thick layer of transparent tissue, with a nerve sensitive to light beneath, and then suppose every part of this layer to be continually changing slowly in density, so as to separate into layers of different densities and thicknesses, placed at different distances from each other, and with the surfaces of each layer slowly changing in form. Further we must suppose that there is a power always intently watching each slight accidental alteration in the transparent layers; and carefully selecting each alteration which, under varied circumstances, may in any way, or in any degree, tend to produce a distincter image. We must suppose each new state of the instrument to be multiplied by the million; and each to be preserved till a better be produced, and then the old ones to be destroyed. In living bodies, variation will cause the slight alterations, generation will multiply them almost infinitely, and natural selection will pick out with unerring skill each improvement. Let this process go on for millions on millions of years; and during each year on millions of individuals of many kinds; and may we not believe that a living optical instrument might thus be formed as superior to one of glass, as the works of the Creator are to those of man?
- (Charles Darwin, On The Origin of Species, chapter 6.)
