User:Kreinbrink.31/sandbox

Evolution of the Eye Edits

Color vision
Five classes of visual photopigmentations are found in vertebrates. All but one of these developed prior to the divergence of cyclostomes and fish. Various adaptations within these five classes give rise to suitable eyes depending on the spectrum encountered. As light travels through water, longer wavelengths, such as reds and yellows, are absorbed more quickly than the shorter wavelengths of the greens and blues. This can create a gradient of light types as the depth of water increases. The visual receptors in fish are more sensitive to the range of light present in their habitat level. However, this phenomenon does not occur in land environments, creating little variation in pigment sensitivities among terrestrial vertebrates. The homogenous nature of the pigment sensitivities directly contributes to the significant presence of communication colors. This presents distinct selective advantages, such as better recognition of predators, food, and mates. Indeed, it is thought that simple sensory-neural mechanisms may selectively control general behaviour patterns, such as escape, foraging, and hiding.

Polarization vision
As discussed earlier, the properties of light under water differ from those in air. One example of this is the polarization of light. Polarization is the organization of originally disordered light, from the sun, into linear arrangements. This occurs when light passes through slit like filters, as well as when passing into a new medium. Sensitivity to polarized light is especially useful for organisms whose habitats are located more than a few meters under water. In this environment, color vision is less dependable, and therefore a weaker selective factor. While most photoreceptors have the ability to distinguish partially polarized light, terrestrial vertebrates’ membranes are orientated perpendicularly, such that they are insensitive to polarized light. However, some fish can discern polarized light, demonstrating that they possess some linear photoreceptors. Like color vision, sensitivity to polarization can aid in an organism's ability to differentiate their surrounding objects and individuals. Because of the marginal reflective interference of polarized light, it is often used for orientation and navigation, as well as distinguishing concealed objects, such as disguised prey.

Focusing mechanism
By utilizing the iris sphincter muscle, some species move the lens back and forth, some stretch the lens flatter. Another mechanism regulates focusing chemically and independently of these two, by controlling growth of the eye and maintaining focal length. In addition, the pupil shape can be used to predict the focal system being utilized. A slit pupil can indicate the common multifocal system, while circle pupil usually specifies a monofocal system. When using a circlular form, the pupil will constrict under bright light, increasing the focal length, and will dilate when dark in order to decrease the depth of focus.

Final Draft
Evolution of the Eye: Marine v. Terrestrial

The evolution of the eye has been a topic of great debate since Darwin’s theory of evolution. Originally, the eye caused uncertainty because of its seemingly perfect design. Today, scientists are in debate about the number of origins for the many morphologies of the eye. A common topic of interest involves the evolution of the vertebrate eye. The eye’s path of evolution has been shaped by many forces. The strongest of these evolutionary forces include natural selection and drift. When discussing the evolution of the eye, one will inevitably consider the different adaptations present in aquatic environments and terrestrial environments. Some of these differences are seen in the preferential light types, photoreceptors, and the shape and structure of the eye. These variations must be considered along with the organism’s current and former environments.

Five classes of visual photopigmentations are found in vertebrates. All but one of these developed prior to the divergence of cyclostomes and fish (Osorio and Vorobyev 2005). However, various adaptations occurred. These adaptations give rise to the most suitable eye depending on the spectrum encountered, the common behaviors, and the photoreceptor noise. The most commonly studied adaptations occur among fish, whose deep-sea habitats are exposed to limited illumination (Osorio and Vorobyev 2005). The visual receptors in these species are more sensitive to the range of light that is available in their level in order to increase the photon catch. However, unlike marine environments, land animals possess relatively homogenous pigment sensitivities. Because of the little variation present, many different species are able to utilize this phenomenon with communication colors (Osorio and Vorobyev 2005). This communication can directly contribute to natural selection as well as sexual selection. Signals involving color have been known to warn of the presence of predators, save prey and predators through warning colorations, and provide a stimulus for young to target when feeding is required, as in young birds. Many organisms also utilize color signals during sexual selection. Color variations can reveal overall health, locality, and even sexual receptivity. Because of this, possessing nearly universal pigment sensitivities would greatly increase an organism’s fitness.

Polarization is the organization of originally disordered light, from the sun, into linear arrangements. Polarization occurs when light passes through slit like filters, as well as when passing into a new medium. Polarization vision is especially useful for organisms whose habitats are located more than a few meters under water. In this environment, color vision is less dependable, and a weaker selective feature, due to refractive alterations as light travels through the water medium. While most photoreceptors have the ability to distinguish partially polarized light, terrestrial vertebrates’ membranes are orientated perpendicularly, such that they are insensitive to polarized light (Cronin et al. 2003). However, fish do discern polarized light, demonstrating that they possess some linear photoreceptors for polarization analysis. Like color vision, sensitivity to polarization can aid in organisms ability to differentiate their surrounding objects and individuals. Because of the marginal reflective interference of polarized light, it is often used for orientation and navigation, as well as distinguishing concealed objects, such as disguised prey (Cronin et al. 2003). Like color vision in terrestrial vertebrates, underwater-polarized vision can be used for communication. Both analogs can be used for hunting, finding individuals of the same species, or warning of apparent dangers. All of which will contribute to an organism’s fitness.

The lamprey and hagfish, more primitive jawless fishes, have been main subjects of observations for the study of the evolution of the eye. Like modern vertebrates, the lamprey possesses a camera eye containing a lens, iris, ocular muscles, and a three-layered retina (Lamb 2011). The hagfish, which shares a common ancestor with the lamprey, commonly occupies the deep ocean floor and is virtually blind. In order to test ancestral relationships, embryonic development is often observed. Studies have shown the lamprey, as well as mammals, possess an ocular embryonic stage similar to that of the hagfish (Lamb 2011). As development progresses, formation of the retina, lens, cornea, and ocular muscles is observed. This information supports the hypothesis of a common ancestor for the aquatic and terrestrial eye, while shedding light on the evolution of the eye as far back as jawless vertebrates. The retina, lens, cornea, and muscles all aid in creating clearer images. This would increase fitness in both water and air by aiding in survival and hunting techniques, making complex eye structures a target of natural selection.

The optical design of a fish eye is relatively similar to the terrestrial vertebrate. However, due to the refractive index of the water, during immersion the cornea plays no role in focusing and refraction. Aquatic vertebrates also possess a more spherical lens and depend on the cortical index and water content of the eye (Jagger 1991). These main differences stem from the light variations in water and air. Just as aquatic animals are more likely to see polarization differences rather than color, their cornea need not play a role in refraction due to the altered light qualities upon striking the air-water boundary (Cronin et al. 2003). However, terrestrial mammals must utilize the cornea and the lens as refractive elements when focusing the light traveling through the air medium (Malmström and Kröger 2006). These differences can be attributed to migration. As discussed above, it is a common opinion that the present terrestrial eyes originated in the water. If an aquatic organism migrated to a new terrestrial environment, they would find that many of their current mechanisms were no longer sufficient. These mechanisms could include locomotion, predation, respiration, as well as vision. The selective pressures in water are distinct from those present on land. Eyes that produce clear images in water would generate distorted vision in air. Because of this, a refractive cornea is beneficial and necessary to produce a reliable image.

Another variable structure in the eye is the pupil slit. In terrestrial animals, this can be observed in order to determine if an organism is utilizing multiple refractive zones of its lens (Malmström and Kröger 2006). A slit pupil indicates the common multifocal system, while circle pupil usually specifies a monofocal system. When using a circle form, the pupil will constrict under bright light, increasing the focal length, and will dilate when dark in order to decrease the depth of focus (Malmström and Kröger 2006). This plays a significant role in natural selection. The ability to adjust focus to multiple planes is crucial to survival in nature. This can be explained using the Red Queen Hypothesis. While both forms are present and successful on land, they can be in competition with one another. When one form would evolve, that organism would gain some advantage by avoiding predation or by becoming a more efficient predator. Both coevolved in an arms race in order to compete with the other.

Another form of aquatic eye can be observed in the cephalopod. The squid eye is optically similar to the other aquatic animals and is also very comparable to terrestrial vertebrates in form and function (Sivak 1982). However, a major difference present in cephalopods is the corneal structure. Unique from the vertebrate eye, the squid’s cornea does not cover the lens, leaving the lens exposed to seawater (Sivak 1982). Because the cornea is not a refractive structure in water, this has no effect on visual acuity. An additional variation present in the cephalopod eye is the possession of a mobile pupil (Sivak 1982). This controls which rays are aiding in the formation of the retinal image. Because of this, any spherical aberrations can be corrected. While significant research has been done in regard to fish eyes, not much has been studied in cephalopods. From what is known, it can be seen that the water environment has played a role in the evolution of this eye type as well. The absence of a complete cornea exhibits the lack of refractive responsibility it plays underwater. The same selective pressures to create a clear imagine in water that affect underwater vertebrates, have also worked on the cephalopod. However, in order to fully understand the differences and similarities between vertebrate and cephalopod eyes, more extensive research is required.

The evolutionary forces working on an individual depends upon their environment. However, some species can occupy multiple or varying environment types. This can have interesting effects on their evolutionary history. An example of this occurs when animals from the land adapt to also inhabit the sea. This resettlement into an open niche has taken place on at least seven different occasions (Reidenberg 2007). Species-specific adaptations occur to make individuals better suited to their new environment. Variations can be seen in a number of anatomical structures including: the cornea, lens, pupil, retina, and ocular muscles.

While it’s clear that the terrestrial eye and the aquatic eye have some distinct differences, not all organisms inhabit one or the other exclusively. This has led to more questions about the efficiency of eyes that are to function in both mediums. One species that has been studied is the penguin, which unlike other aerial birds, live on both land and in water. This can cause some visual concerns in regards to focusing of an image due to the refractive differences between air and water. It is expected that if the birds’ eyes were adapted for aquatic vision that they would consequently be myopic in air (Howland and Sivak 1984). However, penguins are unexpectedly emmetropic in both air and water. Emmetropic means there is no refractive error resulting in a perfectly focused image. This can be attributed to the two refractive elements of a bird’s eye. These structures are the cornea and the lens. Penguins possess a comparatively flat cornea and an effective accommodative mechanism (Howland and Sivak 1984). The lens’s shape contribution is comparatively large due to the insignificant impact of the cornea. The lens shape can be altered by utilizing the large iris sphincter muscle. The development of this iris sphincter is an important adaptation for these animals. If the penguin is to be successful in its niche that overlaps different mediums, it must be able to compete with the organisms that have adapted specifically for one environment or the other. If the penguin were adapted to the air, it would not be able to compete with aquatic life, and vice versa. The iris sphincter muscle allows the lens to adjust for vision depending on the present medium. This adaption is a result of natural selection, which contributes to its overall fitness.

While terrestrial mammals’ eyes are very similar to that of aquatic animals, there are important environmental differences to consider. One of the most important is the refractive index in air and water. In water, a spherical lens is present and no refractive power is contributed by the cornea. However, in air the cornea must aid the lens in focus and refraction in order to get a clear image. Another difference is present in the preferential light types. In air, color vision is often utilized, but in water polarization differentiation is more commonly used. This is due to the natural polarizing affect the air-water surface has on the passing light. This polarization allows under water animals to differentiate otherwise hidden objects. Studying other organisms such as, cephalopods, lamprey, and penguins, can offer more information about the selective pressures each of the environments possesses.

References

Cronin, T., N. Shashar, R. Caldwell, J. Marshall, A. Cheroske, and T. Chiou. 2003. Polarization vision and its role in biological signaling. Integr. Comp. Biol. 43:549-558. Howland, H., and J. Sivak. 1984. Penguin vision in air and water. Vision Res. 24:1905-1909. Jagger, W. 1991. The optics of the spherical fish eye. Vision Res. 32:1271-1284. Lamb, T. 2011. Evolution of the eye. Scientific America. Biology:64-69 Malmström, T., and Kröger R. 2006. Pupil shape and lens optics in the eyes of terrestrial vertebrates. The Journal of Experimental Biology 209:18-25. Osorio, D., and M. Vorobyev. 2005. Photoreceptor spectral sensitivities in terrestrial animals: adaptations for luminance and colour vision. Proc. R. Soc. B. 272:1745-1752. Reidenberg, J. 2007. Anatomical adaptations of aquatic mammals. The Anatomical Record 290:507-513. Sivak, J. 1982. Optical properties of a cephalopod eye (The short finned squid, Illex illecebrosus). J. Comp. Physiol. 147:323-327.

Evolution of the eye Talk Page

1. Comment on previous suggestion "Bias"

I agree that this article seems to focus on the evolution of the camera eye. I feel that outlining the basic characteristics of the different types of eyes would be a great contribution to this article. Including distinctions such as compound v simple, aquatic v. terrestrial, ect. would allow readers to really understand the different layouts’ components and their advantages/disadvantages. The article Eyes lays out some major types from this source.

http://www.annualreviews.org/doi/abs/10.1146/annurev.ne.15.030192.000245

2. This article does touch on some selection forces such as eyespots allowing for circadian patterns, higher resolution aiding in survival/predation, and better recognition with color vision. However, I feel it has some gaps in evolutionary forces and the outcomes. I think the article could benefit from further explanation in the evolutionary baggage category. This topic is very interesting, but I think it could be more informational with some additions as suggested in the “Evolutionary baggage” category of this talk page. The article also points out that the earliest photosensitivity was developed aquatically. In addition to the light filtering effects of water, the refractive indexes would be different for an organism in air. I feel it would be a useful addition to include some information about how these forces worked on the original aquatic eye.

3. I made an addition to the Early Eyes section. I was interested in how the layers allowing for specialization in transparent humour or air and water operation, could be involved in moulting. I thought it could be an interesting addition to include an example.

http://onlinelibrary.wiley.com/doi/10.1111/j.1502-3931.2008.00138.x/abstract;jsessionid=794AD37010C17B485C06DD42029A308E.f03t04

Addition to Evolution of the eye

An example of this can be observed in Onychophorans where the cuticula of the shell continues to the cornea. The cornea is comprised of either one or two cuticular layers depending on how recently the animal has moulted.

Annotated Bibliography

Herring, Campbell, Whitfield, and Maddock. Light and Life in the Sea. Press Syndricate of the University of Cambridge. 1990. http://books.google.com/books?hl=en&lr=&id=nyc9AAAAIAAJ&oi=fnd&pg=PA149dq=evolution+of+the+marine+and+terrestrial+EYE&ots=QqqziUd5Fh&sig=iY6Q3_8om5CqOGLST32ZojWJBx8#v=onepage&q=evolution%20of%20the%20marine%20and%20terrestrial%20EYE&f=true
 * This book discussed light and life of a marine organism. I am particularly interested in chapter 9, Optics of the Eyes of Marine Animals.  This chapter points out that the refraction differences between light in air and light in water.  This difference causes the human eye’s curved air-cornea function to be undesirable.  This chapter also discusses the evolutions of the marine eye as well as the problems encountered by amphibious vertebrates.

Lamb, T.D. 2011. Evolution of the Eye. Scientific American 305:64-69


 * This article discusses the origins of eyes for both compound and “camera-style” forms. It explains the ancestry, as well as the benefits of both.  Smaller organisms, like insects, tend to possess the compound eye.  While larger organisms, such as mammals, utilize a camera-style eye with different photoreceptors.  This article discusses the difficulties of studying the origin of the eye because the eye is a soft tissue which rarely fossilizes.  This article gives insight to the origin of the eye and its functions but does not shed much light on the differing qualities of an aquatic eye compared to a terrestrial eye.

Lamb, T. D., S. P. Collin, E. N. Pugh, Jr. 2007. Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup. Nature Reviews Neuroscience 8: 960-976.


 * This article goes into great detail about the vertebrate’s eye. It discusses the origins and functions of each structure.  It refers to hagfish as having one of the most ancient vertebrate-type eyes.  The hagfish is discussed as the possible primitive form of the eye.  How this eye has evolved is displayed through steps in complexity.  This article does not specifically set out to explain the evolutionary differences between terrestrial and marine eyes.  However, it goes into the benefits of both forms, as well as the types of organisms that possess them.

Land, M.F., D. E. Nilsson. Animal Eyes. New York: Oxford University Press. 2002.


 * This book compares the evolutionary benefits of eye forms for different species inhabiting varying environments. The book contains chapters on aquatic and terrestrial eyes.  It also discusses mirror, compound, and superposition eyes.  I have not been able to check out the book yet.  Relevant results include lens shapes, optics, and origins.

Pettigrew, J. D., S. P. Collin. 1995. Terrestrial optics in an aquatic eye: The sandlance, Limnicthytes fasciatus. Journal of Comparative Physiology A 177: 397-408.


 * This article outlines the major differences most marine species and terrestrial species show. It goes into more detail about the unusual qualities of some aquatic eyes and their origins.  This article specifically discusses the sandlances’ unique eye characteristics including their controversy, how they benefit the organism, and an explanation into the study of how they function properly.

References