Evolutionary development is a very important field of biology because it is now beginning to answer some of the main questions about the diversity of organisms and how such complex organisms such as humans have developed. Many people can comprehend how an organism might make its sight better to meet certain circumstances, but then they might wonder, “How does an organism, through a random mutation, end up with a complete eye? Or a brain?” Afterall, haven’t we been constantly taught in this book that natural selection acts on very small differences to create changes in one trait at a time in an organism? Use the development of the eye as an example of how a single mutation doesn’t account for a complex structure, but rather the accumulation of such mutations. Explain the complete development and differentiation of the eye in different organism. Second, we have also learned that beneficial mutations are rare in occurring, but with enough time they will most likely occur. If we need a whole series of mutations to occur, then the chances that the same complex structures develop from scratch independently among different species is very rare. What actually does happen that accounts for multiple species having similar complex structures? Once again, use the development of the eye to answer the question.
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"Explain the complete development and differentiation of the eye in different organism."
ReplyDeleteThis is incorrect use of English grammar.... no points!!!!
In it’s embryonic stages, the eye in nearly all vertebrates develops in a pattern that produces an ‘inverted’ retina where the initial detection of light happens at the outer portion. The eye is developed from three types of embryonic tissue, including the neural tube (which will form the retina proper and its pigment cell layer), the mesoderm (which produces the corneoscleral and uveal tunics) and the surface ectoderm that becomes the lens of the eye. This process begins around 22 days of development, followed by the formation of the optic vesicles, the beginning of optic nerves and the hyaloid artery and vein.
ReplyDeleteDuring organogenesis, the ectoderm, endoderm and mesoderm develop into the internal organs of the organism. The lens and cornea of the eye differentiate from the ectoderm, and form into the entire structure. The eye, amongst different species, can be classified as either simple or compound. The mutations that occur within eyes do not change its “structure” and function; however, it changes its type. For example, the pinhole eye of the Nautiluses does not have a lens and does not allow the organism to discriminate between objects with an angular separation of less that 11* (meaning that after a certain angle, the image is blurry). However, the pit eye (also known as stemma) is an even more simple eye-spot which only allows the organism to deduce the angle of incoming light. The difference between these two eyes is not their function, but their physical capabilities and their purpose to the organism. Several mutations must have occurred to create this differentiation between a pin hole eye and a pit eye, yet the entire structure was not lost due to the “immortal” genes that are passed on from generation to generation that aid in organogenesis. Even in complete darkness for thousands of years, with “no need” to see, it is likely that the eye would then only “lose” its ability to capture light, but the structure would still remain until there was a selective advantage that favored organisms without eyes (unlikely).
http://education.vetmed.vt.edu/Curriculum/VM8054/EYE/EMBYEYE.HTM
http://en.wikipedia.org/wiki/Eye
Evolution is a cumulative process. The evolutionary development within complex organisms starts with a small, beneficial mutation. Due to natural selection, any mutation that gives the organism a competitive advantage will remain in the gene pool. Over time, more small, beneficial mutations begin to make the structure more complex. Any mutation that puts the organism at a disadvantage, of course, will culled through natural selection. As thousands or even millions of years pass by, these small mutations allow for an extremely complex structure, such as an eye or a brain. Carroll himself likens this process to compounding interest, where even with a small amount of money in the bank and a small interest provided by the bank, "given sufficient time, the growth from the initial number is dramatic after many years of compounding" (43). Similarly, even with small mutations that give only a slight advantage, over time, the evolutionary advantage is immense.
ReplyDeleteIn the case of the complex structure of the eye, evolutionary development began with eyespots. An eyespot consists of a small group of photoreceptor cells. Photoreceptor cells are simple, light-sensitive cells. The eyespots allow for the vague detection of light, signalling to the organism whether it is in a dark area or a light one. This provides a selective advantage because it gives the organism a basic idea of its position and also allows it to predict day and night, which is necessary for circadian rhythms. Organisms such as mussels have these flat, photoreceptor patches, which further demonstates that organisms would develop this type of structure. Evolutionary development continues as mutations cause the eyespots to depress and become more concave. This is a selective advantage because it gives the organism's primitive "vision" more directionality. Light can only stimulate the photoreceptor cell from certain angles, which gives the organism a better sense of where the light is actually coming from. Mutations further depress these light-sensitive cells for this same reason into a cup-shape. This structure can be seen today in some snails and invertebrates, such as flatworms. As the becomes more depressed, it forms a kind of chamber, with the opening of the eye constricting. With a smaller opening, the organism gains an even greater sense of direction, and its vision gains more focus, now able to distinguish specific shapes. This kind of pinhole lens can be seen in the nautilus, a cephalopod. Since the eye basicially consists of a hole leading into photoreceptor cells, the lens formation was the next mutation. Overgrowths of transparents cells protect the eye hole, an obvious advantage. With the eye chamber now separated from the outside environment, it can fill up with vitreous humor, a clear gel for further protection and focus. Further mutations divide the flat lens into two layers, separated by liquid. This lens not only makes the image sharper, the focused light has the added effect of being brighter. Later mutations increase the amount of fluid, or aqueous humor, between the two layers of the lens, making it more concave, which again just focuses the image even more. At this point, the eye is as functional as a human eye, lacking only subtle specializations that natural selection later chooses for.
Looking at this basic outline of the evolution of eye development, it might seem that all complex eyes originated from one source, but it is estimated that image-forming eyes have evolved around fifty to one hundred times independently. Many unicellular organisms have been found with the photoreceptive proteins needed for later development of a photoreceptor cell. These proteins may have had a single origin, but the relative simplicity makes it possible for mutations to have developed them separately. Carroll talks extensively about the repetition of evolution in nature, and these same forces can explain how structurally similar eyes can develop independently given similar situations. The advantage that comes with "vision" or the ability to sense structures around the organism is great enough for independent evolution.
(sources for above)
ReplyDeletehttp://www.ncbi.nlm.nih.gov/pubmed/9310200?dopt=Abstract
http://en.wikipedia.org/wiki/Evolution_of_the_eye
http://www.youtube.com/watch?v=Stb9pQc9Kq0