Monday, April 13, 2009

Eyes

On pages 193-195, Carroll discusses the evolution of the many eyes we see in the world. Given his previous components of evolution; chance mutation, selection, and time, discuss how all these eyes could have evolved so differently as opposed to keeping a more uniform style.
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  1. Kimberly KenyonApril 13, 2009 at 8:04 PM

    Evolution of the eye is suggested to have started that a small spot on an ancestor that detected light gave a selective advantage and random changes allow a sharper image to be seen. This spot on the ancestor over time turned into a retina and soon the eye, over time a lens formed and the eye evolved due to the needs of the organisms.

    There are many factors that may cause eyes to change overtime. It depends on what the eyes are used for. Birds tend to need a further range of vision because they fly above the earth and search for prey beneath them. But blind mole rats’ eyes have become small and covered with fur and skin because its lifestyle involves using its nose and other senses because it is constantly underground or activate at night time. Also, technology may affect the result on eyes. Humans have developed glasses and contacts. Eyes have then used their own genes less and rely on the glasses. Some organisms are incapable of forming images like us, for instance the sea urchin can only detect changes in light.

    Proteins called crystallins are found in lens cells in humans and other fish, frogs, mammals, etc. The crystallins have evolved over time in all species. Crystallins help form the lens and give it its physical properties that are necessary for vision. It is estimated that crystallins were in the ancestors of most organisms with eyes and have evolved through all organisms. The sea urchin (simple eye) has only one crystalline protein to make the lens of the eye. This suggests that humans and other mammals have more crystallins. The protein is found to be controlled by a gene. Mutations in this gene may have caused the evolution of this protein.

    Time is very important for change to occur. Carroll discusses in his book the law “Use it or Lose it” in our bodies. The genes that we don’t use will eventually become fossilized. This means that if our lifestyles do not use our eyes, like the lifestyle of the blind mole rat, then we will eventually lose these genes. Over time, natural selections will occur. Mutations of genes will cause eyes to become better or worse. Whichever is better for the animal will survive and reproduce to pass on the genes. Selection allows this to occur in a population of an organism.

    Eyes have evolved so different to give advantages to different organisms in different ways. Lifestyles have caused the eye to evolve to the organism’s advantage.

    http://www.scienceagogo.com/news/20050822230316data_trunc_sys.shtml
    http://www.teachersdomain.org/resource/tdc02.sci.life.evo.nilssoneye/

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  2. kimberly haApril 13, 2009 at 8:10 PM

    Carroll discusses the different paths that the evolution of the eye took in different species. In the beginning it was widely accepted that the idea of separate routes to creating similar visual aid structures was the reason for the array of different eye types. This is until the idea that instead of the eye being reformed again and again in each species from scratch rather had basic parts in which to work with to create the optical aid. Although many species have differing eyes it is seen that they originated from a most basic structure.
    This can be seen in other structures of differing organisms such as the circulatory system. In fish the circulatory system lacks many important structures that mammals and land animals require. This is also connected with the respiratory system. For fish there are no lungs but rather the skin and gills allow the diffusion of oxygen to occur. This is because in the environemnt of the fish there is no large energy requirement for survival and the gills adn skin are able to allow for diffusion because there is constant moisture due to teh surrounding water. Yet for mammals and land animals there is a higher energy demand because of the need to walk and move to get around. Also since there is a lack of moisture there must be an internal organ that allows for the diffusion of oxygen, such as the lungs. The lungs also allow for more oxygen diffusion, which in turn allows for more energy to be made in the target organs. This allows more energy for the organism which again connects with the energy demands that there are on land creatures. The circulatory and respiratory system was had basic parts that are used in almost all species, but it was modified to fit the needs of the species because of its environment. This is the same for the structure of the eye. Because certain animals may live in different environments they may need their eyes to have certain characteristics that can help them in their own environment. This is when chance mutation and natural selection come into play. A chance mutation may occur that enables the eye to be more useful and better aid survival for the organism in its surroundings. This will give a higher survival rate and so this trait will be more likely passed on to offspring. Based on the advantageous quality of the mutation and the number of offspring, natural selection will come into play and gradually increase the percentage of the population that has this mutation. In this way the species has evolved to better survive in its environment. This explains the different structures of the eye that can be found in differing species. With the developed eye of the bird, they are able to detect ultraviolet light that is unnoticed by humans. This is because it is important on their survival, affecting their mating, feeding their young, and finding food (110). Because it has a great impact on the lives of birds because of their own needs, the eye of the bird has evolved to detect UV light. In the same perception, the eye can be lost and become a fossilized gene because of the idea of "use it or lose it". Those animals that have no need for an eye because of the always dark environment they may live in will eventually have eyes that do not require such intricate parts and instead are rather simple and may sometimes be completely useless. Just as a slight benefit will allow for a characteristic to spread amongst a species, the small benefits that differing structures in the eye brought caused the formation of such unique eyes.
    http://www.nature.com/nrn/journal/v8/n12/abs/nrn2283.html
    http://science.jrank.org/pages/5841/Respiratory-System-Respiratory-system-fish.html
    http://www.globalchange.umich.edu/globalchange1/current/lectures/selection/selection.html

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  3. Jennifer JooApril 13, 2009 at 9:18 PM

    Carroll writes that there are many different types of eyes belonging to different organisms in the animal kingdom. Humans and “other vertebrates have camera-type eyes with a single lens” while “crabs and other arthropods have compound eyes in which many independent unit eyes gather visual information” (Carroll 194). Interestingly, although they are vastly different from humans, squids and octopi also possess “camera-type” eyes. Mayr and Salvini-Plawen suggest that the eye structure has been independently “invented” more than forty times, due to it being a selective advantage for many terrestrial organisms; terrestrial mammals can use it to both locate food and spot predators. As the post above said, “evolution of the eye is suggested to have started that a small spot on an ancestor that detected light gave a selective advantage and random changes allow a sharper image to be seen” (Kenyan).
    One theory is that evolution has found a way to reproduce a useful function for many different organisms, the eye. However, due to recent studies, it has been debated whether eyes were re-invented “from scratch” or whether they developed due to common factors already present in common ancestors and thus, the likelihood of eyes “from scratch” is questioned. Eyes of different organisms generally have more in common than scientists have previously thought; for example, looking at figure 8.2, one can see that the fly and human sequences are nearly identical and the mouse and human sequences are identical.
    Gehring and his colleagues studied the gene that formed the compound eyes of the fruit fly; the eyeless gene was previously discovered, and as its name indicates, it turns off the gene for eye formation in fruit flies. This eyeless gene was then compared to human and mice genes and was found to be highly similar to both of them. The protein, pictured in figure 8.2, was called the Pax-6 gene.
    The Pax-6 raised the question of whether “formation of such different eyes . . . is mere coincidence or a sign of something of greater significance. In other words, scientists questioned whether Pax-6 was due to chance mutation and thus, were selected to be a selective advantage due to time. Mutations are generally changes in the genetic material of a cell; mutation and sexual recombination can create gene variations in a certain group of organisms, exemplifying continuity and change in that group; if a mutation is kept within a group of organisms and proliferates, then it can be kept as a selective advantage, helping the group survive and reproduce. Offspring can only inherit these selective mutations, such as eyes, if the mutations occur in cell lines that produce gametes.
    The Pax-6 of the mouse and fruit fly were shown to be interchangeable; the genes had the same capabilities if not the same coding. The function and sequencing of the gene has been preserved for about 300 million years. The similarities of the Pax-6 gene among many different animals indicate that the animals did not simply stumble upon the gene in nature. There is likely a common ancestor of the animals, a last universal common ancestor (LUCA) from which all animals originated. The LUCA is estimated to have lived 3.5 to 3.8 billion years ago (PDF; http://shiva.msu.montana.edu/courses/mb437_537_2004_fall/docs/uprooting.pdf). The Earth is estimated to be approximately 4.6 billion year old. This amount of time is enough for the LUCA to “split” into different groups of organisms. The Pax-6 gene is not the only aspect that many seemingly different organisms share. There are many ways that the LUCA can be shown to be a common ancestor; for example, all genetic code is based on DNA, which is composed of the four nucleotides, deoxyadenosine, deoxycytidine, deoxythymidine and deoxyguanosine. Thus, it is reasonable to conclude that Pax-6 could have been inherited from the LUCA since all eyes are composed of photoreceptors and pigment cells.
    From the LUCA, organisms that used eyes saw it as a selective advantage. For example, the brownsnout spookfish, Dolichopteryx longipes, (source: http://news.bbc.co.uk/2/hi/science/nature/7815540.stm) lives in the deep sea, about 1000 metres below. The fish has two eyes, each separated into two interconnecting parts; “the spookfish needs one half [of its eyes] to point upwards, to capture faint glimmers of light from the sea surface 1,000m above”. This is an example of how the fish adapted and evolved to meet the survival requirements of its habitat. The example of the spookfish can apply to other fish, and organisms, as well. Not all organisms live in a uniform environment. Thus, they need to embrace certain chance mutations over time in order to meet the requirement to survive and reproduce; different mutations are selective advantages in different environments.

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