Wednesday, April 8, 2009
eyes
On page 194 Carroll examines the large differences in eyes of different species, mainly the single lens human eye, the compound eye of many arthropods, and the three eye system of scallops and clams. Explain how each eye system works and how it is beneficial to each organism. Also think about if there is any change that could happen to any organism eye including structure and placement and explain the possible changes.
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On page 194, Carroll discusses the different types of eyes and how they may have evolved. Humans, having a single lens that acts like a camera. Other species, however, have seemingly different setups. Crabs, for example, have many independent eyes that all compound their information to display what they see. Seeing as how these two examples are vastly different, many cases in the past have believed that different systems of eyes have developed independently of one another. However, recent studies have shown that these eyes are more similar than they appear. Eyes are made of the same materials and the same basic concepts. Nevertheless, different species developed these materials differently to better adapt to their environments.
ReplyDeleteThe type of eyes that crabs and other arthropods are believed to be one of the first examples of compound vision. This setup is called apposition eyes. Since many different images from different eyes are grouped together to form an image, this gives the organism better vision in low-light situations.
Although many scientists have described the human eye as a simple design, in truth, it is complexly developed and has a very intricate design. At the light sensitive area at the center of the eye, there are numerous specialized proteins and protein systems. The first step to vision is the recognition of photons from specialized molecules called 11-cis-retinals. This molecule changes shape when it comes in contact with the photons and creates a positive feedback with another molecule that changes shape called rhodopsin. The whole process ends with an electrical imbalance that sends a message to the brain, resulting in vision. If the specialized proteins that started this reaction didn’t exist or function correctly, this vision system would not work. However, this system is advantageous to humans because of our environment. It allows vision in color and in light. The area that takes in light is not as big as a nocturnal organism, so we can better survive in well lit environments.
Based on detail, organisms develop different attributes based on their environments. For example, male birds that need to mate to reproduce have a higher acuity so they can see farther and sharper. This is a selective advantage because it allows an easier way to find a mate. Another example would be nocturnal insects that need to be able to see with the absence of light. In this case, many have developed larger eyes to take in more light so they can find food or mate in their dark environment.
http://www.detectingdesign.com/humaneye.html#discussion
http://en.wikipedia.org/wiki/Eye
http://www.sdnhm.org/kids/eyes/basics-single.html
Meagan strongly detailed how the human eye works, but I would like to get more into Mitchell question by comparing and contrasting the single lens human eye to the compound eye to the three eye system and how all of these are beneficial to that particular organism.
ReplyDeleteEyes detect light and send signals along the optic nerve to the visual areas of the brain. However, there are many different types of eyes. Each type of eye can fall into two categories, "simple eyes" or "compound eyes." The eyes that are categorized to be simple eyes do not have a reduced level of acuity because they are called simple, because any type of eye adapts to its environment. Compound eyes do have a downfall. Organisms that have compound eyes can not have a resolution better then one degree. In addition, eyes fall into another group, that is based on their photoreceptors and how the cells form those photoreceptors. Photoreceptors are the part of the eye that capture the light, and they can be either ciliated, like found in the cnidaira, or rhabdomic. Rhabdomic photoreceptors are transparent rods that are found in compound eyes like the eyes of arthropods.
There are also subgroups to the simple eye, which include the pit eyes, pinhole eyes, and spherical lensed eye. A pit eye are eye-spots that are set in a pit, or large eye cavity in order to change the angle that the light enters the eye so the organism can figure out where that light is coming from. Pit eyes are one of the first types of simple eye and lack the advance qualities of the some of the other types. A pinhole eye is considered to be an advanced form of the pit eye. It has a smaller aperture and a deep pit. This smaller aperture does lead to a blurry image that doesn't have a lens to be focused on. If the aperture would shrink then lens light could enter the eye. Pinhole eyes are only found in nautiloids. A nautiloid is a group of marine mollusks that are mainly predatory organisms. There have been 2,500 different species discovered. Finally, the spherical lensed eye is yet another improvement to the pit eye through evolution. Because the resolution of the pit eye is so poor, the spherical lensed eye improved it by using a material that has a higher refractive index to form the lens. This reduces the blur. Gastropods and annelids have the most basic form of the spherical lensed eye and they can see much sharper images. The lens has a high refractive index which decreases to the edges of the eye, and this decreases the focal length and allows a sharp image to form onto the retina of the eye. Also, some organisms have more than one lens in their eye. The copeopod Pontella has three lenses. The outer lens has a parabolic surface that allows a sharp image to be formed but doesn't allow for spherical aberration. Copillas have two lenses in their eyes that move in an out to focus the image, like a camera or telescope.
The human eye has three layers of tissue. The sclerotic coat is the tough layer that is the white of the eye. This coat becomes transparent where the cornea lays. The cornea gives light to the interior of the eye and it bands the light rays so they are focused. The tear glands secret tears which keeps the cornea moist and dust-free. Also, the choroid coat is the middle layer of the eye that is pigmented with melanin. This coat reduces the reflection that stray light can cause. The choroid coat also forms the iris, which is pigmented and is the eye color. The pupil is located on the choroid coat and the size of it determines how much light can enter the eye. It is controlled by the autonomic nervous system. When there is little light then it opens wider to allow more light to enter, but when there is a high intensity of light then the pupil becomes very small to keep the light entering the eye to a minimum. The retina is the inner layer of the eye and contains the light receptors called the rods and cones. In addition, the retina has interneurons that processes the signals that the rods and cones take in before sending those images to the brain. This single lens in the human eye is adapted to fit our needs. It can allow all different proportions of light to the enter the eye by the pupil either dilating or being constricted. Therefore, there will never be too much or too little light that could harm or interfere with the image producing process. In addition, we don't have multiple lens because it is not necessary in our environment to have more than one lens. We don't need to hunt down rabbits with our eyes or see the tinniest ant move on the ground, but other organisms may need such specialized eyes in order to survive. Also, some organisms go to sleep right when the sun begins to go down but as humans we can see in relative amounts of darkness as long as there is enough light for our eyes to be fully dilated.
The arthropod eye is made up of repeating units called the ommatidia that each function as a separate visual receptor. Each ommatidium has a lens, a transparent crystalline cone, light sensitive visual cells, and pigment cells. The pigment cells make sure that only light entering the ommatidium parallel reaches the visual cells and triggers the nerve impulses, so each ommatidium is pointed at a single area of space and only provides information for that area that it is focused on. Thousands of ommatidia make up a compound eye. The image produced by a compound eye is a composite of all of the responses from the ommatidia to form a mosaic image or a pattern of light and dark dots. The more dots then the finer and more clear the image appears. The compound eye is beneficial to the arthropod because it is very good at detecting motion. When an object is moved across the visual field of the eye, the ommatidia are turned on and off which creates a "flicker effect". This allows the arthropod to be able to easily detect a moving object compared to a still one. In addition, arthropods may be active in low light situations, so they can concentrate the screening pigments of their ommatidia into the lower ends of the pigment cells. Therefore, light entering the ommatidium at an angle passes another ommatidia and stimulates that one too.
The three eye system is when an organism has three lenses in their eye. It allows for better resolution of light and focusing of that image because the image must pass through more panels then in a single lens human eye.
Through evolution, many organisms' eyes have changed placement on their head. A deer for example has its eyes located on the side of its head so it can better locate predators and prey. By having the eyes on the side they are "watching" from different angles, while our eyes are relatively both pointing straight forward, so to see our sides we must turn our whole head. In addition, some organisms need to be able to detect the tiniest movement in order to capture their prey, so a sharp focusing eye would be essential in order to survive and reproduce.
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CompoundEye.html
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/V/Vision.html
http://www.biology-online.org/2/10_natural_selection.htm