Sujin Ko (sujinko93@gmail.com)
Sunday, April 10, 2011
EYES (fish vs. humans)
In Chapter 9, Shubin describes the two different kinds of eyes; one kind found in invertebrates and the other found in vertebrates (fish vs. humans). These two types of eyes work to increase the surface area where light is gathered in the eye; invertebrates do such by having many folds in the tissue, while vertebrates lots of tiny projections that extend from the tissue. What are some disadvantages and advantages of having the invertebrate type eye? The vertebrate type eye? Why is it that the vertebrate-type eye is more advantageous to humans?
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Some invertebrates like insects have compound eyes, which have many folds in their tissue. Some vertebrates, by contrast, like humans have simple eyes, which have only one lens (http://en.wikipedia.org/wiki/Eye).
ReplyDeleteCompound eyes benefit from having more lenses, as all of the lenses' signals together compose a mosaic image, with each lens representing basically light or dark (http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CompoundEye.html). Since each lens in a compound eye does not have high resolving power or clarity, organisms with compound eyes need to have many lenses in order to see moderately well.
Simple eyes work like a camera lens, with one lens focusing light onto a retina farther back in the eye. Many simple eyes, like those in humans, are highly developed and can focus an image with high accuracy.
One advantage of compound eyes over simple eyes is that they have a very large viewing angle, allowing their organisms to detect fast movement (like a hand swatting at a fly, for instance), and sometimes they can even detect the polarization of light (http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6W66-4M7CMGY-1&_user=10&_coverDate=12%2F31%2F2006&_rdoc=1&_fmt=high&_orig=gateway&_origin=gateway&_sort=d&_docanchor=&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=ec66e8d5baf22b0e65fed3dd6813bc1f&searchtype=a).
Simple eyes, when highly developed, as in humans, can generate images with much greater resolution for the size of the eye than can compound eyes. This is because compound eyes rely on a large number of low-resolution lenses to generate an image, whereas simple eyes can invest their development in the resolution of a single lens.
Simple eyes work well for humans because we benefit greatly from accurate, high-resolution vision. We can quickly turn our heads to effectively increase our field-of-view, and we can also make out fine visual details from a great distance. Compound eyes, which are easier to develop than simple eyes with a human-like resolution, can see a greater area at a given time at a lower developmental cost, albeit at a lower resolution. This works well for flies, for instance, because they do not heavily rely on sight for distinguishing different surfaces or objects, owing to their use of smell and other senses.
- Vincent Fiorentini
(vincent@panatechcomputer.com)
There are many different types of eyes seen in different animals. Vertebrates have camera-like eyes while invertebrate animals have “simple photoreceptor organs” (149) that they use as eyes. Some invertebrate eyes are very similar to vertebrate eyes, like those in the squid and the octopi. The one difference is that a vertebrate eye is “reversed” while the cephalopods’ eyes are not. This means that in vertebrates, the inner layer of the double walled optic cup becomes the retina. So, the light-sensitive portion of the eye (rods and cones) ends up on the outside. Therefore, light has to go through all of the layers before it reaches the rods and cones. In the squid and octopi eyes, it is the opposite where the light sensitive portion is on the inside surface.
ReplyDeleteThe vertebrate eye is advantageous because it has evolved to be able to control the amount of light entering the eye. This was helpful for vertebrates because it allowed them to be able to hunt in either highly lit or dim environments. However, the vertebrate eye does have some disadvantages. First, the eye has a blind spot. This blind spot is caused by “the eyes being an extension of the brain, which, upon formation are pinched off and the optic nerve and blood vessels are limited to go through one small hole at the back of the eye” (http://www.d.umn.edu/~olse0176/Evolution/mammals.html). This part of the eye doesn’t have any photoreceptive organs which causes the formation of the blind spot. This disadvantage doesn’t prove to be a big deal to vertebrates, however, because we have learned to deal with it. Another disadvantage is that the eye’s protective layer, the cornea, is easily damaged. So, a scratch in the eye can cause a lot of damage to a person’s vision. It might have been for this reason that the eyelid developed in vertebrates: to protect the cornea.
The diagram on page 151 of Your Inner Fish shows how the eye has evolved from invertebrates to vertebrates. This helps to show how the vertebrate eye has evolved to become more advantageous by allowing the animals to see clearer. The disadvantage to the invertebrate eye is that animals like the limpet, the nautilus, and the scallop were not able to see as clearly as animals with the more recent vertebrate eye.
Sources:
http://www.d.umn.edu/~olse0176/Evolution/mammals.html
http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab11/Eye/NOTES/EYENOTE.HTM
Danielle Webb (dwebb456@gmail.com)
Shubin comments on the “remarkable” diversity of organs and tissues that animals use to capture light, otherwise known as eyes. Organisms carry out various specific functions from simple photoreceptor organs to complex compound eyes. Eyesight has allowed a variety of organisms to be at a selective advantage. Eyes, whether invertebrate or vertebrate, are all closely related regardless of looks. The universal concept of vision follows the stimulation of receptors, which in turn sends signals to the brain for interpretation.
ReplyDeleteIn 1990, a crucial discovery was made with a mutant fruit fly without eyes. A gene involved nicknamed “eyeless” was mapped out, and results showed that the production of the eyeless gene depended on a transcription factor to control the formation of the eye. This gene is believed to have had a dominant controlling force upon the development of eyes. Gene expression of this specific “eyeless” gene was found to switch development pathways involving several thousand of genes. The eyeless gene was conducted in experiments, which resulted in adult flies with extra eyes and body parts. Scientists were able to also discovery that the eyeless gene sequence was similar to the Pax gene, in mice, that led to the development of tiny eyes. Experiments with this gene also led to eye development at various sites. A gene whose expression usually led to the development of a vertebrate “camera” type of eye led to a more invertebrate “compound” eye, by the mutation of a single transcription factor. The differentiation of the eye between species has followed a common developmental theme. As eyes evolved for the specific and advantageous purposes in organisms, they have all shared a common gene. (http://www.accessexcellence.org/WN/SUA01/master_eye_gene.php)
Sophisticated eyes have evolved by natural selection in abilities to avoid predators and scavenge for food. Specific organisms such as insect and octopus are some of the few species to have evolved compound eyes. Unlike the single-lensed eyes of the vertebrates, compound eyes of flies and other invertebrates are composed of individual lenses called ommatidia. Each ommatidium that descends into the core of the eye has a complex structure (corneal lens, crystalline cone, pigment cells, retinula cells). The brain pieces messages signaled to the brain together to form a “mosaic-like” image of the organism’s surroundings like Vincent has addressed. (http://www.ebiomedia.com/index2.php?option=com_content&task=view&id=154&pop=1&page=0)
In addition, compound eyes have very limited range of vision, and can only see a short distance away. Contrary to this, the vertebrate eye is able to see extreme detail. For an invertebrate to have the magnitude of vision we have, the compound eyes would need to be around 3 feet wide. However, the limited resolution that invertebrates such as insects have may come as a great selective advantage. The compound eye is able to go through much quicker image processing, as it offers a greater “flicker fusion rate”. Especially in flight, invertebrates can assimilate to changes very efficiently. Also, compound eyes are able to see visually at a variety of angles, while camera-type eyes have a blind spot. (http://park.org/Canada/Museum/insects/evolution/evolution.html) (http://www.d.umn.edu/~olse0176/Evolution/mammals.html)
Our camera-like eyes are able to focus on images with much greater magnitude and distance, and are able to focus into images with high resolution. We do not need compound eyes, because we are not high flying or moving as fast as an invertebrate such as a fruit fly. Human’s camera-like eyesight is advantageous in expanding the visual aspects of local environment and see vivid detail. It is unnecessary for species like humans to ever need compound eyes, and our high-resolution sight allows us to be effective scavengers.
Kyle Kim, kkim847@gmail.com