Wednesday, April 6, 2011
Light Receptors
On page 153, Shubin relates human eyes to Old World monkeys. Old World monkeys, like us, had 3 different kinds of light receptors that helped them perceive the world around. Explain how these receptors work and why an organism that has developed vision has a distinct advantage over an organism that doesn't. Shreeraj Patel shreeraj.patel1@gmail.com
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Humans Old World monkeys have three different kinds of light receptors. Each of which is tuned to a different kind of light which allows them to perceive the world around them. According to Shubin, it is possible to trace the origin of color vision by examining the genes that make up these three receptors.
ReplyDeleteMost mammals only have two kinds of receptors for vision which implies that the third receptor (that Humans and Old World monkeys posses) must have been copied from the two receptors and especially evolved for special light. This is similar to what we see in odor receptor genes. It is believed that this evolution must have occurred due “to changes in the flora of the earth millions of years ago” (Shubin 153). Shubin writes that monkeys must have developed this third receptor in order to help distinguish the fruit in the trees so that they can pick the one that is most advantageous to their health. It is estimated that this color vision was formed about 55 million years ago. The increased diversity in forests with respect to color can be related to the development of the third color receptor in monkeys which induced a shift from the monochromatic forest.
“The gene duplication also supports the neutral theory of evolution. Initially, the duplicated genes provided exactly the same opsins and provided little or no selective advantage. However, they did no harm and so were not lost. Once mutations began to set in, shifting the spectral response of the opsins, the genetic variability created by the duplication mutation began to make itself felt.”
After the split of New World and Old World monkeys it is generally believed that early primates came west by rafting events, possibly migrating from one island to another. In any case, that puts a time limit on when the duplication event occurred. Color vision proved an important adaptation, permitting primates to identify more nutritious foods. Leaves are common but not very nutritious, so the ability to spot the most nutritious leaves provided a significant survival advantage.
The selective advantage of trichrome vision is apparently considerable with respect to being color-blind. Color-blindness makes life so much more difficult for them that they don't reproduce well enough to carry the trait on. (http://www.vectorsite.net/taevo_22.html)
-Adnan Jahan
(adnanjahan@gmail.com)
The light receptors, as seen in humans and Old World monkeys, help us to see and distinguish colors. There are two types of light receptors, rods and cones that are present in the retina, which is the inner layer of the eye, and are especially concentrated in the fovea, which is the central point where the light is focused to help us visualize an object clearly. Of the two light receptors, the one that help us distinguish color are the cones. The rods help us distinguish the brightness, and are more sensitive, but they cannot distinguish colors, as the cones can, but are less sensitive. (Campbell, 1101) There are three types of cones, and they each can sense red, green and blue wavelength, each being 565nm, 535nm, and 440nm at peak, and can sense other wavelength, but will stimulate most strongly with each ‘peak’ wavelengths. (http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/V/Vision.html)
ReplyDeleteAs Neil Shubin says in page 153 of Your Inner Fish, the animals before the Old World monkeys only had two types of genes that could distinguish color, which made them have only two types of the cone receptors. This can even be traced in humans as well, as the color blindness of humans can show how the ‘mutated’ gene from the previous animals can still be seen. Most of the color blindness occurs not because one cannot distinguish colors at all, but because one confuses a color from another. The human X chromosome contain two of the three types of the opsin genes, red and green colors. For a human to distinguish from red to green, one needs at least one of each gene, but multiple of the same ‘color’ gene is also fine. However, if one has none of one type of the gene, or if one of the genes is mutated, then one suffers from color blindness. Also, a thing to note, while the red and green vision are both present in the X chromosome, the blue vision opsin gene is present in chromosome 7, which makes the case of color blindness for blue rare. (http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/V/Vision.html) Since two of the three genes that distinguish color are from one chromosome while the other is in a completely different chromosome, it is the red gene that mutated from the green gene that was present in the pre-Old World monkey animals. (http://anthro.palomar.edu/primate/color.htm)
Starting with the monkeys, the formation of the trichromacy was from the selective pressure of their primary food source. As Adnan stated, the mutations of the color gene that was made 55 million years ago became a great advantage over other animals as they could distinguish the colors of the fruits more clearly. As Andrew Smith of the University of Stirling states, the additions of the red colors could help them decide which fruit was ripe and which was not. Also, the trichromacy helped the monkeys who ate leaves predominantly. (http://anthro.palomar.edu/primate/color.htm)
Having a brother who has color blindness, I know that not having to distinguish colors clearly is a very difficult task for them. Even though the animals before the Old World monkeys might not need that extra red pigment to live their lives, the monkeys and us humans need such colors so that we can distinguish what we eat, and how we live.
John Park (wisejsm@yahoo.com)
Both John and Adnan described the advantages to having three different kinds of light receptors. They also both mentioned the problems humans have with colorblindness. If colorblindness serves as such a detriment to humans today, why is it still relatively common? A common type of color blindness is red-green color blindness. This affects a generally small amount of the population. Knowing that colorblindness occurs when a genetic mutation causes a change in one of the three pigments in the retina, this can be defined as variation in a species (1). This variation between those with this mutation, other mutations, and without mutations causes one of the several conditions to be more advantageous than the others. This is known as a selective advantage. In the case of the Old World Monkeys, those that weren’t colorblind had the ability to better determine which fruit was more nutritious. Thus, they were able to grow stronger and reproduce more successfully thus passing down the genes that didn’t code for color blindness. Those with the advantage eventually wiped out those without by being able to out-compete their peers for resources and continue to pass on their genes. This led to the evolution of the species so that all members didn’t suffer from colorblindness. So then why is this condition still present today?
ReplyDeleteOne explanation could be that the evolutionary time scale just hasn’t reached the period where colorblindness is wiped out. But this would mean that to evolve from colorblindness to full vision of all types of colors took significantly longer than the time period it took apes to evolve into humans. This is highly unlikely. However, it could be possible if colorblindness ever served an advantage. An example of this is the ability of those that suffer from red-green colorblindness, or deuteronamoly, to better differentiate between shades of khaki than those with normal vision. As apes evolved to become humans, their environment stayed the same. Often surrounded by dense foliage, it was difficult to find food sources. The ability to determine between shades of khaki, a color more commonly thought of as brown but also known to describe the color of asparagus, would have allowed for the ability to better determine what was a nutritious food source and what was simply grass (1). Perhaps this was a case of coevolution. As apes evolved to be able to better access biotic resources, they needed a way to protect themselves. They could have developed a deeper shade of green to camouflage themselves. Since coevolution is defined as a change in the genetic composition of one organism causing a change in the genetic composition of the other, it is important to be able to prove definitively that this occurred (2). Unfortunately, there is no concrete evidence that this occurred and is simply a possibility. However what is known is that at some point, unless a mutation caused a new form of colorblindness after the evolution of ape to man, colorblindness had to provide some sort of advantage.
1. http://www.nature.com/news/2005/051205/full/news051205-1.html
2. http://biomed.brown.edu/Courses/BIO48/27.Coevolution.HTML
-Robbie Thomashow
(diehardcubsfan93@comcast.net)