Monday, April 6, 2009
Number of Opsins
What is the evolutionary significance of humans having three opsin genes while the majority of other mammals have only two opsin genes while birds and fish have four opsin genes? (pg. 97)Discuss the role of the opsin gene in animals and why more opsin genes could be necessary in birds and fish while fewer opsin genes could be necessary in most mammals.
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The function of opsins are to recognize various wavelengths of light, therefore with more opsins, an organism would have more precise vision, which is obviously important for all predators. A predator needs to be able to efficiently and quickly find its prey in order to survive, especially if the predator must compete with other organisms. For example, it is a huge selective advantage for a bird to have more opsin genes because it is vital for a bird to have precise vision to be able to quickly find prey to fuel its high metabolic rate and to quickly provide food for the hungry chicks. While flying quickly way up high in the sky, birds need good vision in order to find prey, like worms and other insects, all the way down in the soil. Birds with poor vision would have lost the opportunity to find prey, and would not have survived and reproduced, thus birds now have a more precies eyesight, with four or more opsin genes, making them the most suitable to survive. However, mammals and other animals with less opsin genes have other methods of more efficiently finding food. Chimpanzees rely on their aggression to protect their territory from other competition. Chimps don't rely as much on their eyesight, but more on their physical strength. But clearly eyesight is still vital for chimpanzees to be able to see, leaving the chimps with three opsin genes, and not necessary selective for any more than the three opsin genes, while birds with the best vision had a huge selective advantage over the birds with worse vision.
ReplyDeleteEvolution is not a synonym for progression. Rather, evolution is a selection process where genetic variations in a population are screened and selected so only the genetic variation that provides the fittest organisms would remain in the gene pool. The phenomenon of the opsin gene is an example where the selection process has caused evolution to backtrack. The human genome have 3 opsin genes that is responsible for trichromatic vision while most mammalian genome only contain 2 opsin genes responsible for a dichromatic vision. To most people this would make sense because evolutionary speaking, humans are the most advance organisms on this planet. However, having more opsin genes doesn’t make the organism more advance. The most “primitive groups of vertebrates, such as the jawless lamprey have five opsin genes” (Carroll 102) and fish and birds have three or four opsin genes. As evolution progressed, the number of opsin has actually decreased!
ReplyDeleteFishes have retained their three or four opsin gene because the ability to distinguish color is important in the life history of fishes. For example, the sign stimulus for fixed action pattern in stickleback is a red belly. Thus, color vision is crucial for male sticklebacks to defend their territory and nest from other male sticklebacks.
For early mammals, however, the third opsin became more or less disposable. One factor that could account of this lose is the fact that early mammals were nocturnal and dwelled in underground caves. It the dark, it doesn’t make much of a difference if you can only see in all the colors. As a result, the selection pressure on the third opsin gene it relaxed and gene is rendered useless as mutations gradually accumulate in it. However, this third opsin gene resurfaced in old world monkeys a result of the monkey’s lifestyle. Dichromatic animals cannot see the red and green part of the color spectrum and would not be able to distinguish the “younger leaves that are most nutritious and tender, and easier to digest” (Carroll 92) because younger leaves often have a red hue. Monkeys that have trichromatic vision and can distinguish the young leaves from the old would be at a selective advantage to their dichromatic counterparts. Since this third opsin gene evolved early in old world monkeys, most primates, including humans, have trichromatic vision. This time, the third opsin gene remained in our genome because it posed primates are no longer nocturnal and need a colored vision to fully perceive our environment.
Although I agree with Paul Kim that since primates, such as chimpanzees, use territory aggression to obtain food and don’t need a good vision, I still think that color vision is valuable to chimps because it would help chimpanzees distinguish pristine leaves from the mediocre leaves.
Not only does bird have a trichromatic vision, they also have a 4th opsin that allows them to see ultraviolet light. Carroll speculates that the ultraviolet vision allows birds to see colors that we can not, which would play a significant role “in mating, finding food, and even the feeding of young nestlings (Carroll 110). Since this forth opsin enhances the bird’s survival and reproduction, selection pressure would preserve this gene because it would be a selective advantage for birds.
The opsin is a protein within the cone-shaped photoreceptors in the retina of the eye. Each opsin is fine-tuned to a different wavelength of light, and therefore each opsin, when stimulated, produces a different color image. The fact that humans have 3 opsin genes means that humans have trichromatic color vision, what we call "full color vision." In humans, our three opsins are fine-tuned to detect red, green, and blue light wavelengths. Mammals with two opsin genes have only dichromatic vision, while birds with four opsin genes can detect the same colors humans can, and additionally can see in the ultraviolet spectrum.
ReplyDeleteI must object slightly to your phrasing, asking why the evolutions of opsin genes would be necessary. As has been stressed many times, evolution does not happen because species NEED it to happen. However, I can explain why the different numbers of opsins may be ADVANTAGEOUS to those species. As Carroll points out on page 112, many birds use ultraviolet colors in choosing mates. Therefore, having 4 opsins and being able to see ultraviolet would be essential for the mating process. In addition, ultraviolet coloring allows birds to see the mouths of their chicks for feeding, as well as being used in hunting prey. The reason why many mammals have lost good color vision by only having two opsins has to do with the fact that many early mammals became nocturnal and lived in dark areas. As Jia pointed out, if an organism lives in the dark, color vision is not useful, and is not an advantage. Since trichromatic color vision was no longer an advantage, natural selection did not select for trichromatic color vision. In essence, natural selection was relaxed for trichromatic color vision, and as a result it was lost in those mammals.