In Carroll’s discussion of fossil genes, he talks about the reduced reliance of humans on their sense of smell due to the evolution of trichromatic vision. The TRPC2 gene that controls the vomeronasal organ function and the V1r olfactory genes have been fossilized in the human genome. Discuss some other senses that have been relied upon less by organisms with trichromatic vision. What genes have been fossilized to lead to the reduction in reliance on the various senses? Mention the role of mutation and natural selection in gene fossilization as it relates to decreased reliance of certain senses in these organisms.
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ReplyDeleteIt is true that new primates have lost their strong sense of smell due to the evolution of color vision. This probably caused primates to lose their strong sense of taste since they don't need to taste something to know what it is: they can just look at the food and tell its food or not.
ReplyDeleteHowever, the developing new senses such as Trichromatic vision do not always degrade other senses. For example, study done by Andre Fernandez and Molly Morris from Ohio University shows that once the Trichormatic vision was developed, it promoted the red hair and skin through Sexual Selection. In this study, Fernandez was questioning the strict correlation of food choice and color vision while studying howler monkeys in Costa Rica. He recently compiled data on the color vision, social and sexual habits and red skin of 203 different primate species. Fernandez and Morris were able to statistically deduce the probability of ancestors having the same traits, as well if any of the traits were correlated with one another.
As a result, they found that the species that could see red and orange hues were more likely to develop red and orange skin as well as highly social habits that make it easier to visually compare mates. The more social the trichormates are, the more red coloring they show.
This again brings us back to the sexual selection. As the trichomatic vision evolved in primates, it promoted the development of red coloring in our skin or hair which in turn promoted more rigorous sexual selection. Sexual selection, as the GREAT Mr Darwin and Sean Carroll mentions, is a one of the major forces in process of evolution. Even though most primates’ sense of smell got fossilized, they have gained new ability and it helped them to form more recruited social orders and sexual selection.
This example shows important aspect of evolution. As organisms evolve, we are not only moving forward, but evolution itself is deleting genes that we no longer need and helps to develop new genes to survive in a new environment.
Another example of fossilized gene is a human gene called MYH16. On page 134, Sean Carroll explains that the mutation in MYH16 of human gene happened by other genetic pathway. Most likely, as our brain capacity got bigger and the discovery of the fire, the natural selection became favorable to MYH15 mutation.
http://www.physorg.com/news99244352.html
Well first to answer Ryan’s last question gene fossilization is easily recognizable due to the process it undergoes. Under the idea of “use it or lose it” genes that are no longer selectively chosen for will not be preserved through its necessity as a means of prolonged survival. As the gene becomes obsolete, the random mutations that occur will continue to degrade the gene because there is no longer a mechanism to chose for a variation that helps the organism in anyway. Because the gene is no longer used, there in no maintenance of it and mutations begin to fill the genetic code of the gene while it is passed on to successive generations and the mutations begin to accumulate. Carroll states on page 123, “fossil genes are exactly what we would predict to evolve as a consequence of the continuing action of mutation, over time, in the absence of natural selection.”
ReplyDeleteWhile Ryan discusses how the development of trichromatic vision has led to less reliance of other senses and therefore the genes that translate into other senses decay, in many different species there is also gene fossilization in portions of genes coding for vision where more dependence of other senses lead to the unnecessary use of vision. For example in the case of the coelacanth, its natural habitat includes deep underwater caves and it passes along the ocean floor at night looking for food. Like the shifted vision of deep water organisms like the bottlenose dolphins to adjust to the dimmer light that penetrates the water, coelacanths have fossilized MWS/LWS opsin genes for it no longer needs the red-green vision in its natural habitat and lifestyle. A mutation in the gene codes for a stop codon early in its translation and causes the gene to be useless, a fossilized gene. The SWS opsin gene is adequate for the vision of coelacanths in deep water environments and has led to a less reliance on the red-green vision of MWS/LWS opsin gene.
Similar to the lifestyle of coelacanths, the shift to nocturnality in different species has lead to the fossilization or reduction of vision. The example of the blind mole rat demonstrates how the natural habitat and lifestyle has shifted the need of the sense vision due to a reliance of other more necessary senses. While the MWS/LWS gene is still selectively chosen for in the purpose of detecting changes in light, the SWS opsin gene has become completely fossilized. The blind mole rat sports a life underground, where vision is not a necessity for survival. Actually, other selective adaptations have form to accommodate the lifestyle. Instead of the vision, other sense have been heightened such as its way of navigate. The blind mole rat navigates through heightened senses of walking and the use of the Earth’s magnetic field to correctly orient itself in the right direction.
Another important fact to note is the preservation of the gene V1r and its use in pheromone detection. While in humans and most higher primates, the V1r receptors are no longer needed with the large fossilization of olfactory genes, in the howler monkey there is a coexistence of trichromatic vision and pheromone perception. The TRC2 gene that codes for the regulation of ion channels is also intact in howler monkeys. As Sammy stated above, trichromatic vision is used by higher primates for sexual selection as well as choice of food, preferring the reddish younger leaves and ripened fruit for better digestion. In sexual selection, the primates detect color changes of female sexual skins which could possibly have lead to visual-based signaling-sensory mechanism. The preference of this visual sexual selection overlaps the need for a pheromone detection system, but contrary to this conclusion, there still remains the V1r and TRC2 genes coding for a pheromone detection system. Through this finding, it cannot be completely attributed to the development of trichromatic vision as the sole factor in the fossilization of numerous olfactory genes.
http://www.thefreelibrary.com/How+blind+mole+rats+find+their+way+home-a0113896711
http://en.wikipedia.org/wiki/Blind_mole_rat
http://mbe.oxfordjournals.org/cgi/content/full/21/4/697
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=314465