Sunday, April 5, 2009
Owl monkey
When mammals were developed they were originally nocturnal color vision was not necessary. However, most developed mammals currently use color vision and are not nocturnal. Color vision is possible with the presences of a functional SWS opsin gene. This gene is present in mammals including most primates. However, it has been found that in the owl monkey the SWS opsin gene is not functional because a triplet of “TGG” was changed to “TGA”(124). The change in the triplet causes there to be a stop codon that causes the rest of the gene to be unread. Is it feasible that the owl monkey became nocturnal and then the SWS opsin gene became a fossil gene? Why or why not? If so what could have caused the owl monkey to become nocturnal(think in terms of ecology)?
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hen you say that "most developed mammals currently use color vision", I have to protest. Sean Carroll tells us that "nonprimate mammals are impoverished with respect to color vision and opsin genes," and we know that New World Monkeys also do not have trichromatic color vision (Carroll 102-103).
ReplyDeleteThe SWS opsin gene you mentioned is indeed present in mammals, but in most cases it is fossilized; trichromatic primates have redeveloped color vision via duplication of the opsin gene Carroll 105-106). It is AFTER this redevelopment of full color vision that the owl monkey experienced the genetic change you mentioned, rendering it color blind.
It is likely that the owl monkey first became nocturnal, then color blind. If the owl monkey became color blind before it became nocturnal, then color blind monkeys would be at a disadvantage; the trait would still be under selective pressure. However, once the monkey became nocturnal, the trait for color vision was under relaxed selective pressure, allowing it to degenerate. Here again we see the theme of evolution selecting only for what is useful at the time being. Trichromatic color vision was not an advantage to the now nocturnal owl monkeys, thus it was not preserved.
I, like my good friend Mr. Bersin, must disagree with you when you say that "most developed mammals currently use color vision" because developed is a very relative term that should not be used in biology. All organisms are technically "developed" if they are currently living.
ReplyDeleteAs for your first question, it is most probable (and makes the most sense) that the owl monkey became nocturnal before "losing" the SWS gene. Otherwise, a mutation in SWS gene of the owl monkey ancestor that caused it to become color blind would have been a selective advantage if it was diurnal. Once the owl monkey became a nocturnal animal, it could not see in color anyway so there would be no selective disadvantage to losing the SWS gene. Therefore, over the onslaught of time and mutations, the SWS opsin gene slowly became fossilized again due to the lack of selective pressures working for its preservation.
For the second question you posed, which Mr. Bersin forgot to answer, I believe that the owl monkeys (along with most other mammals at the time) became nocturnal due to predation. During the prehistoric times, being a mammal and diurnal would have been a selective disadvantage because the mammals' main predators, dinosaurs, were also diurnal. Therefore, natural selection would have favored a shift towards nocturnal behavior by mammals because therefore the nocturnal mammals would avoid their diurnal predators and have a better chance at survival and reproduction. Therefore, sometime during the paleozoic era, a common ancestor of many mammals including owl monkeys must have become nocturnal.
I believe that the owl monkey must have become nocturnal before it became colorblind (by the fossilization of the SWS gene). This is because, as Mr. Bersin and Mr. Goone so aptly put it, if the owl monkey were to become colorblind before it became nocturnal, the monkeys with the fossilized SWS gene would be at a disadvantage compared to monkeys with full color vision, since the monkeys would still be diurnal. Therefore, since colorblind monkeys would be at a selective disadvantage, natural selection would act to eliminate colorblindness, since colorblind individuals would ahve a lower chance of surviving and reproducing. However, if the owl monkeys became nocturnal before they became colorblind, the SWS gene coding for full color vision would be neither an advantage or a disadvantage. Natural selection on the gene would be relaxed, and since there would be no selective pressures to retain full color vision, mutations in the SWS gene rendering it useless would not be eliminated by natural selection. Therefore, over a long period of time, the SWS opsin gene would be fossilized and no longer functional.
ReplyDeleteFor your second question, I agree with Mr. Goone that predation is a likely factor for the owl monkey becoming nocturnal. If the predators of the monkey were diurnal, then being nocturnal would confer a huge selective advantage, since it would allow the organisms to escape predation. However, there are also other factors that may have led to the owl monkey becoming nocturnal. The owl monkey's food may be easier to obtain during the night than during the day. Certain insects eaten by the owl monkey only emerge at night or are more prevalent during the night. In addition, being nocturnal could eliminate competition to an extent. If the species competing with owl monkeys for similar resources were only active during the day, owl monkeys would face less opposition in gathering resources during the night.