Wednesday, April 1, 2009
Nocturnal Monkeys
On pages 124-127 Carroll talks about how the opsin gene, in owl monkeys, seemed to have "accumulated mutations that render it nonfunctional" (124) and render the monkey color blind. This is due to a lifestyle that the owl monkeys have adapted? What other types of monkeys or similar creatures, who are closely related, also adapted this nonfunctional opsin gene? What kind of interaction with its environment caused this to occur? How exactly can this gene be deemed fossilized and nonfunctional (what kind of a change in genetic code)? Furthermore all these point that a relaxation of a certain gene will lead to its decay. How is this related to us human? What gene or organ, might I say, has become almost nonessential because of our loss of use for it and what was its previous purpose and why dont we need it anymore?
Subscribe to:
Post Comments (Atom)
According to Carroll, the nocturnal lifestyle of the owl monkeys, the only nocturnal monkey of the “higher primates” (124), has made the ability to see color nonessential. Thus, over evolutionary time random mutations, specifically a substation of TGG with TGA, has made the opsin gene non-functional by causing it to terminate prematurely. Most likely owl monkeys have evolved to be the uniquely nocturnal primates because predation levels are lower during the night. Nocturnality also provides another advantage by enabling these monkeys to avoid most sympatric species. The owl monkeys are able to exploit a wider range of habitats while minimizing dangers of injurious encounters with larger primates and the need to defend large territories. Being one of the largest nocturnal frugivores, the owl monkey is able to out-compete most sympatric species active at night.
ReplyDeleteNot only owl-monkeys, but prosimians have also experienced the decay of a gene as a consequence of a nocturnal life style. The fossilization of this gene, however, has occurred in different ways. According to Carroll, instead of a substitution mutation, this gene has undergone deletion mutation: “Each gene has a big chunk of code missing near to beginning of the gene that obliterates the ability to make the opsin” (124).
Just as living a nocturnal lifestyle does not require color vision, so too does an underground lifestyle reject color vision. To explain, blind mole rats have evolved their anatomy and physiology to have eyes that can note detect images because they are covered by a layer of fur. However, fossilization of the rudimentary opsin gene and different mutations enable these rats to use their eyes to regulate their circadian rhythms: their biological clock.
Figure 5.5 of the book, pg. 125, summarizes the cases described above. All of these cases demonstrate the direct correlation between the fossilization of a gene and the habitat/lifestyle of a species.
Natural selection has also specifically targeted which members of a species receive a fossilized/adapted version of the opsin gene. For example, Color vision among New World primate species is surprisingly variable. Some of them are dichromatic and others are trichromatic. In some species, only females can see reds. Research done by Andrew Smith of the University of Stirling in Scotland has shown that male marmosets, tamarins, and spider monkeys only see blues and greens. About 40% of the females in the same species apparently are also dichromatic, but the other 60% are trichromatic. Both male and female howler monkeys are trichromatic. An explanation for the separation of vision is that the Old and New World monkeys became separated 30-40 million years ago and have been traveling down their own evolutionary tracks since then. It is likely that mutations in the X chromosome gene or genes that provide the ability to see red colors occurred after this separation.
Several researchers have proposed hypotheses about the nature of this selective pressure of the variations of vision in primates. Smith believes that trichromacy provides an important advantage for fruit eating species. It can be a valuable aid in determining when fruit is ripe. It also makes it easier to find orange-red fruit against a background of green forest foliage. Dichromatic monkeys are like colorblind humans in that they have difficulty distinguishing visually between green and ripe fruit. While this hypothesis sounds plausible, it may not provide a complete answer because many trichromatic monkeys and the apes predominantly eat leaves. Once again, color may be a valuable clue for such species since the edibility of leaves from the same tree or shrub often varies with their maturity, which can be signaled by color. Peter Lucas of the University of Hong Kong observed that macaques use this as a clue in finding the most desirable leaves to eat. Whether the food is fruit or leaves, the ability to see reds would make it easier to pick out a food target in a green vegetation background. One other very different hypotheses has been proposed by Emily Liman of the University of Southern California. She links trichromacy to the fact that when females of some Old World monkey and ape species are in estrus, they develop reddish sexual skins or swellings. The male ability to see reds would presumably lessen the importance of their relatively poor sense of smell in detecting female pheromones.
Although humans as a species have not inherited a fossilized vision gene like the owl monkeys, about 6-8% of humans today are red-green color blind. Most of them are men. This X-linked inherited condition known as deuteranomaly is due to opsin pigments that are normally sensitive to green light behaving more like the red-sensitive ones. This results in a difficulty in distinguishing between colors in the red and green wavelength ranges. However, people with this condition are at an advantage in differentiating slight variations in khaki colors. This could have been a benefit in the dry grassland environments of East and South Africa where humans first evolved, which may be why this trait has not been deemed injurious by natural selection.
Humans have, though, experienced a “fossilization” of organs in the digestive system (i.e. the appendix). The appendix is an example of a vestigial organ. Humans have a fair number of other vestigial organs such as male nipples, wisdom teeth, tailbones (coccyx), and ear muscles.
The appendix is present in many primates, and primarily (pun intended) used to aid in the digestion of cellulose. Located between the small and large intestines, the appendix and neighboring caecum slows down the body's digestive process. The human appendix has lost this cellulose-digesting ability. Dr. Douglas Theobald argues that while humans do consume some cellulose, the ability of the caecum and appendix to digest it is insignificant. Consequently, plants like grass cannot be digested by humans. While the human appendix has lost its aptitude for digesting cellulose, recent studies have shown that it may play a different, but still trivial, role in the gastrointestinal immune system. Dr. Theobald suggests that the percentage of immune system cells produced by the appendix is not critical when compared with the overall number of lymphoid in the gut. Thus, the positive effects from the human appendix seem to be non-existent.
Natural selection has not disregarded the appendix all together for one reason because the odds of dying from a clogged appendix are merely seven percent, it is likely that this number is just not large enough to kill off all of the carriers. Potential benefits of appendixes in hunter-and-gatherer times are also possible. If 30,000 years ago the appendix produced cellulose-digesting bacteria, humans may have been able to digest grass and other plants which would have been particularly useful in times of famine. Lastly, the energy "wasted" in building the appendix in modern men may also be trivial. Perhaps the energy required is so insignificant that it could not have been better spent elsewhere. In addition, in comparison to other animals, the appendix is in a way fossilized in humans because it is smaller. Thus, the non-plant based diet of animals has rendered the appendix nonfunctional in humans, just as the nocturnal lifestyle of owl monkeys has rendered the opsin gene nonfunctional.
Sources:
http://serendip.brynmawr.edu/sci_cult/evolit/s05/web1/bfremstad.html
http://www.owlmonkey.com/aotus97.html
http://anthro.palomar.edu/primate/color.htm
Nocturnality is a type of niche differentiation that can occur for various reasons. One reason for nocturnality is crypsis, which is the ability of an organism to avoid observation. If the organism is only active at night, then its predator has a much lesser chance of finding the organism and attacking it. Also, an animal can become nocturnal in order to avoid the intense heat of day, especially in the desert. Nocturnality allows animals to avoid losing excess amounts of water during the daytime, making it an adaption for osmoregulation. In the case of the owl monkey, nocturnality probably evolved in order to avoid predation. Since these animals live in the forests of Central and South America, water loss is probably not that big of a threat.
ReplyDeleteAs nocturnality evolved in animals, relaxed selection allowed for the degradation of their opsin genes. Since there was less and less need to see as these animals became nocturnal, the mutations that occurred in the opsin genes were not removed by natural selection. As the number of mutations in the genes added up, the amino acid sequences of the proteins made by these genes differed even greater, and eventually became dysfunctional. Also, as their opsin genes were fossilized, these animals developed high-quality senses of hearing and smell. These new senses helped the animals survive in their habitat without being able to see. Another adaption that the owl monkeys have to survive nocturnally is that they form pair bonds with other members of their species, which increases their ability to find resources and survive without colored sight.
Owl monkeys are the only truly nocturnal monkeys, but many other animals are nocturnal. Many diurnal animals sometimes show nocturnal behavior. Sea turtles often breed during the night in order to prevent predation, which is probably why owl monkeys evolved nocturnality. Full nocturnality probably didn’t evolve in the sea turtle because predation isn’t as big as a threat as it is for other animals, such as the owl monkey. Other animals, such as bushbabies and owls, exhibit nocturnal behaviors.
The relaxed selection in owl monkey opsin genes, which eventually made them dysfunctional, is similar to how relaxed selection made human olfactory receptor genes dysfunctional. According to Carroll, “about half of all our olfactory receptor genes are fossilized and incapable of making functional receptors” (128). Many of our olfactory receptor genes are fossilized because we don’t rely as much on our sense of smell as our ancestors and other mammals. This is probably because our trichromatic vision has allowed us to find resources and detect predators with less reliance on smell, just as how nocturnality has allowed owl monkeys to survive with less reliance on sight.
Sources:
ReplyDeletehttp://www.pnas.org/content/102/41/14712.full.pdf
http://en.wikipedia.org/wiki/Nocturnality
http://www.saveamericasforests.org/Yasuni/Biodiversity/Night%20Monkey.html
sofia should get 20 points
ReplyDelete