Saturday, March 28, 2009
Ultraviolet Rays
On page 91, Carroll states that humans cannot see the ultraviolet range of colors. Why is this? Why is it a selective advantage for us humans to not be able to see in that range of colors? Why can birds and insects see it? Discuss the properties of ultraviolet light and how it helps birds and insects, as well as why it wouldn't help for humans.
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ReplyDeleteHuman's eyes are fine tuned to a specific spectrum of "visible light" that ranges from 400nm to 700nm, and this spectrum is formed because of the three different visual pigments we have, referred to as the SWS, MWS, and LWS opsins. Each of these opsins is fine tuned to a different wavelength that lets us see light that has that wavelength. Since ultraviolet light is out of our wavelength range, below 400, our opsins do not sense it.
ReplyDeleteIt is not necessarily a selective advantage for us to not be able to see UV lights, but rather it is just not beneficial either way. There are actually two possibilities to why humans can not see light, and neither of them includes not seeing the UV light as a selective advantage. First, it is possible that within the hominoids, a mutation that would allow UV light to be visible never occurred. It is well known that in the history of humans, our survival, or the survival of our close predecessors, once depended on gathering and hunting, so it is plausible that enhanced vision would help our species. For this reason, it is most likely that a mutation simply never occurred during the time that we relied on hunting and gathering for survival. We know the mutation occurred in many other animals such as birds and insects, so why did the mutation occur for them? The splits that brought hominoids further from birds and insects occurred long enough ago to give a significant time for both fish and insects to independently acquire this ability after their respective splits. Another option, however, is that such a mutation occurred in a predecessor of humans, but the ability did not give any advantage to the organism, so it simply lost the mutation. The reason this is more likely is that UV light does not play any role in the survival of hominoids. Hominoids are trichromatic, and they acquired this type of vision because natural selection gave those who could distinguish between different food types a better chance of survival. It is clear here that seeing the difference between red and green was most important for the New World primates because it has a direct impact on their survival. Although this might be the reason for hominoids not acquiring UV vision up to this point, there is an answer to why humans will most likely never acquire UV vision in the “near” future. As Carroll hinted at, color blindness in humans poses no real threat to their survival, and for this reason it is a common occurrence. Following this idea, it is clear that the ability to see UV light wouldn’t really help human survival either, so even if a mutation occurred sometime in the future of Homo sapiens, it would most likely be lost to its uselessness for survival.
As for birds and insects that have the ability to see UV light, the relationship between their color vision and survival is much different. Sexual reproduction goes hand in hand with survival of a species, so any trait that aids success of reproduction is favored in a species. In many bird species, both being able to see UV light and produce UV light is a benefit to reproduction because it allows birds to both stand out to the opposite sex and be able to see those who stand out. Just as important to survival is hunting for food, which is a large part of bird’s survival. Being able to see prey easier would give such a large advantage to birds that a mutation of this sort most likely had a very high s value, or selective coefficient, and raised the chance that such a mutation would be retained by the species. Relating back to the reason humans do not possess UV vision, the small effect UV would have on human’s survival, since it does not depend on finding prey, gives it a low selective coefficient to our species.
^ http://www.bio.bris.ac.uk/research/vision/4d.htm
ReplyDeleteCampbell: Biology 6th Edition
Just like Eric had said, the fact that humans cannot see ultraviolet light is not a selective advantage or a selective disadvantage. Throughout the course of evolutionary history, humans and other vertebrates didn’t to have such strong vision because they aren't flying miles about the ground, so other vertebrate have evolved in many other ways that have a stronger natural selection to their habitat. Birds, fish, and amphibians have four types of color receptors in their eyes, while mammals have two types of receptors and primates have three. The four receptors in birds' eyes allow them to detect ultraviolet light and polarized light. They are able to detect these types of lights because they have more light receptors in their retina and more nerve connections between their photoreceptors and their brains. In addition, some birds have more modifications to their eyes that are linked to how they live and where they live. For instance, some birds have a high density of receptors that increase their visual abilities. Also, their eyes are placed to the side of their head so they can have accurate judgment of distances to their prey and so they can detect their predators. In addition, there are nocturnal birds that have tubular eyes with a low number of color detectors, but many rod cell that function better in poor light. Gulls and albatrosses are types of seabirds that have red or yellow oil drops in their receptors that improve their distance vision for hazy conditions that may occur. Therefore, ultraviolet vision has evolved in some bird species but not in others because some birds don't need such acute eyesight because their niche doesn't require it in order for that bird to survive.
ReplyDeleteThe courtship behavior that we previously studies included the powerful preferences of females that shape secondary sexual characteristics and courtship behaviors of males, "for example, the showy displays of peacocks and other male birds during mating season have little to do with direct male-male competition and much to do with advertising robust health to choosy females," (Campbell 1141). Obviously the healthier birds would be the most fit since they would be receiving the right amounts of pigments to color their fanciful bodies. Through ultraviolet light, males may have different pigments showing that we cannot see which is a more accurate predictor of health and female birds would be able to choose the "fittest" males through their preference of color in the ultraviolet realm. Sexual selection pertains to the themes of evolution and heritable information in that sexual selection determined by a female's preferences increases the rate of evolution by selecting for the "fittest" males to reproduce with since "healthy mates provide the best opportunity for producing healthy offspring," (Campbell 1141) and health offspring are those that are most likely to survive and carry the genes/genetic code of their parents. These genes will be able to proliferate (heritable information) and eventually cause those that are not fit enough to reproduce to die off - call it "survival of the fittest" or put it as Carroll states, "making of the fittest." In essence, female (sexual) selection is a pivotal driving force in shaping the evolutionary time frame.
Therefore, humans really have no real need of being able to see ultraviolet light. On the subject of finding food, humanoids, like Eric had said, were used to hunting and gathering. Birds, however, needs extremely good vision in order to spot prey miles away. Moreover, UV vision is an important part of the visual systems of many organisms because it plays many roles in mating, finding food, and feeding. Female blue tits (Cyanistes caeruleus) prefer males whose crests are the brightest and reflect the most UV light. Honeybees use their UV vision to find pollen and conversely, plants have pigments that can be detected in UV vision to attract pollinators like bees.
Sources:
http://jeb.biologists.org/cgi/reprint/209/11/v.pdf
http://beheco.oxfordjournals.org/cgi/content/full/13/1/11
http://www.pnas.org/content/102/18/6391.full
http://www.bio.bris.ac.uk/research/vision/4d.htm
http://www.ncbi.nlm.nih.gov/pubmed/8023459
http://en.wikipedia.org/wiki/Bird_vision
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1617148