Friday, April 3, 2009
Sexual Selection and Ultraviolet Light
In many cases, the forces of evolution have been shaped by sexual selection. In the book, Carroll mentions that "many birds have tuned an opsin to detect ultraviolet light" (110). What mutation(s) occur in birds for the birds to see in the ultraviolet range? How does this ability to detect ultraviolet light connect to how the females select mates? How would the different characteristics (relating to ultraviolet light) of the male be a measure of his fitness? One could relate this phenomenon to the courtship behavior we have learned about in the ecology unit (Campbell 1141). Please examine how sexual selection pertains to the themes of evolution or heritable information.
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The mutations that occur in birds for them to see in the ultraviolet range include a specific site at position 90 in the bird SWS opsin. If the amino acid at position 90 is serine, birds see in the violet range; however if the amino acid is cysteine, the birds see in the ultraviolet range, "just this single amino acid difference shifts the absorption maximum by 35 to 38 nm, a very dramatic shift," (Carroll 111). Carroll has found compelling evidence that the evolution of ultraviolet vision has been affected by sexual selection where mates preferences are determined by colors and patterns visible in UV light.
ReplyDeleteHere's a Journal Article on this study specifically: http://www.pnas.org/content/94/16/8618.full?ck=nck
Here's an interesting explanation for the possible characteristics of the male (bird) as a measure of fitness, that relate to ultraviolet light:
"For many animals, it's not size, but color
Birds may signal their fitness by their color. A male bird that is well fed and strong can accumulate lots of carotenoid pigments from plants to give himself a colorful appearance. When a peacock displays to a pea hen, he is showing her how healthy he is-so healthy, he can afford to squander energy accumulating and arranging pigments in his magnificent tail. But melanin, a dark pigment that many animals can synthesize, also may signal fitness or dominance. For example, says Eaton, the size of the male house sparrow's black throat patch seems to signal his dominance in a flock."
Source: http://www1.umn.edu/umnnews/Columns_SciFri/SciFri_2.11.05_Color_me_essential.html
The healthier the male is, the more pigments it has received and thus he can show his mate, the female, that through reproduction, their genes can be carried as well as their offspring will survive and reproduce.
The courtship behavior that we studied in the autumn describes 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 are 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.
I disagree with Daniel Sheikh’s comment that “birds signal their fitness by their color”. This is a flawed assertion because “humans were assessing the colors of birds” (Carroll 110). With an ultraviolet vision, birds can see colors that humans cannot. Birds have “evolved body markings that reflect light in the ultraviolet part of the spectrum” (Carroll 110). Thus, it is possible that it is the ultraviolet markings that actually play a role in mating. For example, in a research conducted by the University of Bristol, researchers discovered that female blue tits are attracted to the males with the “brightest ultraviolet-reflecting crest” (Carroll 110). Female tits see the bright ultraviolet markings as a sign of fitness in male tits. Since females invest more energy into reproduction, it is natural that they would want to choose a healthy mate. A fit mate would be able to defend and feed her offspring. Additionally, a health mate would also have the “fittest” genes. These genes would be passed on to her offspring and increase the offspring’s chance of survival.
ReplyDeleteFemale birds act as the selection agents of their species. Since females prefer ultraviolet marking on males, only the males with the brightest markings will be able to reproduce. Initially, there were probably variations in the male populations and some males did not have ultraviolet markings. Unfortunately for these individuals, females did not find them attractive enough to mate with. Therefore the non-ultraviolet marking gene died out in the gene pool and all members of the species have ultraviolet markings.
You may also be wondering how are birds able to see ultraviolet light while humans can’t. This is because human SWS opsin have an absorption maximum at 417 nm whiles birds SWS opsins have an absorption maximum at 370 nm. A single point mutation on position 90 of the bird SWS opsin gene can determine whether the bird will have violet or ultraviolet vision. A serine on site 90 will give the bird violet vision while a cysteine will produce ultraviolet vision.