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Welcome to the discussion group for Making of the Fittest by Sean Carroll. Prompts and posts are student generated. This is a collective effort to engage in discussions that connect the theory of evolution with the biological concepts and themes discussed in our course throughout the year.
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Each opsin is encoded by a separate gene. Humans are known to have 3 different visual pigments: tuned to short, medium, and long wavelengths of light. They are known as SWS (417 nm blue), MWS (530 nm green), and LWS (560 nm red). When light reaches a visual pigment, the pigment becomes "excited" and this state causes a "firing" of the photoreceptor cell. The PNS transfer information from the photoreceptors via neurons which are then integrated by the visual areas in the cortex of the brain. In order for an object to be considered colored, "at least two kinds of cone photoreceptors must be triggered," (Carroll 97).
ReplyDeleteThe form/protein structure of an opsin affects the function of an opsin - especially its light absorbing properties - through particular changes in its amino acid sequence. "Advances in understanding the functional properties of opsins have revealed that is is very easy to shift the absorption spectrum of individual opsins by changing particular amino acids...There are just fifteen amino acid differences between the green and red pigments," (Carroll 104). Biologists have discovered that the amino acids at positions 180, 277, and 285 are responsible for the 30 nm difference between the absorption peak of the green versus the red pigments. (See Table 4.1 in Carroll p. 105).
The functions of various opsins, especially that in humans, provide a tremendous selective advantage. The three opsins (SWS, MWS, and LWS) allow for color detection. Such color detection enables us to become attracted to certain colors - for example food. If food is a sickly green as seen by the MWS opsin, a human may not be attracted to it; however, if a food is a bright red (as seen by our LWS opsin), we are most likely attracted to it because brighter colors have a sense of "healthy" as part of their connotative description. If we were prey to some large predator, our color vision would enable us to distinguish the predator's color from the environment's color enabling us to survive and reproduce. Not only are the three opsins providing a huge selective advantage, but the rhodopsin (497 nm) allows us to see in dim light. When humans may be walking at night, it would be a selective advantage for us to see in the dark (somewhat seeing) so that we may survive and reproduce. Thus, the biological theme of natural selection still applies, even here, to the effect that selective advantages provided by opsins can allow us to survive and reproduce; in addition the theme of structure/function applies with the idea that as the structure of a polypeptide is changed, its entire function is also changed.