The presence of five opsin genes in primitive lampreys and four opsin genes in birds, reptiles, and fishes indicates that color vision evolved early on in vertebrate evolution. Non-primate mammals, higher up on the evolution tree than fish or reptiles, only have two opsins. Thus fishes, reptiles, and birds have full color, trichromatic vision while non-primate mammals have dichromatic vision and can only see the visible spectrum from blue to yellow (no red or green). Sean Carroll postulates that the loss of color vision in non-primate mammals was due to the nocturnal lifestyle of early mammals. Color vision might have been unnecessary but was it so detrimental that natural selection would eliminate it from the early mammalian branch? Wouldn’t it have been more advantageous for mammals to fine tune their three or four opsins to fit their nocturnal needs rather than eliminate opsins?
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ReplyDeleteI agree with Karthik in saying the "natural selection doesn't pick and choose what to change" but rather picks the strongest and best suited traits from what mutations have made available. However in this case I think it is more significant to recognize that color vision was not detrimental, but rather it was useless - improving upon it you have been rather pointless. There species lost the SWS posin gene because "it became dispensible to the ancestors of these species" (Carroll 123). As we see in Chapter 5, the gene for color vision became a fossil gene - one that was allowed to debby due to lack of use/necessity. This concept is more easily understood if one contrasts fossil genes to immortal genes. Immortal genes are genes that a particular organism cannot survive without and reproduce without. Fossil genes are genes that have no effect on an organism's ability to survive and reproduce whatsoever. Natural selection becomes "relaxed" (Carroll 123). Thus, as is only natural, "fossil genes are exactly what we would predict to evolve as a consequence of the continuing action of mutation over time, in the absence of natural selection" (
ReplyDeleteCarroll 123). This is a prime example of the true manner in which evolution works - it does not intelligently pick and choose, nor does it move with purpose; it merely lets organisms with an advantage prosper. Here, having color vision did not provide and advantage, so the trait was not preserved.
Generally, natural selection will terminate a certain gene if that gene is detrimental to the lifestyle of any specific organism. For non-primate mammals, having color vision probably was detrimental because, as stated by Sean Carroll, color vision didn't fit the lifestyle of a nocturnal organism. Rather then having three opsins able to detect all general colors, it would be better for nocturnal organisms to have opsins that are more sensitive to shorter (SWS) and medium (MWS) waves of light. Having these opsins would shift the color spectrum for those non-primate mammals more towards violet, allowing them to be able see more clearly in dark environments. One example of how nocturnal organisms with two opsins can be selected for are with nocturnal owls. The owls with two opsins are able to track prey more efficiently because those owls can concentrate more on the movement of the prey rather then trying to process the color of the prey. Owls that can do this will have a higher chance of being selected for because those owls will be able to eat and reproduce.
ReplyDeleteAs for the question if it would have been more advantageous to change the three or four opsins to fit the needs of a nocturnal organism, I believe that natural selection doesn't pick and choose what to change. If a certain trait is harmful or causes an organism to not reproduce, then that trait will slowly be erased. The trait of having two opsins helps nocturnal animals more by allowing those organisms to be able to find food, shelter, and help reproduce.