Wednesday, April 8, 2009
Deja Vu
In Ch. 6, Carroll begins talking about how different species evolve similarly or differently to achieve the same end result. One method he talks about is how “different means” lead to “similar ends” . What were the different means that led to the similar ends of the Arctic and Antarctic fish antifreeze? Why do you think different means were needed to get to the same end result?
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Arctic and Antarctic fish antifreezes were created about 8-12 million years apart from one another. In addition, Arctic fish and Antarctic fish are not immediately related to one another. These two facts make the idea that the antifreeze gene is a common gene unlikely. In fact, when looking at the DNA and the origin of the two genes, it is clear that the two genes are different. Antarctic antifreeze originated from a digestive enzyme. There is no such enzyme in the digestive systems of Arctic fish. Even more important than that is the idea that the two lines of antifreeze contain different separators. In Antarctic fish, repeats of leucine-isoleucine-phenylalanine separate the repeats of threonine-alanine-alanine or threonine-proeonine-alanine. However in Arctic fish antifreeze, the spacers are different. While the two antifreezes are very similar in nature to one another, the two antifreezes contain different spacers and originate in different ways. Because of the different situations (no digestive enzyme in the Arctic fish and difference in time), the two antifreezes were formed differently. Since natural selection occurs randomly, it would be highly unlikely for two identical antifreezes to be created in two independent situations. The reason why the two antifreezes are so similar in nature is because it is known that the threonine-alanine-alanine or threonine-proeonine-alanine repeats are the most effective for preventing ice formation. Over time, natural selection will select the most efficient way for the two fish to create antifreeze.
ReplyDeleteDifferent species which are completely separated can evolve into having similar traits because of the nature of evolution. While we may think that since the antifreeze of the Antarctic and Arctic icefish are similar, they must have a common ancestor, this is clearly not true in this case. Both types of icefish have antifreeze genes composed of repeating sequences of threonine-alanine-alanine or threonine-proline-alanine. However, this gene structure evolved from the genes for a digestive enzyme in Antarctic icefish, while this gene is not present in Arctic icefish. In addition, as David pointed out, the development of the antifreeze gene occurred at different times in each type of icefish. Therefore, the icefishes did not evolve from a common ancestor. The reason that the two types used different means, but ended with the same result, is due to the evolutionary theme that "evolution repeats itself" (36). Natural selection selects for traits that are most advantageous for the species' environment. therefore, if the environments of two species (such as the icefishes) are similar, then natural selection will select for similar traits. This is what happened with the Antarctic and Arctic icefishes, where their environments were similar, so they both evolved to have antifreeze, even though the species were unrelated. The reason why different means were needed to achieve the same result was that the two species were different. Therefore, the two species had different original genes to work with. For example, the Arctic icefish does not have the genes for the same digestive enzyme as the Antarctic icefish used as a basis to evolve the antifreeze. Therefore, by starting with different materials, different avenues are needed for each species to develop the same trait.
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