Monday, April 5, 2010
Do Atavisms have a Purpose?
Coyne discusses atavisms, which is "an anomaly that looks like the reappearance of an ancestral trait" (64-55). He continues by providing examples, such as the legs of whales. Why do you think atavisms occur? Is it because of genetics, evolutionary pressure, or for some other reason? Provide an example of such an occurrence that has been cited or studied that supports your reasoning for why atavisms occur. Also, do you think that atavisms can be evidence of devolution? Provide examples, if any exist, and explain how they apply. Finally, consider atavisms in the context of the mechanism of evolution: natural selection. Is it selectively advantageous for organisms to be able to have atavisms and why?
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Atavisms are remnants of ancestral traits that reemerge due to a developmental error in regulation, particularly of gene expression. Coyne asserts that organisms have "genes that were functional...but were silenced by natural selection" (65). During typical development, differentiation controls what genes will or will not be expressed. This can occur from asymmetric segregation of cellular determinants. In other words, varying cytoplasmic environments can affect the fate of cells. Differentiation can also occur via inductive signaling. This occurs when one cell dictates the fate of another by protein communication. Cells then specialize when proteins travel through the nuclear pores and turn genes on or off. If an incorrect signal is sent due to an unfavorable cytoplasmic environment or an error in transduction, the wrong genes may be turned on. This includes ancestral genes that are unfavorable to the modern organism. Thus, one is left with atavisms like whale legs, snake legs, extra horse toes, and chicken teeth. Overall, problems in regulation are responsible for atavisms.
ReplyDeleteThe presence of an atavism is not a selective advantage. For instance, the coccygeal projection, or human tail, serves no function. It does not help with movement nor is it sexually attractive. Therefore, it is a waste of energy to fabricate the tail. Thus, those with the atavism are less likely to survive and reproduce. Although atavisms may not be advantageous, they do prove the theory of evolution, specifically through continuity and change. Evolution is the slow process of modifying earlier models into something more fit for the environment. Atavisms prove that genes don't just disappear and that the genome is a continuation of previous organisms. Thus, traits are not the result of a designer but of previous organisms. The continuity and change of evolution is clear through the appearance of ancestral features.
Sources:
http://www.talkorigins.org/faqs/comdesc/section2.html#atavisms
http://biology.kenyon.edu/courses/biol114/Chap11/Chapter_11.html
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ReplyDeleteLike Coyne said, an atavism is a “trait produced by the occasional reawakening of ancestral genes that have long been silenced” (Coyne 56). True atavisms must restate an ancestral trait in a fairly similar way. No one can know for certain why atavisms occur, but scientists’ best hypothesis is that they come from the reexpression of genes that used to be functional in ancestors but, like Collons emphasized, natural selection silenced them when they were no longer needed. Present in all animals, these dormant genes can sometimes be reawakened when something goes awry in DNA replication and development. For instance, whales still possess the genetic instructions on how to make legs, and modern horses still have the genes for having more than one toe (hoof). Although these genes are silenced and only on rare occasions are activated, they still reappear sometimes and generate great attention to them. Who wouldn’t be interested to see a whale with a leg!
ReplyDeleteDevolution, also known as backwards evolution, is the notion that a species can change into a more primitive form. Some modern species lost functions and their complexity and seem to be degenerate forms of their ancestors, hence making it appear as though evolution is working backwards (Wikipedia). Personally, I don’t think atavisms are a sign of devolution. Atavisms happen by chance when a DNA sequence is mistakenly changed and activated. A six fingered human doesn’t represent a less complex human, and a whale with legs doesn’t represent a less complex whale. These organisms aren’t moving backwards in their development, they are simply memories of the past that by chance occur in the present. Atavisms don’t seem to be advantageous to species, because like Collons mentioned, they don’t serve a true purpose and are a waste of energy to fabricate. If a human is born with a “tail” or with six fingers, doctors are most likely going to remove that extra body part because it isn’t beneficial to its owner. It will only use energy to maintain itself but it won’t fulfill any true purpose and won’t enhance the survival and reproduction of the organism. This extra energy can be used for other functions, such as defense and reproduction. Natural selection wouldn’t favor these structures because they don’t aid in “survival of the fittest.” I don’t think atavisms are an example of evolution nor devolution; they are simply sporadic “accidents” that occur on random occasions.
http://en.wikipedia.org/wiki/Atavism
http://animals.howstuffworks.com/animal-facts/atavism.htm/printable
http://devolutionmatters.wordpress.com/