Wednesday, March 10, 2010
Atavisms
Coyne explains what atavisms are from pages 64-66 and why they are strong evidence for evolution. "True atavisms must recapitulate an ancestral trait, and in a fairly exact way" (64). These are different from vestigial traits since their occurrence is not seen in every organism of a species, but every once in a while. What are some examples that are provided in the book and how are scientists able to use this as evidence for proving that evolution is true? Also, with all the studies that have been done on birds in the past showing that they can produce teeth, what other organisms may be the target to new experiments and why? On the other hand, if you do not believe scientists will try to bring back past traits from organisms, please explain why. (Hint: think about the definition of evolution and adaptations to the gradually changing environment of present time).
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One example of an atavism the book provides is that of a whale with a rear leg that protrudes from the back of the whale. Atavisms provide the link between a modern species and its ancestors by showing that they share the same genes, they are just expressed differently. The example of the whale helps to prove the theory that whales came from organisms that lived on land since the whale clearly has the genes that can make limbs, they are just not expressed in the majority of whales besides for a few who display the atavism of a rear leg "when something going awry in development" (65). The other example the book provided included horses who have extra toes because their digits continue to develop rather than joining to form one hoof. This atavism helps to show the connection between horses and its 5 toed ancestors because it clearly shows that the organism could be related because they share similar genes. The change in gene shows evolution at work in which that traits that was useful for the ancestors of an organism was "silenced by natural selection when they were no longer needed"(65).
ReplyDeleteWith experiments showing that birds can produce teeth, scientists will soon look to see if there are transitory organism that exist today or have become extinct that show a connection between birds and reptile. Coyne discusses the importance of finding transitory species that can link two completely different organsims because it helps to shows the process of evolution and slow adaptation at work while also linking species that appear to have little in common. According to Creation: the facts of life, the archaeopteryx may be the key link between reptile and birds since it shows characterisitsics from both species. Scientists may also be interested in looking further into the transitory species between reptile and birds in order to see where the gene(s) for teeth was silenced, and why the gene(s) was silenced.
Source:
http://www.answersingenesis.org/home/area/cfol/ch3-vertebrates.asp
In addition to the atavisms that Annie described, another atavism that is discussed in the book is the coccygeal projection, which is more commonly known as the human tail. Early in development, "human embryos have a sizable fishlike tail;" however, some of them "don't regress completely, and a baby is born with a tail projecting from the base of its spine" (65). The types of tails found vary - some are long, some are short, some are just tissue flaps, and some actually have hair, muscles, blood vessels, and nerves (66). The tests on chickens showed the evolutionary relationship between mammals, birds, and reptiles because birds have all the genes necessary to produce teeth, but they don't make them "because a single crucial protein is missing" (66). In the future, scientists can perform experiments using mammal tissue and reptile tissue to show an evolutionary connection between mammals, reptiles, and fish. Other organisms can also be tested so that an evolutionary relationship with fish can be identified. However, some people believe that the research done with chickens and mouse tissue doesn't suggest that birds and mammals are evolutionary connected. Creationists believe that this particular experiment was flawed because "embryonic cells are very flexible in their development. If chick and mouse tissues were taken from different places in the embryo, and then brought into contact, other body parts might develop. This experiment shows that embryonic cells can be manipulated to produce bizarre effects. Further, the similarity of embryonic cells that allowed these strange teeth to develop does not prove common ancestry. Rather, it is evidence of common design by an intelligent Creator who used similar parts to achieve similar ends."
ReplyDeletehttp://www.apologeticspress.org/articles/437
Atavisms are defined, eloquently, by the author as “sporadically expressed remnants of ancestral features” (64). They occur when certain silenced genes are not turned off during development (since by principle of evolution, they are still part of the genome, albeit somewhat degraded from having undergone years of unchecked, chance mutations or general loss of genetic material due to its uselessness in present time). The examples given in the book, which both responses above have addressed fairly clearly (that of the hind leg of the whale, the hooves of a horse, the coccygeal projection in humans, and the chicken teeth) indicate that these modern animals have some connections with their ancestors. Take the horse hooves for example: since they descended from five-toed ancestors, it is seen, through the fossil record, that there is a gradual loss of toes, until modern horses, where only the middle one remains. Occasionally, these toes (which show up during embryonic development) may continue developing so that these horses end up with extra digits including their hooves. Other examples of atavisms include the hind legs on snakes and hind fins on dolphins.
ReplyDeleteThere is an important thing to note, which Coyne states in passing, most likely in assuming its importance, but I believe to be crucial in understanding atavisms; that is, although these atavisms are seen in modern species, without a genetic basis for these traits, they cannot be evidence for natural selection driven evolution. If, when sequencing the genes of these modern atavistic trait-displaying organisms, we cannot find evidence of silenced genes similar to that of the supposed ancestors, then we fail to make any sort of connection. Suppose that a certain trait is seen in some members of a species that is speculated to have originated with an ancestor. Unless it may be seen how the genes have changed from that ancestor to this species, it is quite possible that these genes have undergone a chance mutation to produce this trait. Since, as Coyne puts it, these silenced genes have degraded over time anyways, it is not unlikely that chance mutations may also produce some deceiving traits that we may mislabel as atavisms, when they are not.
The first response does a good job of tying this into the overarching goal of identifying atavisms when stating that “Coyne discusses the importance of finding transitory species that can link two completely different organisms because it helps to show the process of evolution and slow adaptation at work.” There is also something else of importance in this statement – that slow adaptation is a mechanism for evolution. This is why atavisms are such powerful evidence for evolution – that these traits are so unlikely to just “reappear” randomly, if it were not already embedded in the genetic code as a result of natural selection acting upon genes over time.
The above responses have speculated that the experiment with the hen's teeth would possibly be replicated in other experiments in order to find the evolutionary relationships between mammals, reptiles, birds, and fish. Though I do agree that these would be very strong evidence, if these atavisms could be found and experimented upon, in order to identify these changes over time, scientists most likely do not need to design new experiments of this nature, given our ability to sequence and compare genes, unless it is ambiguous in nature, or unless it were to be done at the whim of the scientists. In finding evidence for evolution, scientists only need to show that genes that exist in the modern day organism's genome, and that these genes are silenced, reminiscent of an ancestor who once expressed those genes. As long as this relationship is identified, then this irrefutable evidence for evolution has been found.
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