Monday, March 8, 2010
Vestigial Features or Atavisms?
Starting on page 55, Coyne begins to discuss the "Remnants" of evolution - mainly vestigial features and atavisms. Vestigial features are features of animals that remain from ancestors. Vestigial features have either lost all function or have a new and, usually, reduced function. Atavisms are traits in animals from historical ancestors that are occasionally reawakened through odd gene expression; for example, some humans are born with an atavistic tail (Fig. 14 p63). Both are examples of imperfections that both support the theory of evolution while disproving the theory of creationism. In your opinion, is either observation (vestigial features or atavistic traits) more compelling evidence for evolution than the other one is, or are do both observations support evolution equally?
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While I agree that both vestigial features and atavistic trait show evidence of evolution, I believe vestigial features pose the stronger argument.
ReplyDeleteFirst, in order for atavisms (a random expression of a trait that used to be present in an ancestral species but has disappeared) to prove that evolution has occurred, one must first be aware of ancestral traits. For example, when the baby pictured on page 63 was born with a small "tail," the scientific community all claimed this was clearly an atavism. In this situation, it is easy to say the tail is an atavism because the tail is remnant of the tails our ancestors had. But what if we did not know our ancestors had tails? Or what if a baby (not necessarily a human one) was born with a strange feature, but scientists do not know if past species had this. While scientist may call the strange feature an atavism, one cannot be sure that is feature is remnant of the past; the strange feature could very well be a new never expressed before mutation that is a new selective advantage of disadvantage.It may be difficult to determine from only one individual whether a mutation is an atavism, or a first time mutation when the past of an organism is not completely known.
Vestigial features, on the other hand, are far more uniform throughout a population of a species. Since all individuals of a given species have the specific vestigial feature, it can be concluded that the genes coding for the feature were not transcribed "accidentally." In contrast, atavisms represent random mutations; mutation that are more difficult to say whether are remnants of the past of new first time mutations. When a feature representing an organism's evolutionary past is present throughout a population(instead of randomly within only one individual) there is more solid evidence of evolution. Therefore, vestigial traits (with the large number of individuals showing the trait) are better evidence that evolution has occurred.
I agree with Sammie that vestigial traits are a more compelling evidence for evolution but atavistic traits also can be used to back up evolution. Vestigial features are features that have lost function due to time. The organism has no need for the function anymore due to a selective advantage acquired. For example on p. 57 Coyne explains that the wings on an ostrich is a vestigial trait. The fact that the ostrich has wings but can't use them means that the wings probably descended from a bird that could fly. The ostrich probably doesn't fly because flying takes up to energy. Also most ostriches live on islands where there are no predators so the ostriches don't need to fly away to get away from their predators. Because the ostrich is saving energy, not being able to fly is a selective advantage. A higher being wouldn't have created wings on an ostrich if the wings had no purpose to the bird. The wings must have been passed down from a bird that had wings for flying through evolution. An atavism trait is a trait that is usually not seen but is seen due to the genes being expressed from a previously existing phenotype from an ancestor of the organism. Like Sammie had said this doesn't always prove evolution because creationists could say it's a mutation that has happened. Also if we don't know whom the organism's ancestors are, how are we able to tell if the trait is an atavistic trait. An example of an atavistic trait in humans, found in the website statemaster.com, is that sometimes humans may possess very large ape-like teeth. Most humans don't have large teeth but occasionally some do. Humans have descended from apes/primates so when a human has a gene that expresses large teeth it probably came from our ancestors the apes. If we hadn't known that humans had descended from apes we would have never known that DNA code for big teeth had come from apes.
ReplyDeleteVestigial traits can be used to prove evolution but atavistic traits can only be used to back evolution, once you already know that a certain organism has descended from another organism. Atavistic traits can't be used to prove that the organism descended from a certain organism.
Both Sam and Anika made the point that it is necessary to know an organism's ancestry and ancestral traits in order to realize an atavism; however, I believe that the same argument could be made for vestigial traits. For example, Coyne mentions that "Whales are treasure troves of vestigial organs" (60). Coyne goes on to discuss some of those organs, for instance, hind limbs and pelvic bones. These traits would be considered vestigial, since whales do not have legs. These features are only significant in supporting evolution, having known that whales evolved from terrestrial ancestors, like cows. If a person does not know that whales evolved from cows, then how can hind limbs be proof of evolution?
ReplyDeleteDespite this argument, I believe that vestigial features and atavisms provide tremendous proof of evolution. As Sam pointed out, I think vestigial traits provide greater support because they are present in every individual of a certain species, whereas an atavism is usually a rare occurrence. While reading this chapter, Coyne definitely had me convinced that by using vestigial traits and atavisms we can see that evolution is inevitably true. However, when discussing embryology on pg. 80, Coyne mentioned a possibly atavistic behavior, the "grasping reflex" of infants, that in a way "shook" my belief in atavisms and their reliability. The "grasping reflex" occurs when a person touches an infant's palm, causing the baby to make a fist and grasp. Coyne mentions that a similar grasping behavior occurs in monkeys allowing young monkeys to hold on to their mother's fur for a long time. I felt that this example backtracked a little bit and made me doubt atavistic traits. I do not fully understand how the connection can be made between infants grasping when their palm is touched and the ability of monkeys to grasp tightly. It seemed to me like Coyne (or the person who first brought this idea up) may have been jumping to conclusions, and so this made me doubt the power of atavisms a little bit. When can a trait be considered atavistic? Can any behavior automatically be considered atavistic if something similar occurs in an ancestor? Reading about the "grasping reflex" made me think that maybe atavisms should be strictly physical traits, like human tails.Those were the types of atavisms that really convinced me that evolution happens.
I agree with Sami and Anika in respect to atavisms and previous knowledge. Without knowing what traits our ancestors possessed, it is indeed hard to try and say that a 'different' or unusual trait (like a child being born with a tail) is a result of previous, ancestral traits being washed up. What if the alleged atavism is simply a first-time mutation? Without substantial knowledge about the organisms that came before us, one can’t know.
ReplyDeleteAlso, because vestigial traits happen in most organisms where as atavisms are rare, vestigial traits provide more compelling evidence for evolution. Going along with that, I disagree with Tavor on the point of previous history when it comes to vestiges though. She gives the example of whales having hind legs, and how one would not use this as support of evolution unless the person knew about the whale’s relation to and evolution from land mammals. Though ancestral knowledge would no doubt be beneficial, it isn’t crucial to make the evolutionary conclusion, because of the nature of vestigial traits and organs. Coyne argues that a celestial creator would never give an organism an organ that shows similarities to an organ of a different organism, but has no purpose in this ‘newer’ species. (58-59) What would be the point of including a useless structure? The only reason that makes sense in explaining the presence of vestigial traits is one that describes the evolution of an organism where an ancestor had a certain trait that was no longer a selective advantage in the species that evolved from it. Remnants of once useful organs are important because these organs were ONCE USEFUL in a different organism. If we are strictly using these organs to simply demonstrate evidence of evolution, then unlike Tavor said, it doesn’t matter what preceding ancestor had this vestigial trait. What matters is that modern organisms like whales have, for example, useless remnants of hind legs, because this modern ‘uselessnes’ is very unintelligent design, indicating that the structures must be a byproduct of an organism’s evolution over time.
One of the themes that is relatable here is interdependence in nature. As stated in the COB, “an organism’s phenotype is the synergistic product of genes and the environment.” Traits are passed down through generations from ancestral organisms, like hind legs in terrestrial mammals. However, with the move of whales back to water, the environment change no longer made hind legs a selective advantage. Instead, the mutations that brought about long, strong tails were more useful; accordingly, legs became less and less prominent, turning into a vestigial trait with no apparent usefulness for whales today.