 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
Natural Science Forum / Biology / Paleontology / April 2007Layman's question on Newsweek article
The March 19 Newsweek article on evolution has something I don't get. P. 54, col. 2: "Human DNA and chimp DNA differ by no more than 1.2 per cent, and DNA changes at a fairly regular rate. That lets scientists use this rate to calibrate a 'molecular clock' whose tick-tocks measure how long ago a genetic change occurred." I do not understand how random mutations produce "a fairly regular rate" of DNA change, especially considering that only some of the random changes confer a survival advantage. Are they talking about a long-term probability function over millions of years, or what? Thaks for all explanations. (I will be cross with anyone who thinks I'm a Creationist troll. I'm a Carl Sagan fan and was raised on Bertrand Russell and Tabasco sauce.) > The March 19 Newsweek article on evolution has something I don't get. > P. 54, col. 2: [quoted text clipped - 12 lines] > I'm a Creationist troll. I'm a Carl Sagan fan and was raised on > Bertrand Russell and Tabasco sauce.) You'll find a pretty good article here: http://en.wikipedia.org/wiki/Molecular_clock A lot of people think the assumption that there is a regular turnover of substitutions questionable. What is done, however, is to use known data from the fossil record to identify the youngest date range for calibration.  SignatureJohn S. Wilkins, Postdoctoral Research Fellow, Biohumanities Project University of Queensland - Blog: scienceblogs.com/evolvingthoughts "He used... sarcasm. He knew all the tricks, dramatic irony, metaphor, bathos, puns, parody, litotes and... satire. He was vicious." >>The March 19 Newsweek article on evolution has something I don't get. >>P. 54, col. 2: [quoted text clipped - 19 lines] > A lot of people think the assumption that there is a regular turnover of > substitutions questionable. Which people? And what do you mean by "a regular turnover of substitutions"? > What is done, however, is to use known data > from the fossil record to identify the youngest date range for > calibration. > The March 19 Newsweek article on evolution has something I don't get. > P. 54, col. 2: [quoted text clipped - 8 lines] > random changes confer a survival advantage. Are they talking about a > long-term probability function over millions of years, or what? It's just like any random process. If you add up enough events, the percent standard deviation can get as small as you like. Air pressure is, after all, just a sum of the random collisions of vast numbers of molecules. Now in fact if mutations conferred an evolutionary advantage the clock wouldn't work. It's only those mutations that are neutral (which turns out to be most of them) that are expected to conform to any sort of clock. For such mutations, the mean number fixed in the population per unit time is the same as the mean number that happen per individual in that time. Since this is a poisson process, the variance is equal to the mean, and thus the standard deviation is the square root of the mean. If we're talking about 35 million mutations (the number separating human from chimp), the standard deviation would be around 6 thousand, a negligible percentage of error. > Thaks for all explanations. (I will be cross with anyone who thinks > I'm a Creationist troll. I'm a Carl Sagan fan and was raised on > Bertrand Russell and Tabasco sauce.) This seems like an odd newsgroup to ask a question about molecular evolution in. > The March 19 Newsweek article on evolution has something I don't get. > P. 54, col. 2: [quoted text clipped - 12 lines] > I'm a Creationist troll. I'm a Carl Sagan fan and was raised on > Bertrand Russell and Tabasco sauce.) Most of our DNA is not doing much other than maybe confusing viruses and occupying space. A lot of it is old viruses. Random changes don't seem to make much difference in many of these segments so you can use the average rate of change in them to roughly measure time. If the stuff is active it's a whole other story. > Most of our DNA is not doing much other than maybe confusing viruses and > occupying space. A lot of it is old viruses. Random changes don't seem to > make much difference in many of these segments so you can use the average > rate of change in them to roughly measure time. If the stuff is active it's > a whole other story. Is noncoding DNA used in these studies. That is, do they look at mutations in codons that don't code for an amino acid? The assumption has long been made that this is "junk" DNA with no uses. If that was true, they would make the perfect DNA clock. Recent studies show they may have some selective value as place holders. Although such DNA probably has some uses, I would think their bias with respect to DNA dating would be far less than that of coding DNA. >>Most of our DNA is not doing much other than maybe confusing viruses and >>occupying space. A lot of it is old viruses. Random changes don't seem to [quoted text clipped - 3 lines] > > Is noncoding DNA used in these studies. Sometimes yes, sometimes no. > That is, do they look at > mutations in codons that don't code for an amino acid? Actually, if they don't code for amino acids (or are not stop codons), then they aren't codons at all. A codon is a three-base sequence that, when translated into RNA, is read by a ribosome. > The assumption > has long been made that this is "junk" DNA with no uses. Not true. Most of it is junk, but a small percentage is not, and this has long been known. > If that was > true, they would make the perfect DNA clock. Not true either. There are differences in mutation rate among sequences that would make a clock imperfect even if all evolution were neutral. > Recent studies show they > may have some selective value as place holders. > Although such DNA probably has some uses, I would think their bias > with respect to DNA dating would be far less than that of coding DNA. Maybe. Depends on the coding DNA. There are unfortunately differences in evolutionary rates among taxa and among sequences within taxa, even in neutrally-evolving DNA. >> Most of our DNA is not doing much other than maybe confusing viruses and >> occupying space. A lot of it is old viruses. Random changes don't seem to [quoted text clipped - 9 lines] > Although such DNA probably has some uses, I would think their bias > with respect to DNA dating would be far less than that of coding DNA. Okay you got me. I don't completely trust that the non coding DNA isn't doing something. It is late and my brain isn't working at its best at the moment but I seem to recall a recent article that stressed this point. Some of the "junk" was impacting genetic expression. |
Reply to this Message |
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2009 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.