-Caveat Lector- Activist Mailing List - http://users.westnet.gr/~cgian/ January 28, 1999 Genetic Mutations Accumulating Rapidly, Scientists Say By NICHOLAS WADE Biologists analyzing human genetic data in the DNA data banks have given fresh meaning to the saying that no one is perfect. Harmful mutations have accumulated so fast in the human genome, according to a new study, that the immediate question is why the human species has not become extinct. Although human populations are evidently doing fine, common minor afflictions like weakened eyesight, headaches and stomach upsets could reflect this inherited baggage of adverse mutations. And some biologists fear that as the bite of natural selection is relaxed by medical advances, the mutational baggage could become more significant in the centuries ahead. The effective mutation rate in the human genome is estimated in the new study as being at least 4.2 mutations per generation, of which at least 1.6 mutations are harmful. This is a high number considering that a harmful mutation can be eliminated sooner or later only by the "genetic death" -- death without progeny -- of its carrier. The study also found that humans have retained a much larger proportion of adverse mutations in their genome than have other animals, like mice and rats. The authors of the study, which appears in Thursday's issue of Nature, are Adam Eyre-Walker of the University of Sussex in England and Peter D. Keightley of the University of Edinburgh. The high retention of adverse mutations probably reflects the fact that human populations have been extremely small throughout their evolutionary history. In small populations it is easier for a mutation to become fixed. "Our genome appears to be degenerating in one sense," said Eyre-Walker. But he noted that the seriousness of the adverse mutations was unknown and in any case had been outweighed "by some key adaptations that have made us very successful," presumably intelligence. The new finding is principally of interest to those engaged in human evolutionary history and has little immediate bearing on the genetic health of present-day populations, because the adverse mutations that were found are all probably small in effect, even if large in number. One theoretical implication of interest to evolutionists is that the high mutation rate confirms a long-standing speculation about the purpose of sex. Biologists have often wondered why a species would go to the bother of sexual reproduction when division without sex, the way the amoebas do it, would seem to be more efficient. A favorite answer is that sexual reproduction, in which the genomes are shuffled between generations, is an relatively efficient way of shedding adverse mutations. The high rate of adverse mutation found in the new study confirms that some efficient mechanism -- presumably sex -- is required to remove bad mutations from the genome. "To flush out these deleterious mutations we need sex," Eyre-Walker said. "If we were asexual we would probably be dead." Dr. James F. Crow, a population geneticist at the University of Wisconsin in Madison, confirmed Eyre-Walker's interpretation. "The existence of a high deleterious mutation rate strengthens the argument that a major advantage of sex is that it is an efficient way to eliminate harmful mutations," he writes in a commentary on the paper. The cleansing action of sex arises because bad mutations are brought together and eliminated. Sex has not been completely efficient, however, and many adverse mutations still remain in the human genome. Crow is concerned that the mutational baggage may increase in the future because of higher living standards that allow most infants to reach reproductive age. "Can we keep this up forever?" he wondered. "I don't know." Using DNA sequences now on deposit in DNA data banks, the study compared humans and chimpanzees. Each DNA difference was declared to be a human mutation if in a third species, usually a gorilla, the DNA was the same as the chimp's. Some mutations are inconsequential because they do not change the sequence of amino acids in a protein and therefore have no effect on the organism's survival. By measuring the numbers of inconsequential and of effective mutations, the researchers were able to compute the rate at which mutations entered the genome over the last 6 million years. Copyright 1999 The New York Times Company ______________________________________________________ Get Your Private, Free Email at http://www.hotmail.com DECLARATION & DISCLAIMER ========== CTRL is a discussion and informational exchange list. Proselyzting propagandic screeds are not allowed. Substance�not soapboxing! 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