MadSci Network: Evolution |
Dear Bruce, I am a little confused by your question but I'll try and answer what I understand you to be asking. I think you're asking why individual cells in our bodies don't evolve in the same time scale as the bacteria and viruses infecting us. In order for evolution to happen there are three conditions which must be fulfilled: 1. there is biological variation, 2. these variations are heritable and 3. under selection pressure some of these variations are more advantageous than others. Darwin outlined these three conditions in "The Origin...". You are right in that viruses and bacteria evolve much more quickly than organisms such as humans with longer generation times. Cells within a human do not however evolve independently. Each cell in our body contains the same genetic code, except for mutations that occur within somatic cells, germ cells go on to produce eggs and sperm and it is only gene changes in germ cells that are passed on to the next generation. Gene changes in somatic cells are not heritable, violating condition 2 for evolution. So even if one of your skin cells had a mutation which made that cell more resistant to infection by a wart virus it would make no difference to the germ cells that make sperm containing the unmutated gene, so your children would not have greater resistance to warts. If however one of your germ cells happened to acquire a gene for wart resistance and that germ cell produced a sperm which went on to contribute half of your child's DNA making them resistant to warts, more attractive to the opposite sex and more likely to give you grandchildren as a result, then we could say that evolution was happening. I hope this answers your question, I would heartily recommend Richard Dawkins book "The Selfish Gene" for further explanation of the levels of selection. Yvonne Buckley Moderator's Note: To address the "top down vs. bottom up" part of the question, selection occurs in response to all environmental forces, regardless of their position. Most organisms are under selective pressure from predators, parasites, diseases, environmental changes, etc., such that both "top down" and "bottom up" are inaccurate.
Try the links in the MadSci Library for more information on Evolution.