In a recent thread I posted there was mention of evolution having a specific sense in biology. I just spent an hour reading some evolution primers and I think I have a better sense of how this works at the molecular level but will need to read a lot more till it all hangs together in my mind. However, one thing that immediately stood out is a question about randomness of mutations. I did a little research but couldn't quickly find an answer - most discussions about randomness seemed to focus on rates and locations of change. I think I have misunderstood what is meant by random. My question is pretty basic. If mutations arise in a random fashion and then can be selected for or against, how is it that the same deletrious mutations may be found on an ongoing basis? I apologise if my terminology is not correct, this is from just the briefest of introductions to the topic. What I mean is this. If a mutation to a gene gives rise to an abnormality that decreases an organisms fitness, how is it that the mutation is preserved over time? I am thinking of for example a genetic disease. If a creature has a genetic condition that reduces its fitness wouldn't it follow that the particular mutation would be selected against and eventually disappear from the gene pool? If mutation is random, and selective pressure acts to preserve advantageous changes but not disavantageous changes, shouldn't the case be that a particular condition does not remain evident over time? Or that we should be seeing variations of a condition over time?