Info on mutations for Russ.
The vast majority of mutations are neutral. They neither confer harm nor benefit. There is no evidence that humans, or any other species, are accumulating harmful genetic “mistakes” over time. See references at the end of this post for information about the different kinds of mutations that occur, how they occur, and the effects on the organism.
The human mutations we know most about are detrimental because medical scientists preferentially study illnesses that cause significant morbidity and mortality. Consider the theoretical possibility that a beneficial mutation has occurred in a particular human gene; even if this mutation were identified by a comparison of the mutated gene in a child versus the unmutated version of the same gene in both parents, there is no way that this mutation could ever be recognized as beneficial. If the mutation increased intelligence, strength, longevity or specific disease resistance, this would never be apparent without long-term breeding experiments that could obviously never be done on humans. Therefore, since such beneficial mutations in humans could never be recognized in humans, our ignorance of examples cannot be taken as evidence that they don't exist. However, the experiments necessary to demonstrate a beneficial mutation can be done with laboratory organisms that multiply rapidly, and indeed such experiments have shown that rare beneficial mutations can occur. For instance, from a single bacterium one can grow a population in the presence of an antibiotic, and demonstrate that organisms surviving this culture have mutations in genes that confer antibiotic resistance. In this case origin of the population from a single bacterium allows comparisons of the mutated genes with the corresponding genes from the original bacterium, verifying that the variant sequences were not present before the culture with antibiotics and therefore arose as beneficial mutations.
Some identified beneficial mutations in humans:
Sickle cell resistance to malaria: In general this is an undesirable mutation because the sickle cells are less efficient than normal cells. In areas where malaria is prevalent it turns out to be favorable because people with sickle shaped blood cells are less likely to get malaria from mosquitoes.
Lactose tolerance: This is not universal; it is something that originated in cultures that kept cattle and goats. In these cultures lactose tolerance had a strong selective value. In the modern world there is a strong correlation between lactose tolerance and having ancestors who lived in cultures that exploited milk as a food.
Resistance to atherosclerosis (Arterioscler Thromb Vasc Biol 1998 Apr;18(4):562-567): Atherosclerosis is principally a disease of the modern age, one produced by modern diets and modern life-styles. There is a community in Italy near Milan whose residents don't get atherosclerosis because of a fortunate mutation in one of their forebearers. This mutation is particularly interesting because the person who had the original mutation has been identified.
Note that this is a mutation that is favorable in modern times because (a) people live longer and (b) people have diets and life-styles that are not like those of our ancestors. In prehistoric times this would not have been a favorable mutation. Even today we cannot be certain that this mutation is reproductively favorable, i.e., that people with this mutation will have more than the average number of descendents. It is clear, however, that the mutation is personally advantageous to the individuals having it.
Immunity to HIV: The HIV virus has to attach to molecules that are expressed on the surface of the T-cells. One of these molecules is called CD4; another is CCR5. Some people carry a mutant allele of the CCR5 gene that results in lack of expression of this protein on the surface of T-cells. These individuals are resistant to HIV and AIDS.
You can read more about genes and mutations in these articles:
Are Mutations Harmful?
http://www.talkorigins.org/faqs/mutations.htmlPlagiarized Errors and Molecular Genetics
http://www.talkorigins.org/faqs/molgen/ 
