A beneficial mutation is exactly what it says it is: a mutation in an organism's genome that produces a beneficial effect. Specifically, it affects the organism in some way as to increase its chances of reproductive success, and therefore the chance of the mutation in question being passed along. It is safe to say that the vast majority of mutations in an organism are not beneficial. Often a mutation is neutral, producing neither a beneficial effect nor a negative effect, although it could be a potentiating mutation. Many are negative mutations - producing a disorder in the organism that significantly lowers its survival and reproductive capabilities. In the case of species depending on sexual reproduction, a negative mutation can prevent the embryo being correctly conceived or even the gametes being functional.
 Relevance to natural selection
Despite the odds against an individual mutation being beneficial, the driving force of natural selection means that when one does appear, it can quickly dominate a population. This is the entire point of natural selection, which invalidates any argument that "beneficial mutations" are too rare to ever have any effect.
In 2008 microbiologist Richard Lenski produced a beneficial mutation in the lab, when his E. coli developed a trait that enabled them to utilize citrate as a carbon source. While E. coli has internal cellular mechanisms that can process the citrate, one hallmark of the species is that it cannot transport citrate across the cell wall. This mutation enabled them to do so.
 Vivid examples of beneficial mutations
- Lactose tolerance - why humans with significant European ancestry can digest milk as adults.
- Antibiotic resistant bacteria - at least beneficial from the point of view of the bacteria.
- Radiation resistant fungi (and perhaps other lifeforms) inside Chernobyl
- "German Superboy", an individual example of a human mutation that not only doesn't cause any visible disfigurement or impairment, but if anything will probably make it easier to maintain a muscular physique and/or low weight. These are characteristics that could be considered desirable in the modern day, when food is abundant.
- The ccr5-Δ32 mutation confers HIV-1 resistance to those with a double copy of the allele (homozygous) . The mutation also confers resistance to plague and smallpox while increasing susceptibility to west nile virus.
 See also
- ↑ http://www.cosmosmagazine.com/node/2095/full
- ↑ http://www.msnbc.msn.com/id/5278028/
- ↑ http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1275522/pdf/pbio.0030378.pdf
- ↑ http://www.pnas.org/content/98/18/10214.short
- ↑ http://news.bbc.co.uk/1/hi/7726118.stm
- ↑ Glass WG, McDermott DH, Lim JK, Lekhong S, Yu SF, Frank WA, Pape J, Cheshier RC, Murphy PM. CCR5 deficiency increases risk of symptomatic West Nile virus infection J. Exp. Med. volume 203, issue 1, pages 35–40, January 2006. doi: 10.1084/jem.20051970.