http://openwetware.org/wiki/BIO254:NaturalSelection 2006 revision
Natural Selection is the process through which individuals with beneficial traits are more likely to survive and reproduce. In the next generation, those traits, if they have heritable components, will be more common. Through generations of accumulation, the traits will spread in the population, which can result in speciation and adaptation. On the other hand, individuals with injurious variations will have less chances to survive or reproduce, and their traits will be eliminated from the population.