Evolution refers to the change in a species' inherited traits from generation to generation. These traits are the expression, or "phenotype", of genes which, at least on earth, are stored in DNA. All species on earth are thought to have come about by the mechanism of evolution. Evolution occurs as random genetic changes build up over time, and those that prove to be negative, or harmful, are weeded out as the organisms that have them are rendered less able to compete with the general population. Conversely, the organisms that had received beneficial alterations to their genes would be more likely to pass on those genes to successive generations, which may occur through increased survivorship, improved reproductive success, more abundant or viable offspring, or through other methods.