The passing on of genes is not always exact, parts of the blueprints may be copied incorrectly or in the case of organisms that undergoe sexual reproduction, genes of one parent are combined with the genes of another parent organism. Individuals that are more fit, better suited for their environment, are likely to transmit their genes to the next generation than those individuals that are not well-suited for their environment. For this reason, the genes present in a population of organisms is in constant flux due to various forces-natural selection, mutation, genetic drift, migration. Over time, gene frequencies in populations change-evolution takes place.