Natural Selection
Natural selection is the process by which traits that help living things survive and reproduce tend to replace traits that aren't so helpful. In every generation of a population, some individuals die before they have any offspring, and of the ones that do reproduce, some may be more prolific than others; the ones that are better at passing on their genes have higher fitness. If a particular trait, such as better eyesight, increases one's fitness, that trait will be more common in the next generation, provided that it can be inherited. If this keeps repeating itself, eyesight will improve over time. Natural selection for "new and improved" traits like this is known as positive selection, as opposed to negetive selection in which the best trait is the one that's already common and variants are selected against. Postive natural selection is the basis for adaptive evolution.