 When we think of evolutionary theory and natural selection, we usually think of organisms, say, a herd of deer, in which some deer are faster than others at escaping their predators. These swifter deer will, all things being equal, leave more offspring, and these offspring will have a tendency to be swifter than other deer. Thus, we get a change in the average swiftness of deer over evolutionary time. In a case like this, the unit of selection, sometimes called the “target” of selection, is the single organism, the individual deer, and the property being selected, swiftness, also lies at the organismic level, in that it is exhibited by the intact and whole deer, and not by either parts of deer, such as cells, or groups of deer, such as herds. But there are other levels of biological organization that have been proposed to be units or targets of selection—levels at which selection may act to increase a given property at that level, and at which units increase or decrease as a result of selection at that specific level of biological organization.