Most life forms that have ever existed—over 99 percent, according to some scientists—are extinct. Extinction is, therefore, one of the most basic characteristics of the planet’s ecology. Species disappear because they change through gradual adaptation to such a degree that they can no longer be considered the same species, or because all individuals die off before they can reproduce. Adaptation, consisting of the combined processes of mutation and natural selection as theorized by Darwin and his twentieth-century successors, results in both extinction and speciation, the emergence of novel species. Die-offs tend to affect small populations and/or those with a very limited geographical range—especially island populations—that are vulnerable to unusual climatic events or outbreaks of disease. Extinctions in the normal course of evolution occur at the so-called background rate, which is computed either as the number of species that go extinct in a particular number of years, in Million Species Years (MSY), or as the time intervals during which species survive (Lawton and May 1995).