In the modern science of evolution, adaptations (changes that affect the fitness of an organism) appear slowly, over many generations, by means of natural selection. Because of this, genetic changes usually only spread in a population if they do not lower the fitness of individuals. Neutral changes (changes that don't raise or lower fitness) can spread by genetic drift. In some cases, changes that lower fitness can spread, but large changes that take many steps can only spread if each of the steps raises fitness. The biologist Richard Dawkins describes the situation this way: