The neutral theory of molecular evolution says that genetic drift — random events that affect evolutionary change (see Chapter 6) — accounts for much of the change in DNA. This is the case because most mutations are selectively neutral. In fact, much more variation is neutral than scientists once thought. The variation exists in the DNA, but either it doesn't result in a change at the level of the protein or, if it does result in changes in proteins, these changes don't change the protein's function.
The chance that a mutation will have no effect can vary between different genes. Some proteins, for example, are very tightly constrained in the shapes that they can adopt and still be functional. For these proteins, relatively few mutations are selectively neutral. When scientists examine the mutation rates of different proteins within the same organism, they find that some evolve faster than others.
As stated previously, neutral mutations are neither good nor bad, and when a mutation is neutral, natural selection doesn't act on it. (Why should it? A neutral mutation doesn't help the organism, which would cause an increase in frequency in subsequent generations. Neither does it hurt the organism, which would cause a decrease in frequency.) Therefore, the evolutionary force that acts on these genes is genetic drift. Over enough time, a selectively neutral mutation can reach a frequency of 100 percent in a population just by chance — a situation called fixation. (You can read more about fixation and genetic drift in Chapter 6.)
Many mutations are almost, but not quite, neutral. A slightly deleterious mutation, for example, might still increase in frequency as a result of genetic drift in a small population. Remember that for any given mutation, the chance of fixation (that is, the chance of reaching a frequency of 100 percent) is a function of population size. If population size fluctuates (as it often does), a particular gene may be changing in frequency primarily as a result of natural selection at one time and primarily due to genetic drift at another time.
Two evolutionary forces are at work: natural selection and genetic drift. If a mutation isn't neutral, both natural selection and genetic drift can be the cause of evolution. If a mutation is neutral, only genetic drift can result in a change in the frequency of the gene over time. (This discovery — that random events are evolutionary forces in and of themselves — has been the most important addition to the theory of evolution since Darwin.)
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