When a new mutation is introduced into a population, it occurs in only one individual, and it distinguishes this individual from all others in the population. The mutation may be passed down to further generations, in which case it distinguishes the descendants who carry the mutation from all other individuals. This type of mutation is known as a segregating mutation because it divides the population into two groups: those that carry the mutation and those that don't.
If a mutation is widely spread and eventually is found in every gene in a population, we say it has become fixed. It is no longer a segregating mutation because every individual carries the mutation in its genes. From that point on, the mutation is guaranteed to be passed on to all future individuals.
As an example, consider a gene labeled A. Originally every member of a population is of type AA. Now assume a mutation is introduced, so that one new offspring is of type Aa, where a is the mutated version of A. a is a segregating mutation as long as any individuals of type Aa or AA remain, but as soon as all individuals are of type aa then all future offspring will be aa.
The plot in figure 1 illustrates this process.
The horizontal axis represents time, and the vertical axis represents a percentage of the population. When a mutation enters the population, it occurs in only one individual and is plotted as a point somewhere on the x axis. If the mutation is passed on, for example to four new offspring, then it will be in a higher percentage of the population at the next time step.
Most mutations will soon drop out of the population. Either the individual where the mutation originates will not survive, or if it does survive and mate, by chance it may not pass the mutation to its children. Even then the children may not pass the mutation any further. Mutations that eventually die out show up as inverted ``V'' shapes in the figure: they are introduced, they are passed on to some proportion of the population in the next few generations, and eventually the percentage drops to 0 as the mutation disappears.
Some small percentage of new mutations are passed on successfully. If by chance the mutation continues to spread in each successive generation, it may reach a point when it occurs in every individual, i.e. it becomes fixed. When a mutation is fixed, it is automatically passed on to every succeeding generation with probability 1.0 (unless there is a ``back mutation'' that changes the locus back to its original form, but this is a very rare event).
