A genetic mutation is a physical alteration in the deoxyribonucleic acid (DNA) sequence, representing the ultimate source of all new genetic variation within a species. The time a mutation persists—its lifespan—is not a fixed duration but a dynamic process that depends entirely on the biological context in which it occurs. A mutation’s timeline can range from a few hours within a single cell to millions of years across countless generations. This range is dictated by whether the mutation can be passed to offspring and the evolutionary pressures acting upon it within the population.
Mutation’s Lifespan Within an Organism
The initial persistence of a mutation depends on the specific cell type where the change arose. Mutations are broadly categorized into two types based on their location: somatic and germline.
Somatic mutations occur in non-reproductive body cells, such as skin or muscle, and are acquired throughout an organism’s lifetime. These changes are not heritable and cannot be passed to offspring. The lifespan of a somatic mutation is limited to the life of the affected cell line or the entire organism. If detrimental, the cell line may be purged by cellular repair or programmed cell death. If the mutation confers a survival or replication advantage, such as in cancer, the mutated cell line can proliferate and persist for years.
Germline mutations occur in reproductive cells, such as sperm or egg cells. Since these cells link parent and offspring, a change in their DNA has the potential to be inherited. A germline mutation is the prerequisite for a mutation to extend its lifespan beyond the individual and into the evolutionary timeline of a population.
Initial Inheritance and Population Frequency
For a germline mutation to become a population factor, it must be successfully passed on. The mutation’s frequency within the entire gene pool starts at an incredibly low level. In a sexually reproducing, diploid population of size $N$, a newly arisen, unique mutation is present as just one copy among $2N$ total alleles.
This translates to an initial frequency of $1/(2N)$, which is a tiny fraction in any reasonably sized population. This extremely low starting point makes the mutation highly vulnerable to chance events in its earliest generations.
The immediate fate of the mutation depends on the reproductive success of the initial carriers. If the carrier fails to reproduce, or if the single copy is not passed on due to random chance during gamete formation, the mutation is lost immediately. This precarious early phase illustrates why the vast majority of new mutations are quickly purged from the population within the first few generations.
Evolutionary Forces Determining Persistence
Once a mutation survives its initial generations, its long-term persistence is governed by the primary forces of evolution: natural selection and genetic drift.
Natural selection acts directionally, determining a mutation’s persistence based on its impact on the organism’s fitness. A positively selected mutation, one that provides a reproductive advantage, will increase in frequency relatively quickly as carriers leave more offspring. Conversely, a deleterious mutation, one that reduces fitness, is actively removed from the gene pool. The stronger the negative effect, the faster selection purges it. Mutations with effects that are too small to be noticed by selection are considered effectively neutral, and their fate is largely left to chance.
Genetic drift is the non-directional, random fluctuation of allele frequencies from one generation to the next due to sampling error in a finite population. This random process can cause a mutation to be lost or to increase in frequency irrespective of its fitness effect.
The power of genetic drift is inversely related to the population size. It is a strong factor in small populations where chance fluctuations have a magnified effect. In a small population, drift can overcome even weak positive selection, leading to the accidental loss of a beneficial mutation. A slightly deleterious mutation may also randomly drift to a higher frequency, temporarily persisting against the odds of selection.
The Final Outcomes: Fixation or Loss
The persistence timeline for any mutation ultimately concludes with one of two outcomes: fixation or loss.
Loss occurs when the frequency of the mutation drops to zero, meaning it has been completely purged from the gene pool of the species. This is the most common fate for the vast majority of new mutations, occurring quickly for lethal or highly deleterious variants, but potentially taking many generations for neutral or only mildly harmful ones.
Fixation is the opposite outcome, where the mutation reaches a frequency of 100%, meaning every member of the population carries that genetic variant. A mutation that becomes fixed is considered to have the longest possible lifespan, persisting indefinitely until a new mutation arises at that same genetic location to replace it. This process is the foundation of long-term evolutionary change, where a fixed mutation essentially becomes the new baseline genetic state for the species.
The time it takes to reach either of these end points varies enormously, from a single generation for a devastating lethal germline mutation to millions of years for a neutral change in a large population. For a neutral mutation that eventually fixes, the process is estimated to take an average of $4N_e$ generations, where $N_e$ is the effective population size.