When people hear that a population has evolved, it is natural to think of natural selection. An organism has a trait that improves survival or reproduction, that trait becomes more common, and evolution follows.
That is one important way evolution happens, but it is not the only one.
DNA can change from one generation to the next even when those changes provide no survival or reproductive advantage. Some genetic variants spread through a population largely by chance rather than because natural selection favors them. This process is called genetic drift, and it is central to the theory of neutral evolution.
Neutral evolution does not mean that evolution is random in every respect, nor does it mean natural selection is unimportant. Instead, it describes a major class of genetic changes that are effectively neutral: they neither significantly help nor harm the organisms carrying them. Their fate can then be influenced strongly by chance.
What does it mean for evolution to be neutral?
In biology, evolution means a change in the inherited characteristics of a population across generations. At the genetic level, this can be described as a change in the frequencies of different DNA variants, or alleles, within a population.
Natural selection changes those frequencies because some inherited variants affect survival or reproduction. A variant that consistently gives its carriers an advantage tends to become more common, while a strongly harmful variant tends to become less common.
Neutral variants are different. If two genetic variants have essentially the same effect on an organism’s reproductive success in a particular environment, natural selection has little basis for favoring one over the other. Yet their frequencies can still change.
Suppose a population contains two neutral variants of a gene. By chance, individuals carrying one variant may leave more offspring in a particular generation, while individuals carrying the other leave fewer. If the difference is not caused by the DNA variants themselves, the next generation can nevertheless contain different proportions of the two variants.
Over many generations, chance fluctuations can cause one neutral variant to become common and another to disappear entirely.
That is evolution without natural selection.
Genetic drift is the key mechanism
The main process responsible for changes in neutral variants is genetic drift.
Genetic drift is a random change in allele frequencies caused by the fact that populations have finite numbers of individuals. Reproduction does not produce a perfectly representative genetic sample of the previous generation. Some individuals happen to reproduce more than others, and which individuals reproduce is influenced by many chance events.
In a very large population, random fluctuations tend to have a relatively small effect on allele frequencies. In a small population, the same kind of chance can produce much larger changes.
Imagine a small population in which two neutral alleles are present at equal frequencies. If, by chance, individuals carrying one allele happen to contribute more offspring to the next generation, that allele may become more common. Nothing about the allele itself caused the increase.
The process can continue. Eventually, one allele may reach a frequency of 100 percent, meaning the other allele has been lost from the population. This is called fixation.
A neutral allele can therefore become fixed without ever being advantageous.
Why small populations are especially affected
The strength of genetic drift depends heavily on population size. Chance has a greater influence when relatively few individuals contribute genes to future generations.
This is particularly important when a population goes through a sharp reduction in size, known as a population bottleneck. The survivors carry only a sample of the genetic variation that existed in the original population. Some variants may become much more common simply because they happened to be present among the survivors, while others may be lost.
A related situation occurs when a small group becomes separated from a larger population and establishes a new population elsewhere. This is called the founder effect. The new population begins with a limited sample of the genetic variation of its source population, so allele frequencies can differ substantially from those of the original population.
These processes do not require natural selection to explain the initial change in genetic composition.
Mutation supplies the raw material
Genetic drift changes the frequencies of existing variants, but it does not create new DNA variants. New variants ultimately arise through mutation, which is a change in DNA sequence.
Mutations occur for many reasons, including errors during DNA replication and damage to DNA. Their effects vary. Some mutations are harmful, some beneficial, and many have little or no detectable effect on reproductive success.
A neutral mutation can enter a population and then follow a chance trajectory. It might disappear quickly, remain rare for many generations, or eventually spread through the entire population.
This distinction is important: mutation introduces genetic variation, while drift can change the frequency of that variation.
Natural selection can also act on newly arising mutations, but it is not necessary for every mutation to have an evolutionary fate.
Neutral does not mean “nothing happens”
The word neutral can be misleading. A neutral genetic change is not necessarily meaningless, useless, or biologically invisible.
A DNA change can have no significant effect on reproductive success while still altering the DNA sequence. For example, some changes occur in parts of genes where different DNA sequences can produce the same protein because of the way the genetic code works. Other changes occur in regions where their effects are small or difficult to detect.
Neutrality is therefore about fitness, not about whether a DNA sequence has changed.
In evolutionary biology, fitness refers to an organism’s relative contribution of surviving offspring to future generations. A mutation can be neutral with respect to fitness even if the sequence itself differs from another sequence.
Neutrality is also usually a matter of degree. A variant does not have to be perfectly neutral in every circumstance. A genetic difference may have such a small effect on fitness that, in a particular population, random drift is much more influential than selection. Such variants are often described as nearly neutral.
The molecular clock and neutral change
Neutral evolution became especially important in understanding why molecular differences accumulate between species.
If genetic changes that have little effect on fitness can become fixed through drift, then DNA sequences can diverge over time without every difference representing an adaptation.
This idea helped explain why organisms can accumulate large numbers of molecular differences that do not obviously improve their survival. It also contributed to the development of the molecular clock concept: under appropriate conditions, some classes of genetic differences can accumulate at rates that provide useful information about evolutionary history.
The molecular clock is not a universal stopwatch. Mutation rates and patterns of molecular evolution can vary among genes, organisms, and evolutionary lineages. But the underlying insight remains important: not every difference between species is an adaptation produced by natural selection.
Neutral evolution does not replace natural selection
A common misunderstanding is that neutral evolution is an alternative theory in which natural selection does not matter. That is not what the concept claims.
Evolutionary change can result from several processes, including mutation, natural selection, genetic drift, and gene flow. These processes can operate simultaneously.
Natural selection is especially important when genetic differences have meaningful effects on reproductive success. Genetic drift becomes especially important for variants whose effects are neutral or very small, and its influence can be pronounced in small populations.
The same population can therefore experience both selection and drift at the same time.
Consider a mutation that provides a substantial reproductive advantage. Selection can make its increase in frequency much more predictable than the fate of a neutral mutation. But if the advantage is extremely small, random fluctuations may dominate, particularly in a small population. In that situation, whether the variant spreads can depend heavily on chance.
What neutral evolution tells us about DNA differences
One of the most important implications of neutral evolution is that genetic difference does not automatically imply functional difference.
Two individuals can have different DNA sequences without one having a meaningful biological advantage over the other. Likewise, two species can differ at many positions in their genomes without all those differences being adaptations to different environments.
This does not mean that most biological traits are neutral or that natural selection cannot explain important adaptations. It means that interpreting genetic differences requires care.
Researchers distinguish, among other things, changes that are strongly constrained because they affect important biological functions from changes that appear to evolve with relatively little influence from selection. Patterns of DNA variation can therefore provide clues about the evolutionary forces acting on different parts of a genome.
Can neutral DNA changes accumulate forever?
Not necessarily. A neutral variant can be lost, fixed, or remain in a population for a long time. Its fate depends on population size, reproduction, mutation, migration, and chance.
Even after a neutral variant becomes fixed in one population, new mutations can introduce additional variation. Meanwhile, separate populations can accumulate different neutral changes. If those populations remain separated, their DNA sequences can gradually diverge.
Over sufficiently long periods, this process can produce substantial genetic differences even without a corresponding series of adaptive changes.
Natural selection may still shape other parts of the genome during the same period. Evolutionary history is often a mixture of processes rather than the product of a single mechanism.
The larger lesson of neutral evolution
The central idea is simple but powerful: DNA can change in populations without natural selection favoring those changes.
Mutation creates new genetic variants. Genetic drift changes their frequencies through chance. When a variant has little or no effect on reproductive success, its evolutionary fate can be determined largely by drift rather than by selection.
This gives scientists a way to distinguish two questions that are sometimes treated as the same: Did a genetic difference become common? and Did it become common because it was advantageous?
The first can happen without the second. Evolution does not require every genetic change to be an adaptation, and understanding that distinction is essential for making sense of genetic variation, population history, and the patterns found in DNA.
