Evolution is often described as “survival of the fittest,” but natural selection is only one way populations evolve. In biology, evolution means a change in the inherited characteristics of a population across generations. Natural selection can cause that change, but so can genetic drift, mutation, and gene flow.
This distinction matters because not every evolutionary change makes organisms better adapted to their environment. Some changes are beneficial, some harmful, and some have little or no effect on survival and reproduction. A population can therefore evolve even when natural selection is absent or too weak to explain the change.
Evolution is a change in populations, not individuals
An individual organism does not evolve during its lifetime in the biological sense. Instead, evolution occurs when the genetic composition of a population changes from one generation to the next.
For example, imagine a population of beetles containing a gene with two inherited versions, or alleles. If one allele becomes more common over several generations, the population has evolved. The change might happen because beetles carrying that allele leave more offspring, which would be natural selection. But it could also happen simply because chance caused some beetles to leave more offspring than others.
That second process is genetic drift.
The key idea is that evolution describes the change itself. Natural selection is one possible mechanism producing that change.
Genetic drift can change populations by chance
Genetic drift is evolutionary change caused by random differences in which organisms survive and reproduce. It is especially important in small populations, where chance events can have large effects on the population’s genetic makeup.
Suppose a small population contains equal numbers of individuals carrying two alleles. If a storm, accident, disease outbreak, or other random event leaves some individuals alive and others dead, the survivors may not represent the original genetic proportions. An allele could become much more common simply because its carriers happened to survive.
The same principle applies to reproduction. Two individuals may be equally capable of surviving and reproducing, yet one may happen to produce more offspring. If the difference is unrelated to their genetic traits, the resulting change in allele frequencies is not natural selection.
Over many generations, random changes can cause alleles to become common, rare, or even disappear entirely.
Population bottlenecks amplify chance
A population bottleneck occurs when a population is drastically reduced in size. The survivors carry only a portion of the genetic variation present in the original population.
Because survival in such an event can be largely random with respect to particular alleles, the genetic makeup of the surviving population may differ substantially from that of the original population. When the population later grows, the descendants inherit the genetic composition of those survivors.
The population has therefore undergone evolutionary change without requiring the surviving individuals to have been better adapted.
Founder effects can do the same thing
A related phenomenon is the founder effect. It occurs when a new population is established by a small number of individuals separated from a larger population.
The founders may, by chance, carry unusual proportions of the original population’s alleles. Their descendants can consequently have allele frequencies quite different from those of the source population.
Again, the difference does not require natural selection. It can arise because the founding group was a small and unrepresentative sample of the original population.
Mutation creates new genetic variation
Mutation is a change in genetic material. Mutations are the ultimate source of new alleles, although they do not necessarily cause evolution by themselves.
A mutation that occurs in a cell that contributes to reproduction can potentially be inherited by descendants. If the resulting allele enters a population, it adds genetic variation that was not previously present in that form.
Whether the allele eventually becomes common is a separate question. Natural selection might favor it, oppose it, or have little effect on it. Genetic drift can also change its frequency, particularly when the allele is rare.
Mutations themselves are not produced because an organism “needs” them. They arise through changes in genetic material, and their consequences are subsequently shaped by processes such as selection, drift, and gene flow.
Gene flow can change allele frequencies
Evolution can also occur when individuals or reproductive cells move between populations and introduce alleles into a population that previously had different genetic frequencies. This movement of genes between populations is called gene flow.
For instance, if individuals from one population regularly join another population and reproduce there, alleles carried by the newcomers can become more common in the recipient population. Conversely, alleles already present can become less common relative to the newly mixed gene pool.
No advantage or disadvantage is required. The population changes simply because its genetic composition has been altered by the arrival of genetic material from elsewhere.
Gene flow can therefore produce evolutionary change even when natural selection is not involved.
Not every evolutionary change is an adaptation
One of the most important reasons to distinguish evolution from natural selection is that evolution does not necessarily produce adaptation.
An adaptation is an inherited characteristic that became more common because it contributed to reproductive success in a particular environment. Natural selection is central to the evolution of adaptations.
But many evolutionary changes are not adaptations.
A neutral or nearly neutral allele, for example, may have little effect on an organism’s ability to survive or reproduce. Its frequency can still change because of genetic drift. If it becomes common, the population has evolved even though the change did not make its members better suited to their environment.
This is why describing every evolutionary change as an improvement is misleading. Evolution has no built-in direction toward greater complexity, strength, intelligence, or perfection.
Natural selection and genetic drift work differently
Natural selection and genetic drift can both change allele frequencies, but the underlying processes are different.
Natural selection is nonrandom with respect to fitness. If a heritable trait consistently causes its carriers to leave more surviving offspring in a particular environment, the associated alleles tend to increase in frequency.
Genetic drift is random with respect to fitness. Allele frequencies can change because of chance differences in survival and reproduction, even when the alleles themselves do not affect fitness.
The word “random” here does not mean that every outcome is equally likely or that biology operates without rules. It means that, relative to the effects of particular alleles on reproductive success, the sampling of individuals from one generation to the next can produce chance changes in allele frequencies.
In real populations, selection and drift can operate simultaneously. An allele might be beneficial and therefore favored by selection while also being affected by random fluctuations. Which force has the stronger effect depends partly on the circumstances and population size.
Evolution can occur even when natural selection is weak
The contrast between selection and drift is particularly important for understanding small populations.
In a large population, random fluctuations may have relatively small effects on allele frequencies. In a small population, the reproductive success of just a few individuals can substantially alter the genetic composition of the next generation.
An allele that is neutral may therefore increase dramatically by chance in a small population. A beneficial allele can even be lost through drift before natural selection has a chance to make it common.
This does not mean natural selection stops operating in small populations. Rather, the relative influence of chance can become much greater.
Natural selection is not the only source of evolutionary change
A useful way to organize the mechanisms is to ask what causes allele frequencies to change:
| Mechanism | How it changes a population |
|---|---|
| Mutation | Introduces new genetic variants |
| Genetic drift | Changes variant frequencies through chance |
| Gene flow | Moves genetic variants between populations |
| Natural selection | Causes heritable variants affecting reproductive success to change in frequency |
These processes are not mutually exclusive. Mutation supplies new variation; gene flow moves variation among populations; drift changes frequencies through chance; and natural selection can favor some inherited variants over others.
Together, they provide a much fuller picture of how populations change genetically.
Sexual reproduction can reshuffle existing variation
There is another process worth separating from natural selection: genetic recombination.
During the formation of reproductive cells and the production of offspring, genetic material is reshuffled. This produces new combinations of alleles even when no new mutation has occurred.
Recombination by itself does not necessarily change the frequency of alleles in a population, so it is not usually considered an independent force that changes allele frequencies in the same way as selection, drift, mutation, or gene flow. Its major evolutionary importance is that it creates new combinations of existing genetic variants on which selection and other processes can act.
This distinction helps prevent another common misconception: generating genetic differences and changing their frequencies are related but not identical processes.
Evolution does not require organisms to become better
The phrase “evolutionary progress” can create the impression that evolution must push populations toward improvement. It does not.
Consider a neutral allele that becomes common because of genetic drift. Nothing about the process requires the resulting population to be better adapted. Likewise, gene flow can introduce alleles that are advantageous, harmful, or effectively neutral in their new environment.
Even natural selection does not produce universal improvement. A trait can be favored in one environment and disadvantageous in another. Selection favors traits according to their effects on reproductive success under particular conditions.
Evolution is therefore better understood as change in inherited population characteristics, not as a march toward an ideal organism.
Why natural selection remains especially important
Saying that evolution can happen without natural selection does not diminish the importance of natural selection. It explains what natural selection actually is.
Genetic drift can alter allele frequencies by chance, mutation can introduce new variants, and gene flow can move variants between populations. Natural selection has a distinctive role because it can produce consistent adaptation to environmental conditions: inherited traits that improve reproductive success tend to become more common.
That is why natural selection is indispensable for explaining many features of organisms that appear well matched to their environments. But when the question is simply “How can the genetic composition of a population change?”, natural selection is only one answer.
Evolution can happen whenever inherited variation changes in frequency across generations. Sometimes that change reflects differences in reproductive success. Sometimes it results from chance, the introduction of new mutations, or the movement of genes between populations. Natural selection is one mechanism of evolution—not a synonym for evolution itself.
