Evolution is often described as a process in which organisms become better suited to their environments. That description is useful, but incomplete. Evolution can also happen because of chance.
Two of the most important mechanisms that change populations over generations are natural selection and genetic drift. Natural selection is nonrandom with respect to which inherited traits tend to leave more offspring. Genetic drift, by contrast, changes the genetic makeup of a population through random sampling. Both can alter allele frequencies—the proportions of different versions of a gene in a population—and both can shape the evolution of species.
The key difference is simple: natural selection favors some inherited variants because they affect survival or reproduction, while genetic drift changes their frequencies by chance. In real populations, the two processes often operate at the same time.
What genetic drift means
To understand genetic drift, it helps to start with a basic idea from population genetics. An allele is a particular version of a gene. A population may contain several alleles for the same gene, and each generation passes only a sample of those alleles into the next generation.
That sampling is not perfectly predictable. Some individuals leave more offspring than others even when there is no meaningful difference in their inherited traits. A particular allele can therefore become more common simply because its carriers happened to reproduce more successfully, while another allele becomes less common because its carriers happened to leave fewer descendants.
That random change in allele frequencies is genetic drift.
Drift is especially powerful in small populations because random events can affect a large fraction of the population. In a very small population, for example, the accidental loss of a few individuals can eliminate alleles entirely. In a large population, the same number of chance losses may have little effect on overall allele frequencies.
Genetic drift does not require an allele to be harmful, beneficial, or even relevant to survival. A neutral allele can rise in frequency, decline, or disappear entirely through drift.
Natural selection is different
Natural selection occurs when inherited differences among individuals affect their chances of surviving, reproducing, or contributing offspring to future generations.
Suppose a population contains an inherited trait that makes its carriers more likely to survive and reproduce in a particular environment. If that advantage is passed genetically to their offspring, the associated alleles will tend to become more common over generations.
The process is not conscious or goal-directed. Organisms do not evolve traits because they need them, and environments do not intentionally select particular individuals. Instead, individuals with heritable characteristics that increase reproductive success tend to leave more descendants.
Natural selection is therefore nonrandom with respect to fitness. The environment systematically favors some heritable variants over others under particular conditions.
This distinction from drift is crucial. Natural selection can produce adaptation because differences in reproductive success are associated with inherited traits. Genetic drift has no such requirement.
Chance versus selection
The simplest comparison is:
| Genetic drift | Natural selection |
|---|---|
| Changes allele frequencies through random sampling | Changes allele frequencies through differences in reproductive success |
| Can affect neutral alleles | Acts on heritable traits that influence fitness |
| Particularly influential in small populations | Can be powerful in populations of many sizes |
| Does not inherently produce adaptation | Can produce adaptation to an environment |
| Can randomly increase or decrease an allele | Tends to increase alleles associated with higher fitness in the relevant environment |
The word chance can be misleading if it is taken to mean that drift has no biological causes. Drift results from ordinary events such as which individuals reproduce, which offspring survive, and which organisms happen to migrate or die. What makes drift random is that, with respect to the allele being considered, these sampling events do not consistently favor one variant because of its effect on fitness.
Natural selection can also involve chance at the level of individual events. An animal may randomly encounter a predator, for example. But if an inherited trait consistently changes the probability of surviving such encounters, the resulting difference in reproductive success can produce selection across generations.
So evolution is not divided neatly into “random evolution” and “nonrandom evolution.” Chance affects evolutionary outcomes, while selection can systematically bias those outcomes.
How a population can evolve through drift
Imagine a small population in which two alleles, A and B, are present in roughly equal proportions. Assume neither allele affects survival or reproduction.
By chance, individuals carrying A may happen to produce more offspring in one generation. Their descendants then make up a larger share of the next generation. In another generation, the opposite might happen. Over time, one allele may become common and the other may disappear.
Nothing about the alleles themselves had to cause the change.
Once an allele is lost through drift, it cannot be restored by drift alone. This is one reason genetic drift can reduce genetic variation within populations. Drift can also cause different populations of the same species to become genetically different from one another as each population experiences its own random changes.
Bottlenecks and founder effects
Two situations make genetic drift particularly visible: population bottlenecks and founder effects.
A population bottleneck occurs when a population is sharply reduced in size. The surviving individuals carry only a sample of the genetic variation that existed before the population decline. Alleles that were common before the bottleneck can become rare afterward, while rare alleles can become disproportionately common simply because of which individuals survived.
A founder effect occurs when a small group establishes a new population. The founders carry only a subset of the genetic variation in the source population. The new population can therefore have allele frequencies quite different from those of the larger population from which the founders came.
In both cases, the important point is sampling. A small group is not necessarily genetically representative of the population it came from.
Why population size matters
Genetic drift is generally stronger when populations are small. Random fluctuations in allele frequencies become more pronounced when each individual’s genetic contribution represents a larger fraction of the next generation.
This does not mean drift is absent from large populations. It means its effects are typically weaker relative to the effects expected from random sampling in a small population.
Population size also interacts with natural selection. A mildly beneficial allele may increase because of selection, for example, but strong drift can sometimes overwhelm weak selection, particularly in small populations. Conversely, powerful selection can dominate random fluctuations under appropriate conditions.
Evolutionary change is therefore often the result of selection and drift acting together, rather than one mechanism operating in isolation.
Natural selection does not always mean rapid adaptation
Natural selection is often associated with obvious adaptations, but selection does not guarantee that a population will become perfectly suited to its environment.
Several factors can limit or alter selection. Environmental conditions change. Different traits can involve trade-offs. A trait that improves one aspect of survival may reduce another. Genetic variation may be limited, and other evolutionary processes can counteract selection.
Most importantly, natural selection acts on existing heritable variation. It does not create a useful trait because an organism needs one.
A population can also experience stabilizing selection, directional selection, or disruptive selection, depending on how different trait values affect reproductive success. Selection can favor an intermediate trait, push a population toward one end of a range, or favor multiple extremes.
Drift can matter even when selection is present
It is tempting to think of natural selection and genetic drift as competing explanations: either a trait spread because it was advantageous, or it spread by chance. In many cases, both processes contribute.
Consider a newly arising allele that provides a modest advantage. Natural selection may favor it, but its early fate can still be influenced strongly by chance. If its first carriers fail to reproduce, the allele may disappear before selection has much opportunity to increase it. If it happens to reach a higher frequency, selection may subsequently have a stronger effect.
This is particularly important for new mutations. A beneficial mutation is not guaranteed to spread through a population. Its initial survival can depend partly on chance, after which selection may favor it if its effects increase fitness.
Genetic drift and natural selection can produce different patterns
The consequences of the two mechanisms can sometimes be distinguished by the patterns they produce.
Natural selection tends to produce changes associated with differences in fitness. If a particular inherited characteristic repeatedly improves reproductive success in a given environment, alleles associated with that characteristic can consistently increase.
Drift, in contrast, can produce changes without a consistent relationship to fitness. A neutral allele can become common in one population and rare in another simply because the populations experienced different random sampling events.
Drift can therefore contribute to divergence among populations, even when those populations experience similar environments and the alleles involved have little or no effect on fitness.
A related process: gene flow
Another major influence on allele frequencies is gene flow, the movement of alleles between populations through migration and reproduction.
Gene flow differs from both drift and natural selection. It can introduce genetic variation into a population or make previously separated populations more genetically similar. Its effects depend on which individuals move and reproduce.
Mutation is another distinct process. Mutation creates new genetic variants, while selection, drift, and gene flow change the distribution of variants among and within populations.
These processes are not interchangeable. Mutation supplies new variation; selection can favor variants that increase fitness; drift changes frequencies through chance; and gene flow moves variants between populations.
Is evolution random or predictable?
Neither answer captures the whole picture.
Mutation is random with respect to an organism’s needs, meaning organisms do not direct mutations toward traits they require. Genetic drift is also fundamentally stochastic: chance sampling can push allele frequencies in different directions.
Natural selection, however, is not random in the same sense. When an inherited trait consistently increases reproductive success under particular environmental conditions, selection tends to increase the associated variants.
Even so, the overall course of evolution can remain difficult to predict. Environments change, mutations arise unpredictably, populations interact, and drift introduces randomness. Selection provides a directional force, but it operates within a world that contains substantial chance.
The most useful distinction is therefore not “random evolution versus purposeful evolution.” It is chance versus differential reproductive success as causes of changes in allele frequencies.
Genetic drift can make an allele more common without making it better. Natural selection can make an allele more common because, in that environment, it contributes to greater reproductive success. Both are genuine evolutionary mechanisms, and understanding their interaction is essential to understanding how populations change over time.
