Evolution is defined by changes in the genetic makeup of a population over successive generations. Natural selection (NS) and genetic drift (GD) are two primary forces driving this evolutionary change. NS is known for shaping adaptations, while GD is characterized by random fluctuations in genetic traits. Although often taught as opposing concepts—one directional, the other non-directional—they share fundamental similarities. Both mechanisms act on the gene pool and rely on the same starting conditions to function.
Shared Mechanism: Altering Allele Frequencies
The most fundamental similarity is that both natural selection and genetic drift cause microevolution by changing the relative frequency of alleles within a population’s gene pool. Microevolution is defined as any change in allele frequencies across generations, a definition both mechanisms fulfill. Allele frequencies remain constant in a population not experiencing evolutionary pressure, a state described by the Hardy-Weinberg equilibrium principle.
Natural selection shifts frequencies by favoring alleles that provide a reproductive or survival advantage, causing those beneficial alleles to become more common over time. Genetic drift also shifts allele frequencies, but it does so purely by chance events, such as random differences in reproductive success or sampling errors during gamete formation. Although the cause of the change is different—adaptive versus stochastic—the result is a quantifiable shift in the proportion of specific alleles from one generation to the next.
This shift in frequency can culminate in fixation, where an allele’s frequency reaches 100% and all other variants are lost from the population. Both NS and GD can drive an allele toward this point. NS generally fixes an advantageous allele if its selective advantage is strong enough. GD, particularly in small populations, can also lead to fixation randomly, potentially fixing a neutral or even detrimental allele. Regardless of whether the process is directed by fitness or driven by chance, the gene pool is permanently altered by the loss or establishment of a specific genetic variant.
Shared Prerequisite: Reliance on Genetic Variation
A shared feature is the requirement for pre-existing genetic variation within the population for either process to operate. Both natural selection and genetic drift act upon the diversity of alleles and genotypes already present in the gene pool. If a specific gene locus is monomorphic—meaning every individual carries the identical allele—there is no variation for either process to affect.
Natural selection requires variation because it acts as a filter, favoring individuals whose heritable traits allow them to survive and reproduce more successfully than others. Without different alleles producing different phenotypes, there is no differential success for selection to act upon. The presence of multiple forms of a gene, or polymorphism, is the substrate upon which selection operates.
Genetic drift also relies on this underlying variation because it involves the random sampling of alleles from one generation to the next. If there is only one type of allele, the random sampling process will simply pass on that single allele type every time, resulting in no change in frequency. Drift can only cause a random fluctuation in frequency if multiple alleles are present in the initial population to be sampled by chance. The diversity of alleles provides the raw material that both forces manipulate.
Shared Scale: Operating at the Population Level
Both natural selection and genetic drift are phenomena that occur at the level of the population, not the individual organism. They describe changes in the collective gene pool over time, which is why they are studied under the field of population genetics. An individual organism is born with a fixed set of genes and cannot experience evolution during its lifetime.
Natural selection operates on the differential reproductive success of individuals within a group. The cumulative effect of these individual successes or failures across generations results in the evolution of the population. The pressure is felt by the individual, but the resulting change is measured as a shift in the population’s allele frequencies.
Similarly, genetic drift is a change in the population’s allele frequency due to chance events affecting which individuals reproduce. An individual’s survival or reproductive output may be random, such as being the only survivor of a natural disaster. However, the consequence is a change in the genetic makeup of the next generation of the population, distinguishing both mechanisms from processes like mutation (gene level) or development (individual lifespan change).