Evolution is driven by multiple forces that shape a species’ genetic makeup. While natural selection acts on differences in survival and reproduction, genetic drift is another major force that operates independent of fitness. Understanding why the process is called “drift” is key to grasping its profound influence on biological change.
Defining Genetic Drift as Random Allele Change
Genetic drift is the change in the frequency of gene variants, known as alleles, within a population that occurs purely by chance. Unlike natural selection, which systematically favors alleles that enhance an organism’s fitness, genetic drift is a non-selective process. The “drift” refers to the haphazard, non-directional fluctuation of allele frequencies from one generation to the next. Alleles may become more or less common solely because of random events related to mating, survival, or inheritance, not because they offer any advantage.
The Mechanism of “Drift”: Sampling Error
The underlying cause of genetic drift is a statistical phenomenon called sampling error. When a new generation is produced, it receives a random “sample” of alleles from the parent generation’s gene pool. If a population is large, this sample is highly representative of the parental allele frequencies, much like drawing thousands of marbles from a well-mixed bag.
However, in a small population, the chance selection of which individuals reproduce, and which of their two alleles are passed on, can lead to a non-representative sample. This random deviation from the expected proportion in the next generation constitutes the drift. For instance, if a population has a 50/50 split of two alleles, a small group of parents might, by chance, pass on more of one allele to their few offspring. This statistical accident causes the allele frequency to shift randomly upward or downward. The smaller the population size, the greater the potential effect of this sampling error, causing rapid fluctuations in allele frequencies across generations.
Consequences of Random Allele Shifts
Over many generations, the cumulative effect of these random shifts is a reduction in the overall genetic diversity within the population. Genetic drift causes alleles to be either completely lost or to become “fixed,” meaning they reach a frequency of 100% in the population. Once an allele is fixed, every member of the population carries that gene variant, and the gene locus no longer contributes to genetic variation. This loss of heterozygosity makes the population more genetically uniform.
These shifts occur regardless of the allele’s effect on the organism. A beneficial allele may be lost, or a slightly harmful one may become fixed simply through the randomness of inheritance in a small population. Once an allele is lost or fixed, genetic drift ceases to act on that specific gene. The probability that an allele will eventually become fixed is equal to its frequency in the population at that time, assuming drift is the only force acting upon it.
Real-World Manifestations of Drift
Genetic drift is amplified in specific ecological scenarios, most notably the population bottleneck and the founder effect. A population bottleneck occurs when a species experiences a sudden, drastic reduction in size due to an event like a natural disaster or overhunting. The few survivors carry only a small, random subset of the original gene pool, leading to an immediate loss of genetic diversity.
The Northern elephant seal illustrates this effect; it was hunted down to approximately 20 individuals in the late 19th century. Although the population has recovered to tens of thousands, its genetic diversity remains low.
The founder effect describes a situation where a small number of individuals split off from a larger population to establish a new, isolated colony. The new colony’s gene pool is limited to the alleles carried by those founders, which may not represent the original population.
For example, the Amish community in Pennsylvania was established by a small group of founders. Due to their reproductive isolation, certain rare alleles, such as the one causing Ellis-van Creveld syndrome, became much more prevalent than in the general population. Both effects demonstrate how chance events can rapidly alter the genetic landscape of a species.