The founder effect is a concept in population genetics where a new population is started by a small number of individuals. These “founders” carry only a small fraction of the original population’s genetic variation. The new population can therefore differ significantly from the parent population in its genetic makeup. This phenomenon is a special case of genetic drift, which encompasses random changes in the frequency of gene variants in a population.
The Mechanism of the Founder Effect
The process begins with a large, genetically diverse parent population containing a wide array of alleles, which are different forms of a gene. From this larger group, a small subset of individuals migrates to a new location and becomes reproductively isolated, meaning they no longer interbreed with the original population.
By pure chance, the allele frequencies in the founder group can be very different from those in the parent population. An analogy is drawing a small handful of marbles from a large bag containing many colors; the handful is unlikely to have the same color proportions as the entire bag. This random sampling of genes leads to an outcome where certain alleles that were rare in the parent population may become much more common in the new one.
Examples in Human Populations
The founder effect is well-documented in several human populations, often leading to a higher prevalence of certain genetic disorders. One of the most-cited examples is the Old Order Amish of Pennsylvania. This population was established by approximately 200 German immigrants and has remained largely closed to intermarriage with outside populations.
A condition among the Old Order Amish is Ellis-van Creveld syndrome, a rare form of dwarfism characterized by short limbs, extra fingers or toes, and heart defects. The high incidence of this autosomal recessive disorder has been traced back to a single couple, Samuel King and his wife, who immigrated in 1744. Because the gene was present in the small founding group and the population remained isolated, the allele became more concentrated over generations.
Another example is found among the Pingelapese people of Micronesia. A significant portion of this population experiences a rare form of total color blindness called achromatopsia. This high frequency is the result of a typhoon in 1775 that left only about 20 survivors. One of the survivors, the ruler at the time, is believed to have been a carrier of the recessive allele for the condition. Today, nearly 10% of the Pingelapese population has achromatopsia, and another 30% are carriers, a stark contrast to its rarity in the general global population.
The Founder Effect vs. The Bottleneck Effect
The founder effect is often discussed alongside another mechanism of genetic drift known as a population bottleneck. A bottleneck occurs when a population’s size is sharply reduced for at least one generation due to a random event, such as a natural disaster, disease, or overhunting. This event randomly kills a large portion of the population, leaving behind a small group of survivors.
While both processes result in a decrease in genetic diversity, their primary distinction lies in the cause. The founder effect is a consequence of migration, where a small group establishes a new population in a new location. The bottleneck effect, on the other hand, involves a drastic reduction of a population in its original location without any migration. The northern elephant seal, which was hunted to near extinction in the 19th century, is an example of a species that has gone through a population bottleneck.
Broader Evolutionary Significance
The founder effect has implications beyond isolated human groups and can contribute to evolution. It is considered a potential contributor to speciation, the process by which new species arise. This occurs through a model of speciation known as allopatric speciation, which involves geographic isolation.
A founding population is not only genetically different but also faces new environmental pressures in its habitat. This combination of a distinct gene pool and different selective forces can accelerate divergence from the parent population. Over time, this may lead to the formation of a new species, demonstrating how random chance can initiate significant evolutionary change.