Genetic drift and the founder effect

Genetic drift and the founder effect are two related processes that describe how allele frequencies in a population can change over time, primarily due to chance rather than selective pressure. These processes are especially impactful in small populations.

Genetic Drift

Genetic drift is a random process by which the frequency of an allele in a population changes due to chance. This happens because, by chance, some alleles may be passed on to more offspring than others, leading to an increase in their frequency within the population. This is distinct from natural selection, where alleles become more common because they confer a selective advantage in survival and reproduction.

Key aspects of genetic drift include:

• Randomness: Unlike natural selection, which is a non-random process that leads to adaptation, genetic drift is purely random.

• Impact on Population Size: Genetic drift generally has a greater effect in smaller populations, where chance factors do not tend to even out across the whole population as they do in larger populations.

• Effect on Genetic Diversity: Genetic drift tends to decrease the genetic diversity within a population by causing the loss of alleles. This reduction in diversity can be problematic, as a population with low genetic diversity may be less able to adapt to changing environments and selection pressures, increasing its risk of being wiped out by events like disease.

• Evolutionary Change: If an allele is consistently passed on more often by chance, it can become more common, leading to evolutionary change over time. This can lead to significant differences in allele frequencies between isolated populations, potentially resulting in reproductive isolation and speciation if enough differences accumulate.

• Examples: An instance of genetic drift is seen in human blood group frequencies among Native American tribes, like Blackfoot Indians being mainly group A and Navajos mainly group O, where blood group doesn't affect survival but differences arose by chance in past smaller, isolated groups. The falling frequency of pink eyes in an island froghopper population, compared to a constant frequency on the mainland, is also attributed to genetic drift due to the island's smaller population size.

The Founder Effect

The founder effect is a specific type of genetic drift. It occurs when just a few organisms from a population start a new colony, leading to a significant reduction in the genetic diversity of the new population's gene pool compared to the original population.

Key aspects of the founder effect include:

• Mechanism: A small group of individuals, carrying only a subset of the alleles from the original population's gene pool, separates and establishes a new population.

• Allele Frequency Discrepancies: The frequency of alleles in the new colony may be very different from those in the original population. An allele that was rare in the original population might become much more common in the new colony purely by chance, because the founding individuals happened to carry it.

• Reduced Genetic Diversity: The new population starts with a smaller gene pool, meaning it has reduced genetic diversity. This can make the population more susceptible to various threats because it may lack the necessary advantageous alleles to adapt to future environmental changes.

• Consequences for Genetic Disorders: The founder effect can lead to an unusually high incidence of certain genetic disorders in the new population, especially if the founding individuals happened to carry recessive alleles for those disorders.

• Examples: The Amish population of North America, descended from a small number of Swiss migrants, exhibits little genetic diversity and a high incidence of certain genetic disorders, including Ellis van Creveld syndrome, which is believed to be a result of the founder effect. The Mauritian pink pigeon population, which fell to less than 20 individuals, is also cited as an example where genetic drift (including the founder effect) could significantly impact its already small gene pool and increase extinction risk. Similarly, studies on Anole lizards in the Caribbean after a hurricane demonstrated how the leg length distribution in new island populations was influenced by the alleles of the few founding individuals, independent of natural selection.

Both genetic drift and the founder effect are significant factors in evolutionary change, especially in shaping the genetic makeup and diversity of populations, often leading to divergence and, in some cases, the formation of new species through processes like allopatric speciation.