Genetic Drift, Founder Effect, and Bottlenecks

Genetic drift is a random change in allele frequencies from one generation to the next due to chance, and it has the strongest effect in small populations; the founder effect and population bottlenecks are two situations in which a small number of individuals gives rise to a population with reduced genetic variation. In brief: unlike natural selection, drift is not related to fitness. Over time, it can cause an allele to be lost (frequency 0) or fixed (frequency 1) in the population, and it reduces heterozygosity. Gene flow (migration) and mutation also change allele frequencies: gene flow tends to make populations more similar, and mutation adds new alleles.

Random events are an important source of evolutionary change, particularly for rare alleles and small or endangered populations. This guide explains genetic drift and why population size matters, describes the founder effect and bottlenecks with examples, covers gene flow and mutation as mechanisms of evolution, compares the mechanisms in a table, works through numerical examples and corrects common mistakes.

What is genetic drift?

In every generation, the alleles that are passed on come from a sample of the parents’ gametes, and a sample can differ from the whole by chance. Think of a bag with 50 red marbles and 50 blue marbles, from which you draw 10. You will often not get exactly 5 of each color. In the same way, the allele frequencies in the next generation differ somewhat from those in the previous generation by chance alone. The smaller the sample (the population), the greater the effect.

The standard deviation of the change in the frequency of an allele in one generation is approximately √(pq ÷ 2N), where N is the number of breeding individuals. For an allele at a frequency of 0.5, the typical change is about 0.11 when N = 10, 0.035 when N = 100 and 0.011 when N = 1 000. Drift is therefore a strong force in small populations and is negligible in very large ones.

The consequences are:

• Allele frequencies wander randomly over generations.
• Eventually, one allele is fixed (frequency 1.0) and the others are lost. The probability that a neutral allele becomes fixed equals its initial frequency, so an allele with a starting frequency of 0.1 has a 10 percent chance of eventually being fixed and a 90 percent chance of being lost.
• Genetic variation (heterozygosity) is lost. The fraction of heterozygosity left after t generations is about (1 − 1/(2N))^t.
• Different small populations drift in different directions, so they become genetically different from each other.
• Drift can increase the frequency of harmful alleles, or remove helpful ones, in small populations.

What is the founder effect?

The founder effect occurs when a new population is established by a small number of individuals from a larger population. The founders carry only a part of the genetic variation of the source population, and rare alleles in the source can have a much higher or lower frequency in the new group.

• Amish and Mennonite communities: the Old Order Amish in Pennsylvania descend from about 200 founders who migrated in the eighteenth century. A recessive form of dwarfism with extra fingers (Ellis-van Creveld syndrome) is much more frequent among them than in the general population, as one of the founders carried the allele. Mixed with high rates of marriage within the group, this raised its frequency.
• Tristan da Cunha: a small Atlantic island population founded by a handful of people in 1816 has a high frequency of asthma and a form of eye disease (retinitis pigmentosa), traced to the founders.
• Islands: populations of species on islands often differ from those on the mainland, in part because of the founder effect, followed by adaptation.
• Introduced species: invasive species often start from a few individuals.

What is a population bottleneck?

A bottleneck is a sharp reduction in the size of a population by an event such as a disease, a natural disaster or hunting. The survivors are a random sample of the original gene pool, and rare alleles are likely to be lost. Even if the population recovers its size, the reduced genetic variation remains for a long time.

• Northern elephant seals: hunted to about 20 individuals in the 1890s. The population has recovered to over 100 000, but the genetic variation is very low.
• Cheetahs: have very low genetic diversity, which is thought to be due to one or more bottlenecks around 10 000 years ago and more recently. Individuals accept skin grafts from one another, which suggests genetic similarity.
• Greater prairie chickens in Illinois: declined from millions in the nineteenth century to about 50 birds in 1993, with a fall in egg hatching rate. After birds from other states were brought in, the hatch rate recovered.
• Humans: genetic studies suggest that the ancestors of non-African populations passed through a bottleneck when they left Africa, so the genetic diversity outside Africa is lower than within Africa.

What are gene flow and mutation?

Gene flow is the movement of alleles between populations by migration of individuals or of gametes (pollen, spores). It adds alleles that were not present in the receiving population, and it tends to reduce the differences between populations. It can counteract drift and selection. Even a low rate of migration, about one migrant per generation, is enough to stop populations from diverging by drift. Gene flow from crop plants to wild relatives, and from hatchery fish to wild populations, are practical concerns.

Mutation is the original source of new alleles. The mutation rate per gene per generation is very low (of the order of 10⁻⁵ to 10⁻⁶), so mutation alone changes allele frequencies slowly. Its importance is that it provides the new variation on which selection and drift act. Together, selection, drift, gene flow, mutation and non-random mating are the five mechanisms that can change allele frequencies.

Mechanisms of evolution compared

Worked example: drift in populations of different sizes

Consider an allele at a frequency of 0.5 in three populations of different sizes (N = number of breeding individuals).

1. Typical change: for N = 10, √(0.5 × 0.5 ÷ 20) = √0.0125 = 0.112. For N = 100, √(0.25 ÷ 200) = √0.00125 = 0.035. For N = 1 000, √(0.25 ÷ 2 000) = 0.011.
2. Heterozygosity left: (1 − 1/(2N))⁵⁰. For N = 10: (0.95)⁵⁰ = 0.077. For N = 100: (0.995)⁵⁰ = 0.778. For N = 1 000: (0.9995)⁵⁰ = 0.975.
3. Interpretation: a population of 10 loses over 90 percent of its heterozygosity in 50 generations, so it is almost entirely homozygous. A population of 1 000 loses only 2.5 percent.

The founder effect

An allele is present in the source population at a frequency of q = 0.01. A group of n founders is chosen at random. The chance that none of the founders carries it is (1 − q)²ⁿ, since each founder has two copies of the gene.

1. For 2 founders: chance of at least one copy = 1 − 0.99⁴ = 3.9 percent. The allele is most likely absent from the new population.
2. For 10 founders: 1 − 0.99²⁰ = 18.2 percent.
3. For 50 founders: 1 − 0.99¹⁰⁰ = 63.4 percent.

If a copy is present in the founders, it may be at a much higher frequency than in the source. For example, one copy among 10 founders (20 gene copies) is a frequency of 0.05, five times the source frequency. In a small group, the allele is thus either lost or becomes much more common than before. This is the reason for the high frequency of certain rare diseases in isolated communities.

Why does this matter for conservation?

Small populations of endangered species suffer from drift and inbreeding. Loss of genetic variation reduces the ability of the population to adapt to disease or climate change, and increases the expression of harmful recessive alleles (inbreeding depression). The Florida panther was reduced to about 20 to 30 animals in the 1990s and showed defects such as heart problems and poor sperm quality. In 1995 eight female pumas from Texas were released into the population, and the number of panthers increased to more than 100 in following years, with fewer defects, an example of “genetic rescue” by gene flow. Conservation biologists use the effective population size (N_e), the size of an ideal population that would show the same drift as the real one, and it is usually much smaller than the census size. A rule of thumb used in conservation is that an N_e of 50 is needed to avoid inbreeding in the short term, and 500 or more to keep the evolutionary potential in the long term, though these numbers are debated.

How was drift discovered?

Sewall Wright developed the theory of genetic drift in the 1930s, and the term “Sewall Wright effect” was once used. Ernst Mayr described the founder effect in 1942 in his work on speciation. Motoo Kimura proposed in 1968 the neutral theory of molecular evolution, which says that most genetic differences between species and within species arise from the random drift of neutral mutations, not from selection. The theory remains influential, though selection is also known to be important for traits that affect fitness.

Which mistakes are common?

Mistake 1: Saying that drift favors the fittest

Drift is random and has no direction in relation to fitness.

Mistake 2: Thinking that drift is as strong in large populations

Its effect is inversely related to population size. In very large populations it is negligible.

Mistake 3: Confusing the founder effect and the bottleneck

A founder effect starts with a small group that colonizes a new place. A bottleneck is the shrinking of an existing population.

Mistake 4: Assuming that a recovered population has recovered its genetic variation

Numbers can recover fast, while lost alleles do not return, unless mutation or gene flow supply them again.