Genetic Drift

Genetic drift is one of the key evolutionary factors that changes gene frequencies in populations. Unlike adaptive selection, it acts purely by chance. In this chapter, we focus on what is specific to genetic drift, how it works, and in which situations it becomes especially important.

What Is Genetic Drift?

Genetic drift is the random change of allele frequencies in a population from one generation to the next, caused by chance events in reproduction.

It is non-directional: drift does not “favor” better-adapted or less-adapted alleles.
It is a sampling effect: each generation is like a random sample of gametes (eggs and sperm) from the parental generation.
It acts on all loci, not only on those affecting fitness.

Under genetic drift, even neutral or slightly disadvantageous alleles can increase in frequency or become fixed, simply because of random fluctuations.

Drift as a Sampling Process

Imagine a population with two alleles at a gene: $A$ and $a$.

Suppose the frequency of $A$ is $p = 0.5$ and $a$ is $q = 1 - p = 0.5$.
The next generation is formed from gametes produced by the current generation.
The number of copies of $A$ and $a$ in the offspring is random, even if all individuals have the same expected reproductive success.

Mathematically, if a population has $N$ diploid individuals (thus $2N$ gene copies at that locus), and the frequency of allele $A$ in generation $t$ is $p_t$, the number of $A$ alleles in generation $t+1$ is a random variable that can be modeled as:

$$
X \sim \text{Binomial}(2N, p_t)
$$

The new allele frequency is then:

$$
p_{t+1} = \frac{X}{2N}
$$

This binomial sampling inherently generates random deviations from $p_t$. Over many generations, these deviations accumulate, sometimes leading to fixation or loss of alleles.

Drift Depends Strongly on Population Size

The strength of genetic drift is inversely related to the effective population size $N_e$ (see below):

In large populations, random fluctuations tend to average out, so allele frequencies change slowly and slightly.
In small populations, random deviations are stronger, so allele frequencies can change rapidly and dramatically.

The variance (statistical scatter) in allele frequency due to drift per generation is:

$$
\text{Var}(p_{t+1}) = \frac{p_t (1 - p_t)}{2N_e}
$$

Key points:

The smaller $N_e$, the larger the variance.
Maximal variance occurs when $p_t = 0.5$ (both alleles equally common).

Effective Population Size ($N_e$)

The effective population size $N_e$ is the size of an idealized population that would show the same amount of genetic drift as the real population.

$N_e$ is often smaller than the actual census size $N$ because of:

Unequal sex ratios (e.g., very few breeding males, many females).
Unequal reproductive success (a few individuals leave many offspring, many individuals leave few or none).
Fluctuations in population size over time.
Overlapping generations and age structure.

Only $N_e$, not the census size, determines the actual rate of genetic drift.

Consequences of Genetic Drift

Genetic drift has several characteristic effects on genetic variation within and between populations.

1. Loss of Genetic Variation

Within a single population, drift tends to reduce genetic diversity over time:

Alleles are randomly lost.
Eventually, if no new variation is introduced, every locus tends toward fixation of one allele.

For neutral alleles (no fitness differences):

The probability that an allele will eventually become fixed is equal to its current frequency.

A neutral allele with frequency $p$ has fixation probability $p$.

The probability that it will be lost is $1 - p$.

Thus, rare neutral alleles are more likely to be lost than common ones.

A rough approximation for the expected time to fixation of a neutral allele (in generations) in a diploid population is on the order of:

$$
\text{Time to fixation} \approx 4 N_e
$$

(for an allele already present in the population and destined to fix).

2. Random Fixation and Loss of Alleles

Because of drift:

Even alleles with no advantage or slight disadvantage can become fixed.
Conversely, slightly beneficial alleles may be lost before selection can increase their frequency.

This introduces an element of chance into evolutionary outcomes: repeated evolution in replicate small populations can proceed along very different genetic paths, even under the same environmental conditions.

3. Increased Genetic Differences Between Populations

While drift reduces variation within each population, it can increase variation between populations:

If several small populations are isolated from each other, drift acts independently in each one.
Over time, their allele frequencies diverge, sometimes leading to:

Different fixed alleles at the same loci.
Distinct genetic “signatures” of each population.

This divergence by drift is one of the processes that can contribute to genetic differentiation and, together with isolation and selection, can play a role in speciation.

4. Interaction With Inbreeding

Genetic drift tends to increase inbreeding, because:

As alleles are lost, individuals in the population are more likely to inherit the same allele from both parents (become homozygous).
In small, isolated populations, drift and inbreeding reinforce each other:

Drift reduces diversity.
Lower diversity leads to higher probability of mating between genetically similar individuals.
Inbreeding increases homozygosity and can expose deleterious recessive alleles.

The combination of drift and inbreeding can contribute to inbreeding depression and an elevated risk of extinction in small populations.

Special Forms of Genetic Drift

Although drift is always operating to some degree, its effects become particularly visible in specific demographic situations.

Bottleneck Effect

A population bottleneck occurs when a population undergoes a sudden, drastic reduction in size (e.g., due to natural disasters, disease, overhunting, habitat fragmentation).

Consequences:

Only a random subset of the original gene pool survives and reproduces.
Rare alleles are often lost simply by chance.
Allele frequencies after the bottleneck can differ sharply from pre-bottleneck frequencies.

This is a strong, one-time pulse of genetic drift.

Even if the population later grows large again:

The genetic variation usually remains lower than before.
Certain alleles may be gone permanently.
The population may show signs of reduced heterozygosity.

Bottlenecks leave a characteristic genetic pattern: low diversity and certain alleles at high frequency that were previously rare.

Founder Effect

The founder effect is a special case of drift during the founding of a new population by a small number of individuals.

Example situations:

A few individuals colonize an island.
A small group migrates into a new, isolated habitat.
A new captive breeding population is established from few founders.

Characteristics:

The genetic makeup of the new population is determined by the alleles carried by the founders—a sample of the source population.
Allele frequencies may differ markedly from those in the original population just by chance.
Rare alleles in the source population can become common or fixed in the new population if the founders happen to carry them.

Biological consequences:

Founder populations often have reduced genetic diversity.
They may show higher frequencies of certain genetic diseases (if founders carried rare deleterious alleles).
Over time, drift and selection in the new environment can accentuate differences between founder and source populations.

Drift vs. Selection

Genetic drift and natural selection both change allele frequencies over time, but in fundamentally different ways.

Key Differences

Cause:

Drift: random sampling of alleles during reproduction.
Selection: systematic differences in survival or reproduction based on phenotype (and thus genotype).

Directionality:

Drift: nondirectional; can increase or decrease any allele’s frequency without regard to fitness.
Selection: directional; tends to increase alleles that confer higher fitness and decrease those that reduce fitness.

When Does Drift Dominate?

Whether drift or selection has the stronger effect depends on the relative sizes of:

The selection coefficient $s$ (strength of selection against or for an allele).
The effective population size $N_e$.

A common rule of thumb:

If $|s| \ll \frac{1}{2N_e}$, drift tends to dominate: selection is too weak relative to random fluctuations.
If $|s| \gg \frac{1}{2N_e}$, selection tends to dominate: fitness differences are strong enough to overcome random noise.

This has important implications:

In small populations, even mildly deleterious alleles can increase or fix by drift because selection is relatively weak ($N_e$ small).
In large populations, selection is effective in shaping allele frequencies, and drift mainly affects strictly neutral alleles or those with extremely small $|s|$.

Drift and Neutral Theory

Genetic drift plays a central role in the neutral theory of molecular evolution, which emphasizes:

Many molecular changes (e.g., in DNA or protein sequences) are selectively neutral or nearly neutral.
The rate of substitution (fixation of new neutral mutations) in a population is approximately equal to the neutral mutation rate, independent of population size.

Basic idea:

In a diploid population with size $N_e$, the number of new neutral mutations per generation at a locus is:
$$
2 N_e \mu
$$
where $\mu$ is the mutation rate per gene copy per generation.
Each new neutral mutation has a fixation probability of:
$$
\frac{1}{2 N_e}
$$
Thus, the rate of neutral substitution (number of neutral fixations per generation) is:
$$
2 N_e \mu \cdot \frac{1}{2 N_e} = \mu
$$

This shows how mutation and drift together can explain a steady rate of molecular change over time, without invoking selection for every substitution.

Genetic Drift in Conservation Biology

Genetic drift is especially important in the context of small and endangered populations.

Risks in Small Populations

In threatened species, populations often have small $N_e$ due to:

Habitat fragmentation (isolated subpopulations).
Previous bottlenecks.
Unequal sex ratios or skewed reproduction.

Consequences of strong drift in such populations:

Rapid loss of genetic variability.
Increased inbreeding and exposure of deleterious recessive alleles.
Possible inbreeding depression, reduced fertility, and survival.
Reduced ability to adapt to environmental changes or new pathogens because of low genetic diversity.

Management Implications

Conservation strategies often aim to counteract drift by:

Increasing effective population size (e.g., habitat restoration, enlarging population sizes).
Facilitating gene flow between subpopulations (e.g., wildlife corridors, managed translocations) to introduce new alleles.
Avoiding repeated bottlenecks and minimizing skewed breeding (e.g., careful breeding plans in zoos).

Understanding genetic drift is therefore crucial not only for evolutionary theory but also for practical conservation and management of biodiversity.

Summary

Genetic drift is the random change in allele frequencies due to sampling effects in finite populations.
Its strength increases as effective population size decreases.
Drift leads to:

Loss of genetic variation within populations.
Random fixation and loss of alleles, including neutral and weakly selected ones.
Increased genetic differences between isolated populations.

Special forms of drift include bottleneck effects and founder effects, where strong random changes in allele frequencies occur because of drastic reductions in population size or the founding of new populations by few individuals.
The balance between drift and selection depends on $N_e$ and the selection coefficient $s$.
Drift, together with mutation, underpins the neutral theory of molecular evolution and has major implications for conservation biology.

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