Random processes can change a population’s genetic makeup without any “best” phenotype being favored. Over generations, mutation, genetic drift, and gene flow can shift allele frequencies, sometimes pushing populations onto different evolutionary paths.

Core idea: evolution can be random

Evolution is often described as changes in a population’s allele frequencies over time. In addition to non-random natural selection, several random processes can alter allele frequencies:

• Mutation introduces new alleles unpredictably.

• Genetic drift changes allele frequencies by chance (sampling error).

• Gene flow moves alleles between populations via migration and reproduction.

When populations experience different combinations of these processes (or experience them at different rates), their allele frequencies can become increasingly different, leading to population divergence.

Allele frequencies: what is changing?

Because allele frequencies track alleles in the whole gene pool, they are measured across the population, not in a single individual.

A key point for AP Biology: random processes can change $p$ over time even if no phenotype has a consistent survival or reproductive advantage.

Mutation: random origin of new alleles

Mutation is the ultimate source of novel genetic variants. Its effects on allele frequencies often follow these patterns:

• Directionless with respect to “need”: mutations do not arise because organisms require them.

• Typically small per generation: many mutations are rare, so frequency shifts can be slow at first.

• Can be neutral: many mutations have no immediate effect on reproductive output, yet they still alter allele counts.

• Can alter multiple levels: changes may affect proteins, gene regulation, or chromosome structure, producing new alleles that may later rise or fall due to other processes.

Even when a new allele appears, whether it persists is strongly influenced by chance (drift) and movement between populations (gene flow).

Genetic drift: chance changes, especially in small populations

Genetic drift reflects sampling error: the alleles passed on are a random sample of the previous generation’s gene pool.

Genetic drift produces random “walks” in allele frequency across generations, with much larger fluctuations when population size is small. The multiple trajectories emphasize that identical starting frequencies can lead to different outcomes by chance, including fixation ($p=1.0$) or loss ($p=0.0$). Source

Important features include:

• Stronger in small populations: fewer reproducing individuals means greater random fluctuation in allele frequencies.

• Can cause fixation or loss by chance: an allele may reach frequency 1.0 (fixed) or 0.0 (lost) without being beneficial or harmful.

• Reduces genetic variation within a population over time when alleles are lost, which can further amplify random change in later generations.

A bottleneck reduces population size so that the surviving gene pool is a non-representative sample of the original, shifting allele frequencies by chance. The diagram highlights how some alleles can be completely lost after bottlenecks, lowering genetic variation and making subsequent drift effects stronger. Source

• Different outcomes in different populations: two isolated populations can drift in different directions purely by chance, contributing to divergence.

Drift is random with respect to fitness, but its consequences (like reduced variation) can be biologically significant.

Gene flow: allele movement between populations

Gene flow changes allele frequencies by adding or removing alleles from populations. Its effects depend on how much movement occurs and how genetically different the populations are:

• Introduces alleles into a population, potentially increasing variation (especially if incoming individuals carry alleles not currently present).

• Homogenises populations when migration is frequent, making allele frequencies more similar and reducing divergence.

With gene flow among subpopulations, allele frequencies that begin far apart move toward a common equilibrium value over successive generations. The curves show how migration reduces differences between populations, counteracting divergence that would otherwise accumulate from drift and mutation. Source

• Can overwhelm drift in small populations if enough migrants contribute alleles each generation.

• Asymmetry matters: if migration mainly goes one direction, allele frequencies may shift more in the recipient population than in the source.

Whether gene flow promotes similarity or contributes to differences depends on migration rates, reproductive success of migrants, and the existing allele frequencies in each population.

How these processes drive population divergence

Populations tend to diverge when random processes push allele frequencies apart faster than they are pulled together:

• Mutation generates different new alleles in different populations over time.

• Genetic drift changes frequencies unpredictably and independently in each population, especially when population sizes are small or few individuals reproduce.

• Reduced gene flow allows differences created by mutation and drift to persist and accumulate, increasing divergence across generations.

• High gene flow generally counteracts divergence by repeatedly mixing alleles and equalising allele frequencies.