Explain how genetic drift, gene flow, the founder effect and bottlenecks change allele frequencies in a gene pool

What this dot point is asking

SCSA wants you to define a gene pool and explain the non-selective forces that change allele frequencies, distinguishing random processes (drift, founder, bottleneck) from gene flow. A strong answer says why each effect is stronger in small populations.

The gene pool

A gene pool is the total collection of alleles present in all the individuals of a population. Evolution can be described as a change in the allele frequencies of a gene pool over time. Natural selection is one cause of such change, but several other processes also shift allele frequencies, sometimes without any survival advantage being involved.

Genetic drift

Genetic drift is random change in allele frequencies from one generation to the next due to chance alone. Because real populations are finite, not every allele is passed on in exact proportion. By chance, some alleles become more common and others rarer, and rare alleles can be lost entirely.

Gene flow

Gene flow is the movement of alleles between populations when individuals migrate and breed, or when pollen or seeds are carried between plant populations. Gene flow tends to make populations more genetically similar and introduces new alleles into a population. A barrier that stops gene flow, such as a mountain range or ocean, allows populations to diverge.

The founder effect

When a small number of individuals leave a population and establish a new one, they carry only a sample of the original gene pool. The new population's allele frequencies may differ markedly from the parent population purely by chance, and some alleles may be over-represented while others are absent. This is the founder effect, a special case of drift.

The bottleneck effect

A bottleneck occurs when a population crashes sharply, for example through disease, drought or hunting, leaving only a small number of survivors. The survivors carry only a fraction of the original genetic diversity, often not a representative sample. Even if the population recovers in numbers, its genetic diversity stays low. Reduced diversity leaves a population less able to adapt to future change.

Putting the forces together

Natural selection, drift, gene flow, founder effects and bottlenecks all change allele frequencies, and they often act at the same time. Selection is directional and non-random; drift, founder effects and bottlenecks are random; and gene flow connects populations. In conservation biology these forces explain why small, isolated Australian populations of threatened species need careful genetic management to avoid losing diversity.

Why this matters for continuity

These processes show that evolution is not only about selection. A small Australian population recovering from a bottleneck, or a founder population on an island, can diverge from its parent population by chance alone. Understanding them is essential for conservation, because a species with a depleted gene pool may survive in the short term yet lack the variation needed to adapt to future threats.