How can a gene version spread without being more useful?

A gene variant can become common even when it provides no survival or reproductive advantage. In a finite population, the gene copies passed to the next generation form a sample of the copies present in the previous one. Chance differences in that sample cause genetic drift.

An allele is a version of a gene or genetic site. Imagine a simplified population with equal numbers of two neutral alleles, shown as blue and orange tokens. Neutral means that, under the conditions being considered, neither affects reproductive success. The next generation need not inherit exactly equal numbers. By chance, more copies of one allele may be represented.

Repeating that sampling over generations can move the frequency farther from its starting point. Frequency is the fraction of gene copies represented by an allele. The direction is not predetermined: a second population beginning with the same frequencies may drift the other way. The colored tokens in the diagram represent gene copies, not an assumption that every organism carries only one gene copy.

Sampling effects are proportionally stronger when fewer copies contribute to the next generation. If a storm leaves only a small, chance subset of a population, the survivors may carry a different mixture of alleles. This is a population bottleneck. A small group establishing a new population can likewise bring an unrepresentative sample, called a founder effect. Neither requires that the surviving or founding variants were better suited to the environment.

Over time an allele may be lost, while another reaches fixation: all copies at that site in the population have the same version. In a closed model with no mutation or immigration, a lost allele cannot reappear through sampling alone. Real populations may receive new variants through mutation or gene flow, so the closed-model condition matters.

Genetic drift differs from natural selection in the cause of the frequency change. Selection involves consistent differences in reproductive success associated with inherited traits. Drift involves chance sampling. Both operate at the same time, and a change in observed frequency is not enough by itself to identify which explanation dominates. Population size, repeated observations and the relationship between genotype and reproductive success help researchers assess the evidence.

The relevant population size is also more subtle than a head count. If only a small number of individuals produce most offspring, fewer independent gene copies contribute to the future than the total population might suggest. Unequal contribution can therefore strengthen drift.

The revealing consequence is that evolution is not always a march toward improvement. Some inherited patterns reflect historical accidents in who reproduced and which copies happened to be passed on. Understanding those accidents helps explain why small populations can lose genetic variation even while every remaining individual appears healthy.