Genetic Drift

Genetic drift is a change caused by chance in how common a gene version, called an allele, is in a population. Chance can make a version more common or cause it to disappear; losing a version reduces genetic variety. Drift especially affects small, isolated populations. Ask this: When you notice a change, could chance help explain it?

Genetic drift changes the frequency of an allele—the proportion represented by a particular gene version—through random chance. A version can disappear, reducing genetic variation, or become much more common. It can even become fixed, meaning it is the only version remaining at that gene location in the population. These are possible outcomes, not guaranteed ones. [1]

For a hypothetical sampling example, imagine ten cards representing gene copies: seven red and three blue. Draw a card, record its color, return it, and repeat ten times to represent a new generation. Your record may contain different proportions, or even no blue cards. This is a simplified illustration of random sampling, not evidence from an actual population.

The card example assumes every card has the same chance of being drawn. Natural selection is a different mechanism: inherited differences affect survival or reproduction and therefore influence which gene versions are passed on. [2] A mutation is a change in an organism's DNA sequence; this is different from a chance change in the frequency of an existing version. [3] A change in frequency alone does not tell you which mechanism caused it; the example isolates chance by assuming no version is favored.

The glossary emphasizes that drift primarily affects small, isolated populations. [1] In the card illustration, fewer draws give chance outcomes more influence over the resulting proportions. However, population size alone does not supply a numerical probability or a timetable for losing a version. The supplied source also gives no basis for claiming that a particular survival advantage guarantees fixation.

As an organizational analogy, imagine a small team losing its only practitioner of a useful work habit after a chance departure. The habit might disappear without anyone judging it ineffective. This illustrates how chance loss can change what remains, but workplace practices are not alleles, and this hypothetical analogy is not biological evidence.

• NHGRI defines genetic drift as random fluctuations in the frequency of an allele, or gene version, within a population. The supplied excerpt offers a glossary explanation rather than results from a specific empirical study. [1]
• Qualitative scope: The glossary identifies small, isolated populations as especially affected by drift and states that its effects can sometimes make traits overwhelmingly frequent or cause them to disappear. [1]
• Possible outcomes: The narration explains that drift can eliminate existing gene variants and reduce genetic variation; initially rare alleles can also become more frequent or fixed. These are possibilities, not predictions for a particular population. [1]
• The University of California Museum of Paleontology explains natural selection through variation, differential reproduction and inheritance. This distinguishes selection from random sampling alone. [2]
• NHGRI defines mutation as a change in an organism's DNA sequence, distinct from a change in the frequency of an existing gene version. [3]

The supplied glossary gives no measured effect sizes, loss probabilities, generation counts, or threshold at which a survival advantage guarantees fixation. It therefore cannot support numerical forecasts from population size alone. The card example is a simplified model assuming equal sampling chances, and the team example is a hypothetical analogy. Neither is empirical evidence. A change in frequency by itself does not establish its cause.