Dérive génétique

Genetic drift is a change in allele frequency due to chance events rather than trait advantage. Unlike natural selection, which favors traits improving survival or reproduction, genetic drift alters allele frequencies randomly. For example, a strong wind might remove individuals by chance, not because they lack beneficial traits. This random process can shift allele frequencies significantly, especially in small populations.

Small populations experience stronger genetic drift because random sampling creates larger proportional changes in allele frequencies. When a population is reduced, the remaining members may not represent the original genetic diversity, creating a sampling error. In large populations, many individuals can be lost while maintaining sufficient genetic diversity for other evolutionary mechanisms to act. This makes small populations more vulnerable to random allele frequency shifts.

The bottleneck effect occurs when a population is suddenly reduced in size due to natural disasters or overhunting. The surviving individuals carry only a fraction of the original population's alleles, reducing genetic diversity. This is a form of genetic drift where random chance determines which alleles survive. The new population's allele frequencies may differ dramatically from the original population.

The founder effect happens when a small group leaves a larger population and establishes a new population elsewhere. Because this founding group carries only some original alleles, the new population has reduced genetic diversity. The alleles present in founders become overrepresented by chance alone, not because they provide advantages. This exemplifies how genetic drift can shape populations through random sampling rather than selection.

Yes, genetic drift can alter frequencies of advantageous, neutral, and harmful alleles alike. Unlike natural selection, which typically removes harmful traits, genetic drift changes allele frequencies purely by chance. A harmful allele might increase in frequency randomly, or a beneficial allele might disappear. This random process operates independently of whether alleles benefit or harm the population.

A sampling error occurs when a sample does not represent the population from which it is derived. In genetic drift, when part of a population is eliminated, remaining members may represent only a fraction of the original genetic diversity. Larger samples are typically more representative, which is why small populations experience sharper reductions in genetic diversity. This sampling error is central to how genetic drift reduces variation.

Yes, evolution can occur through genetic drift alone, without natural selection. Genetic drift is a change in allele frequency due to chance, not trait advantage. Over time, random shifts in allele inheritance can cause one allele to become much more common or disappear entirely. This demonstrates that evolution is not always driven by selection favoring beneficial traits.