Why are the bottleneck effect and founder effects considered genetic drift?

Genetic drift is the change over time of the relative frequencies of the alleles in a population (due to chance). It occurs especially in smaller populations.

The bottleneck effect is a situation that occurs when the number of individuals in a population is dramatically reduced (the founder effect also may fall into that, since only a small number of individuals are there to "found" a new colony/population.) In both cases, the population sizes are very small.

In both these cases, the individuals "left" due to this reduction is random; so no specific allele or characteristic is favored.

Because of this, there may be significant variations of the allele frequencies in the new population. This will, over time, change these frequencies in this population (compared to that of the original one): since these variations may cause one allele to have an exceptionally high frequency, the allele frequencies in the population may shift towards that one allele.

Likewise, if there are exceptionally low frequencies of one allele, the allele frequencies may shift against that one allele, or it may disappear from that population altogether.

Hope this helps!

The bottleneck effect and founder effect are considered forms of genetic drift because they both result in a random change in allele frequencies within a population due to chance events rather than natural selection. In the bottleneck effect, a population undergoes a dramatic reduction in size, leading to a loss of genetic diversity. In the founder effect, a small group of individuals establishes a new population, leading to a genetic composition that may differ from the original population. Both processes can lead to changes in allele frequencies that are not driven by natural selection, thus constituting genetic drift.

The bottleneck effect and founder effect are considered genetic drift because they both involve random changes in allele frequencies within a population due to chance events rather than natural selection. In the bottleneck effect, a sudden reduction in population size dramatically decreases genetic diversity, leading to a smaller gene pool. This can result in certain alleles becoming more or less common in the population purely by chance. Similarly, the founder effect occurs when a small group of individuals establishes a new population in a new area, leading to a loss of genetic variation compared to the original population. In both cases, genetic drift leads to changes in allele frequencies over time, which are driven by random sampling rather than adaptive evolutionary processes.