Genetic drift, gene flow, and natural selection may sound similar or even confusing to some. All three are mechanisms in the evolutionary process that have to do with alleles and/or gametes, but there are several significant differences.
Discussions about genes and natural selection usually include the term allele. An allele is just one version of a gene found at the same place (locus) on a chromosome. An example of an allele is the color of a bird’s feathers. In sexually reproducing organisms, alleles occur in pairs because the offspring receive one from each parent.
In genetic drift, alleles change frequency within a population due to random sampling. As a result, it does not produce adaptations. Two mechanisms cause genetic drift. The first is the bottle effect. This is genetic drift in a population after it has gone through a catastrophic event like a flood.
The bottleneck happens when the allele frequency of a main trait in the original population is reduced because so many individuals carrying the allele have died. This causes most of the surviving population to die off, leaving a few random individuals as survivors. The other mechanism is called the founder effect.
This is when a few members of a population break away and create their own group. Because of the random sampling that created the new group, the allele frequency can dramatically shift depending on the selective pressures place on the individuals.
Gene flow differs from genetic drift because it is the transfer of alleles or gametes from one population to another. It happens when a population migrates or becomes geographically isolated. This is different from the genetic drift seen with the founder effect where the new group is formed in an area that does not have an existing population.
Natural selection is like genetic drift but with one major difference—it’s not random. And unlike genetic drift which can be helpful, detrimental, or have no effect, natural selection represents only positive change/adaptation.
Also, natural selection is influenced by changes in environmental conditions while genetic drift is random and based on luck. One major way gene flow is different from natural selection is that gene flow helps keep alleles in a population homogenized while natural selection increases genetic variation and always moves toward creating new species.