How, when and where does drift happen?
Genetic drift is change in allele frequencies in a population from generation to generation that occurs due to chance events. To be more exact, genetic drift is change due to “sampling error” in selecting the alleles for the next generation from the gene pool of the current generation. Although genetic drift happens in populations of all sizes, its effects tend to be stronger in small populations.
So drift happens all the time but its effects are more noticeable in small populations and two ways that populations become small have their own names: bottleneck effect and founder effect. Bottlenecks occur when some catastrophe reduces a population to a small number of individuals, a partial extinction. The result is that individuals carrying alleles (alternative copies of genes) occurring at low frequency may be eliminated altogether or so reduced in number that those alleles are lost for good in the next generation or two and thus genetic diversity is lost. Founder effect is where a small sample of a large population (or the extreme example of a single individual) becomes permanently isolated from the main population, such as by being transported to a new environment devoid of the main population. Again, alleles present at a low level can be lost completely in the isolated small population.
I don’t think there is controversy that genetic drift is a real phenomenon. What I have had difficulty in seeing is how genetic drift contributes to evolution as a whole. Being, by definition, a random process and one that, in small populations, reduces diversity, I think I’m correct that drift does not contribute to selective adaptation. But perhaps, where small subsets of populations go through a bottleneck, it creates a consequent springboard for subsequent adaptation in the surviving population.
But I place myself in the dummy category regarding drift and appreciate any corrections as necessary in my text above. This isn’t the first time I’ve expressed skepticism as to the importance that drift should be accorded so my apologies for being a slow learner and I look forward to input from those more informed than me.
I should credit
Berkeley University University of California, Berkeley for the diagram above.