Genetic drift is a mechanism of evolution that occurs by random chance rather than natural selection. In genetic drift, a population experiences a change in the frequency of a given allele, prompted by random luck rather than a need for adaptation. This differs from natural selection, in which allelic frequency is altered based on the fittest genes surviving to reproduce and the weaker genes dying off. Genetic drift tends to be a phenomenon amongst smaller populations, while natural selection holds sway in larger populations.
An allele, or genetic variant, is a component of a gene that produces a certain trait. Imagine there are both red worms and white worms in the same population. If a red worm mates with a white worm, each will pass one allele to its offspring, red or white, to form a gene. The dominant, or strong, allele will determine which trait the baby worm presents. If white is dominant, the baby worm will be white, if red is dominant, the baby worm will be red, and if the baby worm receives two of the same recessive alleles, it will exhibit that recessive feature. Genetics is far more complex than this example allows, but this is the general concept.
Now imagine these worms live in a swamp filled with red mud and are surrounded by birds that want to eat them. The red worms are more likely to survive because they are camouflaged by the mud and won’t be as easily seen by predators. Therefore, more red worms will live to reproduce and more red alleles will be passed on to offspring, increasing the red allelic frequency. More white worms, which are easily seen by birds, will be eaten before they are able to pass on their genes, thus decreasing their allelic frequency. This is natural selection.
Now, imagine there are ten red worms and ten white worms with equal chances of surviving to reproduce. A tree falls on the swamp, killing eight worms; six white and two red. Then suppose two white worms and one red worm get sick and die. By chance, there are now seven red worms and only two white worms left. This is an example of genetic drift.
Genetic drift can also occur through a random sampling error. A sampling error occurs when a sample exhibits different results than the entire population would. For example, say there are fifty red worms and fifty white worms in a population, and scientists randomly select ten worms to observe. Because the sample is smaller, the alleles passed on in the group of ten may not even out as they would in a group of one hundred. Also, if the group contains more red worms than white, the presentation of alleles in the offspring will be skewed.
Genetic drift becomes fixed when one allele replaces another entirely or one allele dies off. Imagine the seven red worms and two white worms left in the swamp after the tree catastrophe and sickness killed the other eleven worms. As the worms reproduce, less white worms will appear until finally there are no white worms left. Genetic drift will then be fixed, because all future generations will be red.
Because genetic drift works much more rapidly in small populations, a population bottleneck or founder effect can increase the process of genetic drift. A population bottleneck occurs when a population suddenly undergoes a dip in size. The tree falling on the swamp and killing nearly half the worm population is an example of the bottleneck effect. A founder effect occurs when a small portion of a population becomes isolated from the rest of the group and evolves separately.