Genetic drift?

Genetic drift is a mechanism of evolution that occurs by chance rather than natural selection. In genetic drift, a population experiences a change in the frequency of a given allele, brought about by chance luck rather than a need for adaptation. This differs from natural selection, where allele frequency is altered based on fitter genes surviving reproduction and weaker genes dying off. Genetic drift tends to be a phenomenon among smaller populations, while natural selection dominates in larger populations.

An allele, or genetic variant, is a component of a gene that produces a particular trait. Imagine that there are both red and white worms in the same population. If a red worm mates with a white worm, each will pass on an allele to its offspring, red or white, to form a gene. The dominant, or strong, allele will determine which trait the little worm has. 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 gets two of the same recessive alleles, it will show that recessive trait. Genetics is much more complex than this example allows, but that’s the general idea.

Now imagine that these worms live in a swamp filled with red mud and are surrounded by birds that want to eat them. Red worms are more likely to survive because they are camouflaged by the mud and won’t be easily seen by predators. Therefore, the more red worms that live to reproduce, the more red alleles will be passed on to the offspring, increasing the red allele frequency. Other white worms, which are easily seen by birds, will be eaten before they are able to pass on their genes, thus decreasing their allele 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. So suppose two white worms and one red worm get sick and die. By chance, there are now seven red worms and only two white ones left. This is an example of genetic drift.

Genetic drift can also occur through random sampling error. A sampling error occurs when a sample shows different results than those of the entire population. For example, suppose 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 match up as they would in a group of one hundred. Also, if the flock contains more red than white worms, the presentation of alleles in the offspring will be distorted.

Genetic drift sets in when one allele completely replaces another or an allele dies. Imagine the seven red worms and the two white worms left in the swamp after the tree catastrophe and disease killed the other eleven worms. As the worms reproduce, fewer white worms will appear until there are no white worms left. Genetic drift will then be corrected, because all future generations will be red.
Because genetic drift works much faster in small populations, a population bottleneck or founder effect can increase the process of genetic drift. A population bottleneck occurs when a population suddenly experiences a decline in size. The tree falling into the swamp and killing almost half of the worm population is an example of the bottleneck effect. A founder effect occurs when a small part of a population is isolated from the rest of the group and evolves separately.

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