What is Genetic Drift?

In reading some of the arguments Dr. Sanford gives for creation, I noticed that one of his main points is that most mutations are harmful (which is not true) and that even those mutations that aren’t harmful usually aren’t beneficial enough to spread through a population. But is this true?

First, we have to understand a bit about how mutation works. Traits are coded for by genes, which are made up of DNA. All DNA is composed of four base pairs: adenine, guanine, cytosine, and thiamine, commonly abbreviated A, G, C, and T. A mutation occurs when base pairs are added, deleted, or changed. These mutations can be positive, negative, or neutral (they produce no effect).

Another important piece of information is that every organism (technically every eucharyotic organism, but we won’t go into that) possesses two copies of each gene. Because most organisms are diploid (their chromosomes come in pairs) they have a copy of every gene on each chromosome. Further, these copies can be different from each other. We call these copies alleles.

Our alleles are completely separate from each other, so if a mutation happens on one allele, it doesn’t affect the other. And the way that genetic expression works is that if the two alleles of a gene are different from each other, only one gets expressed as an actual trait. The expressed allele is called dominant, and the other is called recessive. If both alleles are the same, then of course they both code for the same trait, so they both get expressed. This is the only way for recessive alleles to get expressed.

Many mutations are recessive, which means that even if an organism gets an extremely negative mutation, it probably won’t affect them at all, because it won’t be expressed. This means that mutations can be passed down from generation to generation, and as long as only one of the alleles is mutated, everyone will be fine.

Now, let’s see what happens if an organism acquires a recessive beneficial mutation. If they reproduce, their offspring each have a 25% chance of also inheriting that mutation. Let’s say this organism is a rabbit, and this rabbit has a hundred children over her lifetime (not an excessive estimate at all, by the way). This means that she’ll have 25 children with a mutated gene.

Each of these 25 offspring can each have 100 offspring of their own, which can then have 100 offspring, and so on. We see that very quickly, mutations can accumulate in the rabbit population. Also notice there is no selection pressure. This mutation doesn’t do anything. It spreads through the population entirely through statistics.

This is genetic drift. Genetic drift is a powerful force that doesn’t even require natural selection to spread mutations. It’s also completely ignored by Dr. Sanford, to his detriment. Genetic drift allows mutations that may only be slightly beneficial to spread through the population, even when natural selection has little to no impact on that spread.

One of Dr. Sanford’s major points is that those rare beneficial mutations don’t give enough of a genetic advantage to take hold in the population. As we’ve seen, that’s completely untrue. As a geneticist, Dr. Sanford should know better.


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