Today I'm going to talk about genetic drift. And as usual, there's new vocabulary to learn. So we've talked about genes before and the fact that in most animals, there are two copies of any one gene in any given individual. Now the sequence of those two copies doesn't have to be exactly the same. There can be differences. Slightly different variations of the same gene are called alleles. A common example is blood type. ABO blood type is based on a single gene, and there are A, B, and O alleles.
Now what about genetic drift? What does it have to do with alleles? In a field called population genetics, scientists study the genetic makeup of entire populations of organisms, and a common measure of this genetic makeup is allele frequency - how common some alleles are compared to other alleles. In humans, you can think about this in terms of how common blue eyes are compared to brown eyes, or attached ear lobes versus unattached ear lobes. On the scale of populations, evolution can be defined as change in allele frequencies over time. Genetic drift is one explanation scientists have for why allele frequencies change over time.
Genetic drift is why increasing the population size of endangered organisms is important. Whenever genes are passed from one generation to the next, there is some level of randomness as to which allele in an given individual will be passed on to the next generation (during sexual reproduction, each individual in a mating pair only passes on a single allele for each gene). So how close allele frequencies stay the same over the course of multiple generations is dependent on the population size. Why? Take this completely abiotic example. You flip a coin. If you flip the coin only once, then the frequency of tails is either going to be 1 or 0. If you flip the coin ten times, then the frequency of tails might be something more like .7 or .3. The more times you flip the coin, the more likely that the frequency of tails will end up to be 0.5. Genetic drift works the same way - the larger the population, the less variation in allele frequencies from generation to generation. Conversely, the smaller the population, the more variation in allele frequencies. If a population is small enough, you can even lose alleles from the population, because they just didn't make it into the next generation.
Genetic drift is part of neutral theory, because what the allele does is of no consequence to the effect genetic drift has on it.
If you want to play around with some simulations that illustrate the principle of genetic drift, here are some links:
http://darwin.eed.uconn.edu/simulations/jdk1.0/drift.html
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