Okay. So now let's talk about genetic drift. This is important particularly for the case of Samir, who assumed an infinite population size, because real populations aren't infinitely large. Right? Populations can't be infinite. There isn't an infinite number of individuals in any population. And because there is a limitation on population size, it can't be infinite. Since populations are actually finite, only a limited number of offspring will be produced, and those offspring only represent a subsection of the total alleles in a population. What this means is that even though the population is finite, there's an unlimited number of gametes, say, 3,000 gametes per person, which is actually much lower than the actual number. However, not every gamete that's created will produce an individual. And because not every gamete produces an individual, each generation only contains a sample of alleles. A small subsection of alleles from the previous generation are passed on to the next. These are the ones that stay in the gene pool. But because there is a loss of alleles, essentially, not every allele is passed on, there's this phenomenon called genetic drift.
How does genetic drift happen? Take yourself as an example. You have many alleles that represent your genetic material. When you create gametes, whether sperm or eggs, there's a random selection of alleles. Remember, there's random assortment, so you only get half of your total alleles in every single egg or sperm. Millions of eggs and sperm are produced, but we don't have millions of children. Thus, only a small subsection of all the alleles will actually be passed on to the next generation, and that's completely by chance, just randomly chosen. Moreover, the more offspring that you have, the more alleles that are passed on, and this is again, all by chance. If the number of gametes produced is small, which is true for humans, then a smaller number of offspring is produced. This increases the chance that the gametes will differ from the entire gene pool.
There's this entire parental gene pool that contains all the genes from both of our parents, but we only inherit half of those. And there's a great chance that this half will differ significantly from the frequencies found in the parents. This is referred to as sampling error. It is a deviation from the expected ratio due to a limited sample size. For example, if I have 5 pennies and 5 nickels and I choose 2 of them, there's only a small chance that I'll choose 1 penny and 1 nickel, representing the frequency found initially. There's a higher chance I would choose 2 pennies or 2 nickels. This is an example of sampling error, and the same thing happens with alleles. We only inherit half of the alleles from our parents, and only some of these produce an individual. Because we're producing only a small number of individuals, there's a very high occurrence of what's called genetic drift. Genetic drift is this change in allelic frequencies. As illustrated earlier, the frequency of half pennies to half nickels changed to either 2 pennies or 2 nickels when I picked that sample. Similarly, genetic drift is this change in allelic frequency due to random choices, the random disappearance of genes in a small population.
Genetic drift occurs at a much higher rate in small populations, and when allelic frequencies are equal. Now, in Hardy-Weinberg assumptions of infinitely large populations, genetic drift doesn't occur. However, in real situations where populations are finite, genetic drift happens due to random occurrences. Sometimes we might randomly choose more blue alleles over red, and sometimes vice versa. This affects different alleles and genes in finite populations, especially in smaller populations producing a small number of offspring, happening at a very high rate.
Genetic drift can lead to the fixation or loss of an allele. Fixation occurs when all individuals in a population are homozygous for one allele, for instance, if all individuals have two copies of the blue allele, it's fixed. Loss occurs when no individual in a population carries an allele, like when the red allele is completely lost. These effects of genetic drift can restrict genetic variation, which is crucial for the adaptability and survival of species.
Selective pressures like the founder effect and the bottleneck effect can also cause genetic drift. The founder effect occurs when a new, smaller population is formed from a few individuals who do not carry all the alleles from the original, larger population. This reduces variation in the new population. The bottleneck effect occurs when there's a significant reduction in population size due to environmental events or other catastrophes, which results in a loss of genetic variance.
Overall, genetic drift is a crucial concept in understanding population genetics, particularly in non-infinite populations, a core aspect of the Hardy-Weinberg principle.
With that explained, let's now move on.
