So Hardy Weinberg exists in this ideal world of these five specific assumptions. But reality isn't like that, right? And these assumptions are way too restrictive to exist in real life. So we're going to go through each of these five and talk about what happens in real life populations. So the first one is selection, right? No selection for Hardy Weinberg, but in real life, there is selection. And that selection is called natural selection. And natural selection is when organisms have genes or alleles that allow them to survive better and allow them to reproduce. And the reason that natural selection exists is simply because there's a struggle for survival. There are many organisms that are born on this Earth. There are so many organisms born on this Earth, not all of them can survive. Because not all of them can survive, there have to be a few that are more apt to survive and reproduce than others. And those few usually have some kind of gene that allows them to survive and reproduce better. So because not everyone survives, the ones that do have particular phenotypes that allow them to, and they can pass that on to their offspring. That's how selection pretty much works. Now there are many different types of selection. One is called directional selection, and that is when an allele moves in one direction towards what's called fixation or loss. Loss makes sense, right? That allele will be completely lost in the population. But fixation, what does that mean? The exact opposite of loss. So fixation, fixed alleles, are alleles found in every organism. So when an allele is fixed, that means every organism in that population has only that one allele. There are no other allele options for them. And so Directional Selection moves what could be two or three or four alleles toward one direction, whether they are fixed in a population or they're lost from the population. Then, there's positive and purifying, which are separate selections. So what happens in both of these is that there's some kind of mutation. Right? So when there's a mutation, that can either be beneficial, or it can be harmful, or it could potentially be neutral. But in selection, it either has to be beneficial or harmful. So when a mutation is beneficial, positive selection says, okay, that's great. We need this. It's beneficial. It helps the organism survive and reproduce, so let's make it a higher frequency. Purifying selection says that the mutation is harmful. It makes the organism less likely to reproduce, so we're going to get rid of it. We're going to remove it from the population. And then, finally, there is balancing selection. And this is when there are two or more alleles, but essentially they're both useful. They're both beneficial to the population. And so they are maintained at some kind of equilibrium, where they both exist and selection sort of weeds out many other alleles for these, to balance these. The majority of the organisms have these alleles. So an example of this would be directional selection, where the red represents before. So this is before over here, and the blue represents after. Before Directional Selection, there's a really high amount of this one phenotype, less of this intermediate, and a very low amount of the phenotype. After selection, this one, the allele for phenotype 1, has been lost, and this one has been gained, moving towards fixed. Right? And, after, this means that there's a low amount of phenotype 1, intermediate of intermediate, but a very high amount of Phenotype 2, and that's due to Directional Selection, where this allele becomes lost and this one is gained at a higher frequency. Then there's stabilizing selection. This is also kind of like balancing selection, where before there was sort of this broad allelic frequency distribution over three different phenotypes. Whereas, after the selection, the alleles for this intermediate phenotype here in the middle have been selected for, and that intermediate phenotype is usually a mixture of multiple alleles, and so those multiple alleles have been selected for. Now, let me disappear so we can talk about this. Another measurement of natural selection includes fitness. And what fitness is, it's a measure of how well an individual's genetic makeup contributed to the next generations. And so that says, how well are those genes passed on? There are two kinds of fitnesses. There's absolute fitness, and this just says the number of offspring an individual has. Right? The more offspring, the more genes you passed on. The second type is relative fitness, and that's the fitness, or the number of offspring an individual has, compared to another individual. So if we say that we have a brown fish and a blue fish, and the brown fish produce two offspring, and the blue fish produce ten. Well, the relative fitness is two to ten. Right? And so that compares how fit the blue fish was to the brown fish. So it says brown. Sorry for my handwriting. And so fitness is another way to measure selection. Now selection exists, right? Natural selection exists all the time. We can actually see it in our lifetimes. And it is a reason why, because this exists, it is also why Hardy Weinberg equilibrium is not a good representation of current populations living at the time, and instead, it's just an estimation. So that's selection. Now, let's turn the page.
Table of contents
- 1. Introduction to Genetics51m
- 2. Mendel's Laws of Inheritance3h 37m
- 3. Extensions to Mendelian Inheritance2h 41m
- 4. Genetic Mapping and Linkage2h 28m
- 5. Genetics of Bacteria and Viruses1h 21m
- 6. Chromosomal Variation1h 48m
- 7. DNA and Chromosome Structure56m
- 8. DNA Replication1h 10m
- 9. Mitosis and Meiosis1h 34m
- 10. Transcription1h 0m
- 11. Translation58m
- 12. Gene Regulation in Prokaryotes1h 19m
- 13. Gene Regulation in Eukaryotes44m
- 14. Genetic Control of Development44m
- 15. Genomes and Genomics1h 50m
- 16. Transposable Elements47m
- 17. Mutation, Repair, and Recombination1h 6m
- 18. Molecular Genetic Tools19m
- 19. Cancer Genetics29m
- 20. Quantitative Genetics1h 26m
- 21. Population Genetics50m
- 22. Evolutionary Genetics29m
21. Population Genetics
Allelic Frequency Changes
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