Okay. So now we're going to talk about non-random mating, which is the last part of this seminar, labeled as 'M'. In Hardy-Weinberg, there has to be random mating, but in real life, that doesn't actually happen, and non-random mating due to phenotypes, which are caused by different alleles, occurs in every organism on Earth, including humans. It's called assortative mating when individuals choose mates based on phenotypes. There are two types, positive and negative. It's positive mating when it's chosen based on similar phenotypes, so how similar the two mates are phenotypically, and it's negative when it's based on if females and males have any kind of assortative mating based on smell. What they did is they got males to wear white T-shirts, and they lived in these shirts essentially for a few days. They exercised, they got them real sweaty, real smelly, it was nasty. Then they put them in plastic bags and gave them to women to rate, based on attractiveness. It turns out that women prefer the odor of males who have a certain genotype, and that genotype is very different from them. The alleles that they were looking at were these alleles called MHC. You may or may not know what they are. It doesn't really matter what they are. Just know that they're involved in the immune system. Females always chose men with different MHC molecules, and I could tell this simply by the odor that those MHC molecules were different in the men. There's obviously something in the odor of probably both females and males that allow humans to differentiate different potential mates. This is an example of non-random mating. There are examples of this all throughout the animal kingdom: different mating dances, feathers or colors, locations, geographical locations, all of this is an example of nonrandom mating.
So, there is another type of non-random mating, which is isolation by distance. Two populations that live in different areas that are either separated by continents or countries or even something as simple as a mountain or a stream, aren't going to mate with each other because they can't get there. They're not like humans with planes that can fly all over the world. Generally, organisms are restricted to a very small geographical location. When there are two populations or more populations that are separated from each other for an extended period of time due to selection or genetic drift or any of these things that we've talked about previously, genetic variations begin to develop between these new populations. When those variations get to a certain point, we call this speciation, which is the creation of a new species. Speciation occurs through reproductive isolation, so isolating two populations that now can't reproduce with each other. There are two types of reproductive isolation, there can be prezygotic, so this is before the formation of the zygote, something that prohibits them from mating, and that reduces breeding. Try to think of some things that could be prezygotic, preventing the organisms from mating. Right? It could be isolation by distance, which is what we're talking about. It could also be that the organisms don't have the genitalia to be able to mate, or that they would never think of each other as mating populations to begin with. For instance, hummingbirds and bears are never going to look at each other and say, that's probably a good mate. And so, those are all prezygotic mechanisms. Then there are postzygotic mechanisms, and these are things that after the zygote is formed, prevent further reproduction. Usually, what happens is there's some kind of offspring created from these mates, but they're either inviable, meaning that they die, or they are born but they're infertile because they're sterile and they can't reproduce. So if you can't make more offspring, then it's considered reproductive isolation. An example of this is a mule, which is a mating between a horse and a donkey. Mules are an example of postzygotic isolation because they are sterile, and they cannot reproduce, and so you can't get more mules by mating mules, and therefore, it's an example of reproductive isolation.
Another form of random mating or non-random mating is inbreeding, which I know is kind of a taboo subject among humans, but essentially it's the mating between relatives. Inbred individuals are much more likely to be homozygous for harmful recessive alleles. A lot of genetic diseases are recessive genetic diseases, and inbred individuals are much more likely to be homozygous for those and are much more likely to have recessive genetic diseases. It's called inbreeding depression when inbreeding leads to a reduction in vigor or reproductive success, which would happen in the case of a recessive disease. If someone has a recessive disease, they're much less likely to reproduce, and be healthy enough to reproduce. We usually think of this as super taboo in humans, but actually, inbreeding in plants, especially through self-fertilization, can actually be a positive process. But generally, inbreeding is non-random mating, right? Because you're mating with your family. You can measure inbreeding through \( f \), and that's the probability that two alleles in an individual trace back to the same ancestor. And, of course, inbreeding is much more common amongst small populations because there's not that diversity of choice of mates. It's very limited, and so it's much more common there. So the inbreeding coefficients for something like a father-daughter is 25%. So there's a 25% chance that if you choose two alleles from this offspring, it will go back to the same ancestor, and that's very high, especially when you look at recessive genetic diseases. That's a really high inbreeding coefficient. You can look through the rest of these and see that, you know, as you get further away, with second cousins for instance, it's 1.56%. All of these are examples of inbreeding, an example that happens especially in the animal kingdom more often. This is how all these assumptions that Hardy-Weinberg makes in its formulas aren't necessarily that relatable to real life situations. So with that, let's now move on.
