We've said that a major assumption of Hardy Weinberg is random mating. So now we want to take a closer look at what happens with non-random mating. Non-random mating, we're just going to define as when certain genotypes are more likely to mate with each other. Now remember for Hardy Weinberg, we said it's like taking all the alleles in the population and putting them in one bag, or you can think here it's like taking all the gametes from the population and putting them in one bag and then just shaking up that bag and seeing what comes out for the next generation.
Well, if certain organisms are more likely to reproduce with each other, it's not like taking all those gametes, putting them in one bag, and shaking it up. What's going to happen is that this alters genotype frequencies. It's going to push that population out of Hardy Weinberg equilibrium. But importantly, we said it's not going to alter allele frequencies, and that's important because a change in allele frequencies is a measure of evolution.
So non-random mating on its own will push a population out of Hardy Weinberg equilibrium, but it will not cause evolution. It's important to note here, and a little bit of misconception sometimes people have, we are not talking about sexual selection when we're talking about non-random mating; we're still assuming that everyone gets to mate. All the same alleles are still getting passed on. They're just not getting paired together in a random way. Sexual selection is about the ability to obtain mates. We're going to take a closer look at that when we talk about natural selection.
For non-random mating though, we're going to say that this happens a lot just because organisms are often more likely to mate with, we're going to say here, relatives or related individuals, and that's often just due to proximity. Organisms are more likely to mate with organisms that live close to them. For example, this map shows the distribution of white oak trees, and we can see here in green the white oak trees are found basically in most of the Eastern United States.
Now, oak trees are wind-pollinated. Wind pollination, that sounds pretty random to me. Just where the wind blows. But we have this question here: Is an oak tree in Maine likely to mate with an oak tree in Texas? Well, it turns out pollen only travels in the wind something like 200 meters. So these trees up in Maine, they are not going to mate with the trees in Texas. The wind just is not going to blow that far. These trees are mating with those organisms in their immediate proximity. Over time, that leads to inbreeding. And inbreeding, we're just going to say, is mating between relatives. When that happens, the way that this population gets pushed out of Hardy Weinberg is that it increases homozygosity. And homozygosity is just a measure of how many homozygotes there are in the population.
Populations with inbreeding have more homozygotes than you would otherwise expect. This leads to what we call inbreeding depression. Inbreeding depression is a fitness decrease, due to an increase of homozygotes with deleterious recessive alleles. Rare recessive alleles, in a population, their phenotypes are normally masked because they're paired with the more common dominant alleles. But if there's inbreeding, if you have excess homozygotes, those rare recessive alleles are more likely to be paired with each other. And if those alleles are deleterious, well, those phenotypes get exposed, and it reduces the fitness of those individuals. You may think fitness difference. That sounds like it's going to lead to evolution.
It will, but it's going to require natural selection to then change the allele frequency. The non-random mating just made them more homozygous. Natural selection can then push the allele frequency and cause the population to change afterward. Well, practice has more coming up. I'll see you there.
