okay so we're talking about the hardy-weinberg model and the hardy-weinberg model establishes the conditions under which a population remains stable it's a mathematical model describing the relationship between allele frequencies and genotype frequencies and under the conditions of the model uh evolution does not occur so we have a situation in which allele frequencies and genotype frequencies remain constant if the conditions of hardy weinberg do not apply then we have evolution and this figure shows the conditions under which hardy-weinberg conditions are not met so if we have a small population we have genetic drift we have a circumstance where chance events cause changes in allele frequencies we have mutation that produces variation that changes allele frequencies we can have gene flow as organisms move into or out of the populations carrying alleles with them we can have non-random mating if organisms mate preferentially based on a genotype and we can have natural selection to start with you remember random mating is one of the conditions of hardy weinberg now random mating is not quite as much fun as it sounds like random mating simply means that every organism in the population has an equal probability of being selected as a mate by every other organism so if we have non-random mating that means that organisms are selected as mates either directly or indirectly based on their genotypes and the result of this is an excess of homozygous genotypes relative to hardy-weinberg expectations and there are a number of ways that non-random mating can be can occur we can have inbreeding and inbreeding occurs when organisms tend to mate with related individuals and this can cause an excess of homozygotes and and in some cases this excess of homozygous can result in decreased fitness which is known as inbreeding depression and there is an increased probability of alleles being identical by descent and so because alleles are identical by descent you were more likely to be homozygous you're more likely to have the same allele from your mother and from your father because they got them from related individuals now out crossing is the opposite of inbreeding and that's also non-random if you tend to avoid mating with related individuals that's also non-random mating so remember random mating would mean that you don't care whether you're uh you select a mate that's related to you you're just as likely to select a mate that is related as a mate that's not that's not related so out crossing is also a form of uh non-random mating if we select a mate directly based on genotype that's assortative mating now to go back to just briefly to inbreeding and outcrossing inbreeding and outcrossing selects a mate based on their genotype in the sense that related individuals or unrelated individuals are likely or less likely to share a genotype with us but when we talk about assortative mating we're selecting based directly on genotypes so positive assortative mating is the preferential selection of mates with similar genotypes negative assorted mating is the preferential selection of mates with different genotypes and i should point out that when we talk about assortative mating either positive or negative the organism is not necessarily thinking about oh i want to go out and select a mate that has the same phenotype the same genotype that i do whether the organism is consciously aware of it or not or is even making a conscious choice if the mating is preferential that's positive assortative mating so plants can have positive assorted mating they're obviously not making a conscious choice of their mates but if your pollen is more likely to to be cross-pollinated with a plant with a similar phenotype as yours that's positive assortative mating whether you consciously are making that choice or not now if you have type o blood you know do you preferentially select mates with type o blood well you may not consciously be aware of that but is there some factor or is there something else that makes you select a mate based on blood type that you may not be aware of and if so then that's positive assortative mating or if you select a mate that has a different blood type whether you know that you're doing it or not that's a negative assortment of mating now genetic drift and we have a whole chapter coming up on genetic drift so i'm not going to go into great deal detail on in all of this in a small population chance events have a huge effect on allele frequencies and the reason is simply that you have a small sample size if you toss a coin and expect it to come up heads half the time and you toss the coin ten times you're not going to get five heads and five tails all the time you're gonna sometimes get eight heads and uh and two tails and sometimes you'll get ten heads and no tails and dorm 10 tails and no heads and those events are going to happen fairly often when you toss a coin 10 times but if you're tossing the coin a thousand times or 10 000 times you're going to always get much much closer to a 50 50 ratio if that's what you expect with a with a coin toss you may not get 500 heads and 500 tails every time but you might get 510 and 490 one time but you're not going to get 100 and 900 in a large sample size so in a large population chance events are not going to a large have a large effect on allele frequencies also in a small population we can only have a small fraction of the genetic variability so genetic drift results in rare alleles being eliminated so we get less variation in small population so when a population goes through a bottleneck when there's only a few individuals we'll get less variation now in a natural population when we look at genetic variation uh we can consider polymorphism polymorphism morph means form poly means many genetic polymorphism is the variation the genetic variation within a population a gene is considered polymorphic officially polymorphic if the frequency of the common allele is less than 99 so if the frequency of the common allele is greater than 99 or if it's fixed in other words if it's 100 percent or greater than 99 somewhere between 99 and 100 we say that the population is monomorphic at that locus in classical mendelian genetics we typically think of polymorphism at the phenotypic level we look at the phenotype of the organism we look at these these fruit flies these drosophila and when we're looking at the phenotype we're looking at what does the fly look like we look at the the genes and how it affects the phenotype and we say oh yes all of these loci are monomorphic we have these very common alleles and we say this very common allele we call it the quote the wild type and in mendelian genetics we have this thing called a wild type because the idea is that if you go out in the wild and catch wild fruit flies or wild whatever that's though what a wild fruit fly looks like it looks like this this one here on the left and then you may have a mutant fruit fly which is very rare like this one on the on the right with the vestigial wings so you have wild type alleles and rare mutant alleles we're looking at the phenotypic level that's fairly common but when we start looking at more at the genotypic level using molecular techniques we start seeing a lot more variation and now we're going back again to this idea of population thinking thinking about the variation among individuals in a population