Evolutionary Genetics Lecture (3) PDF

Evolutionary Genetics Lecture (3) Transcribed on February 5, 2025 at 9:30 AM by Minutes AI Speaker 1 (00 03) Hey, everyone. So in our next seven weeks of class, we're going to be learning a lot about evolution. We're going to start with evolution at the population level. So a population is a gr...

Evolutionary Genetics Lecture (3) Transcribed on February 5, 2025 at 9:30 AM by Minutes AI Speaker 1 (00 03) Hey, everyone. So in our next seven weeks of class, we're going to be learning a lot about evolution. We're going to start with evolution at the population level. So a population is a group of. Speaker 2 (00 15) Organisms that are all of the same. Speaker 1 (00 17) Type that might be interbreeding with each other. So a bunch of organisms of the same species within populations, we see variation. That variation is going to change over time, which is evolution. This variation is closely linked to the formation of new species. And the success might be interbreeding. Learning a lot about evolution. We're going to start with evolution at the population level. So population is a group of organization. Speaker 3 (00 55) It. Speaker 1 (01 40) That are all of the same type that might be breeding with each other. So a bunch of organisms of the same within populations, we see variation, that variation is going to change over time, which is evolution. These notes were taken with Minutes AI (https://myminutes.ai) : : : : : This variation is closely linked to the formation of new species and the success of these different species and different kinds of habitats. So we can see this variation here in this group of birds, the Galapagos finches. Speaker 2 (02 33) This is a famous group of finch. Speaker 1 (02 34) Species that was studied by Darwin on his visit to the Galapagos Islands and was critical in helping him develop his theory of natural selection. So we're going to learn about how we describe this vision variation within populations, as well as what causes populations to change. We'll see some evidence for that from the finches as well as a number of other species. So in this video, we're going to start looking at the genetics of populations and see how we describe variation within. Speaker 2 (03 06) Populations and how we can use Punnett. Speaker 1 (03 09) Squares as well as some other methods. Speaker 2 (03 11) To track the inheritance of traits across. Speaker 1 (03 15) Generations of a population. So to look at inheritance, our simple understanding of trait inheritance comes from Gregor Mendel and his work with pea plants. And the basic idea here is that within a gene you have multiple different alleles, and those different alleles are responsible for different phenotypes. These notes were taken with Minutes AI (https://myminutes.ai) : : : : : : So on these chromosomes, there's a gene that codes for color. And you could have an allele that leads to purple phenotype or an allele that leads to a white phenotype. An offspring are going to inherit one allele from their mother and one allele from their father. This is because they inherit one chromosome from their mother and one chromosome from. Speaker 2 (05 46) Their father as a result of the. Speaker 1 (05 48) Separation of homologous pairs of chromosomes during meiosis. So meiosis determines the alleles that go into the gametes and therefore the alleles that the offspring inherits. And then this offspring is going to be diploid. So meiosis determines the alleles that go. Speaker 3 (07 35) It. Speaker 1 (08 25) Into the gametes, and therefore the alleles that the offspring inherits. And then this offspring is going to be diploid, right, with one homologue from one parent and one from the other parent. And the dominant allele will be the one that determines the phenotype of the offspring. In this case, the offspring diploid, right, with one homologue from one parent determines the alleles that go into the gametes and therefore the alleles that the offspring inherits. And then this offspring is going to be diploid, right, with one homologue from one parent and one from the other parent. And the dominant allele will be the one that determines the phenotype of the offspring. These notes were taken with Minutes AI (https://myminutes.ai) : : : : In this case, the offspring has a heterozygous genotype, and so the dominant allele is the purple allele, since that's the one that shows up in the phenotype. So you should be pretty comfortable with. Speaker 2 (09 11) This idea of inheritance of traits over. Speaker 1 (09 14) Generations and how meiosis works with that and how the genotype contributes to the phenotype. Right? Pretty comfortable so far. Now, these kinds of traits that Mendel. Speaker 2 (09 25) Looked at are discrete traits. Speaker 1 (09 27) Discrete traits are those where the phenotype has either or distinct alternatives. So you can have purple flowers or white flowers, you can have yellow seeds or green seeds, or you could have more than two options. Like you could be blue or green or orange. But there's always alternative, distinct options. There's nothing in between. These kinds of traits are very often qualitative. So things like color and shape are often going to be discrete traits. And usually they are controlled by war one single gene. So there is one gene with two alleles that produce two different phenotypes or three alleles that produce three different phenotypes. These notes were taken with Minutes AI (https://myminutes.ai) : : : : Speaker 2 (10 10) And these traits. Speaker 3 (10 26) It. Speaker 1 (11 02) A pretty simple inheritance where that one gene gets passed phenotypes or three alleles that produce three different phenotypes and controlled by one single gene. So there is one gene with two alleles that produce two different phenotypes or three alleles that produce three different phenotypes. And these traits have pretty simple inheritance where that one gene gets passed down and then determines the phenotype. A little bit more complicated are quantitative traits. Quantitative traits are those where the phenotype can vary anywhere along a continuum. So there's not only one option or another, there's a whole range of possible options. So continuous variation. So for example, in these amphipods, you're not only 300 micrograms or 900 micrograms, they can be anywhere from 300 to 900, anywhere in between. Speaker 2 (13 28) You're not only short or tall, you. Speaker 1 (13 30) Can have any range of possible heights. So these are usually going to be quantitative, where we're talking about a numerical trait. And you do see at the population level, often a bell curve for quantitative traits. So there's an average that's the most common within the population, and then it kind of ranges away from the average. Now, these kinds of traits are usually going to be polygenic, meaning they are controlled not by one gene, but by many genes. These notes were taken with Minutes AI (https://myminutes.ai) : : : : : And so it's not not only one gene that determines the phenotype you're going to get. It's the combination of a number of different genes. So, for example, color in humans is a quantitative trait, because you're not only perfectly white or perfectly black, you can be anywhere in between this huge range of color variations. And that is absolutely a polygenic trait. It's actually controlled by over 100 different gene products in humans. So quantitative traits, it's a little bit harder to track their inheritance, but you should know that a lot of traits are quantitative. Now, if we want to predict how a particular trait is going to get inherited from parent to offspring, we can do that pretty simply using punnett squares. Punnett squares represent meiosis and fertilization to allow you to predict the possible genotypes and therefore phenotypes of offspring. And so for one set of parents, if you were going to make a punnett square, you would first consider what are the gametes that those parents are capable of producing. So here we have two pea plants. One of them is homozygous for smooth seeds, which is dominant. One of them is homozygous for wrinkled seeds, which is recessive. When these individuals it produce, they produce gametes. And each of those gametes will have one of their homologous chromosomes. Speaker 3 (16 40) It. Speaker 1 (17 31) Because remember, we separate out homologous one of if you were going to make a punnett square, you would first consider what are the gametes that those parents are capable of producing. So here we have two pea plants. One of them is homozygous for smooth seeds, which is dominant. These notes were taken with Minutes AI (https://myminutes.ai) : : One of them is homozygous for wrinkled seeds, which is recessive. When these individuals reproduce, they produce gametes. And each of those gametes will have one of their homologous chromosomes. Because, remember, we separate out homologous chromosomes in meiosis. We're going to take those gametes produced by meiosis and put them on chromosomes. Because, remember, we separate out homologous chromosomes in meiosis, we're going to take those gametes produced by meiosis and put them on the sides of the square as the the bulgametes used for the offspring. So the homozygous dominant parent can produce one dominant allele from one homolog or a different dominant allele from the other homolog. The homozygous recessive parent can produce a gamete with a recessive allele from one homolog or a gamete with a recessive allele from the other homolog. And then in the squares of the Punnett square, we're going to fertilize the different possible gametes to produce different possible offspring. So you take one sperm from one parent and one egg from the other parent, put them together, and this is the genotype you're going to see and therefore the phenotype you're going to see for this particular offspring. Speaker 2 (20 58) So the Punnett square just represents the. Speaker 1 (21 01) Formation of gametes and then the fertilization of those gametes to produce offspring. Now, if we wanted to look at probabilities of getting particular offspring, we have to consider the probability of getting particular alleles. Speaker 2 (21 15) And on the sides of the square. Speaker 1 (21 17) Where we're putting those gametes, there would be a 50, 50 chance of getting one allele or the other allele because the parent just has two homologous chromosomes and you're gonna get one or the other. These notes were taken with Minutes AI (https://myminutes.ai) : : : : So there's 5050 chance of getting the first one or the second one. So 50 the sperm, 5050 chance for the egg for the offspring. Then if you want to know the chance of one particular offspring, it would be the 5050 chance for the sperm times the 5050 chance for the egg. 0.5 times 0.5 gives you 0.25. So there would be a point. Speaker 3 (21 59) It. Speaker 1 (22 25) Probability of getting each of these one times the 5050 egg for the offspring. Then if you want to know the chance of one particular offspring, it would be the 5050 chance for the sperm times the 5050 chance for the egg. 0.5 times 0.5 gives you 0.25. So there would be a 0.25 probability of getting each of these one individual offspring. You should be fairly comfortable with this kind of Punnett square. So hopefully this is just that you fully understand it. Do a little practice problem. Mate together two heterozygotes, fill in the gametes they're going to produce and the offspring that they can produce and then figure out what is the probability that their offspring will actually express the dominant allele in their phenotype. Okay, See if You can do some practice. Now that we understand how a Punnett Square predicts variation for one set of parents, the next step is to try to predict variation for an entire population, not just one set of parents, but the whole gene pool. And we will pick up with that in the next video. Speaker 2 (23 36) This is going to scale that up. These notes were taken with Minutes AI (https://myminutes.ai) : : : Speaker 1 (23 37) To predict inheritance of traits within entire populations. And so we're looking at the genetics of the whole population, not only of one set of parents. Speaker 2 (23 48) Now, in order to do this, you. Speaker 1 (23 50) Do need to be comfortable with some vocabulary. So make sure you can clearly define for yourself the gene pool of a population, allele frequency, genotype frequency, and phenotype frequency within that gene. So if you're not really comfortable with these ideas, pause the video for a second and look it up for yourself, because we are talking about proportions of things within the gene pool. So you got to be really clear on the difference between allele, genotype, and phenotype. Right? They each mean something different, so keep it straight. Okay, now let's see how we can predict inheritance by walking through an example with a hypothetical population. This is a population of flowers, okay? Thousand plants in a meadow. And in these plants, they're going to show incomplete dominance. For flower color. Incomplete dominance is when the dominant allele does not completely hide the recessive allele. So the heterozygote is going to have an intermediate phenotype as opposed to just the dominant phenotype. And this plants, they're going to walk you through an example with a hypothetical population. These notes were taken with Minutes AI (https://myminutes.ai) : : : This is a population of flowers, okay? Thousand plants in a meadow. And in these plants, they're going to show incomplete dominance. For flower color. Incomplete dominance is when the dominant allele does not completely hide the recessive allele. Speaker 3 (27 03) It it's. Speaker 1 (29 07) The heterozygote is going to have an intermediate phenotype. Speaker 2 (30 24) As opposed to. Speaker 1 (30 25) Just the dominant phenotype. And this means that you can actually induce genotype from the phenotype completely accurately. So in this species, we have two alleles, a red allele and a white allele. Traditionally, for incomplete dominance, we use superscripts instead of just uppercase and lowercase. So that's just because we're doing incomplete dominance. If it was complete dominance, you could just use uppercase and lowercase as usual. Now, if you are homozygous for the red allele, that will produce a red flower. If you are homozygous for the white allele, that will produce a white flower. If you are homozygous for the red allele, that will produce A red flower. If you are homozygous for the white allele, that will produce a white flower. If you are heterozygous, then you'll get an intermediate phenotype, which would be pink in this case. These notes were taken with Minutes AI (https://myminutes.ai) : : : : And so you go to this meadow and you find a thousand plants, and you count up how many of each color you see, you find 625 red flowers, 310 pink flowers, and 65 white flowers. And you want to know what does the gene pool actually look like? What are the allele frequencies within this population? Well, in order to calculate these allele frequencies, we can use a chart. And I want to preface this by saying this chart only works if you know how many. Speaker 3 (34 02) It'S. Speaker 1 (35 27) Individuals you have. Of all three genotypes in this species, we know how many individuals are homozygous red, how many individuals are homozygous white, and how many individuals are heterozygous. If you know all three, you can use this chart. Okay. Speaker 2 (35 44) And so the chart basically is just. Speaker 1 (35 45) Going to count up the number of alleles that are present in each of these different individuals within the gene pool. So starting with a red allele, if we want to count up the number of red alleles in the red flowers, well, they are homozygous red, they have two red alleles, there are 625 of them. So you multiply that by two, and that's a total of 1200. Well, they are homozygous red. So starting with a red allele, if we want to count up the number of red alleles in the red flowers, well, they are homozygous red, they have two red alleles, there are 625 of them. These notes were taken with Minutes AI (https://myminutes.ai) : : : : So you multiply that by two, and that's a total of 1250 red alleles, since each of those red flowers carries two red alleles. Now, the pink flowers, because they are heterozygous, they each carry one red allele. So 310 pink flowers is 310 red red alleles. Now, the white flowers don't carry any red alleles. So the total number 300. So starting with the red allele, if we want to count up the number of red alleles in the red flowers, well, they are homozygous red, they have two red alleles, there are 625 of them. So you multiply by that by two, and that's a total of 12 red alleles, since each of those red flowers carries two red alleles. Now, the pink flowers, because they are heterozygous, they each carry one red allele. So 310 pink flowers is 310 red alleles. Now, the white flowers don't carry any red alleles. So the total number of red alleles in the gene pool would be 1250 plus 310. The frequency would be that number of alleles out of the total number of alleles for this population, we have a thousand plants. They are diploid, so each individual carries two alleles. So the total number of alleles in the gene pool is 2002 for every individual since they're diploid. So you add up the red alleles, divide by 2000 the total number of alleles, and that gives you a frequency of 0.78. In other words, 78% number of alleles out of the total number of alleles. For this population, we have a thousand plants. They are diploid, so each individual carries two alleles. So the total number of alleles in the gene pool is 2002 for every individual since they're diploid. So you add up the red alleles, divide by 2000 the total number of alleles, and that gives you a frequency of 0.78. In other words, 78% of the alleles in the gene pool are the red allele. These notes were taken with Minutes AI (https://myminutes.ai) And we could do the same thing for the white allele. Speaker 3 (39 00) It. Speaker 1 (39 42) The red flowers don't have any white alleles. The Pink flowers have one white allele each, so that's 310. The white flowers have two alleles each, so 65 times two gives you 130 white alleles. So you add up all the white alleles, again divide by the total number of alleles, which is 2000. And that tells you that the frequency of the white alleles is 0.22. So 22% of the alleles in the gene pool are white. So now we know the relative frequencies of each of these two alleles in the gene pool. Now, notice that these two numbers, 0.78 and 0.22, add up to 1. Why do they have to add up to 1? Think about it and see if you can puzzle out why these two frequencies must add up to one. Okay, now, if these are the frequencies that are present in the population, can we then predict what the next generation of this population will look like? Yes, we can do it with a punnett square. But this time, instead of using individual alleles for our punnett square, we're going to use frequencies of alleles for the punnett square, because we're not doing only one set of parents. We're doing the whole gene pool. So we're going to plug the allele frequencies that we just calculated into a Punnett square. So There is a 0.78 frequency of the red allele, a 0.22 frequency of the white allele. And remember before, when we were looking at probabilities for one set of parents, we had like 0.5 times 0.5. And you multiply them together to get. These notes were taken with Minutes AI (https://myminutes.ai) : : We're going to do the same thing here. There is a 0.78 chance of getting one red allele. There is a 0.78 chance of Getting another red allele. So the chance of getting two red alleles would be 0.78 times 0.78 or 0.61. The chance of getting one red allele is 0.78. The chance of getting one white allele is 0.22. So the chance of getting a red and a white would be 0.17. Or you could get a white and a red, which would also be 0.17. The chance of getting two white alleles would be 0.22 times 0.22 or 0.2. And so these are the relative genotype frequencies that we expect to see in the next generation. 0.61 homozygous red, 0.34 heterozygous, and 0.05 homozygous white. Now, notice that these genotype frequencies also add up to 1. If you add up those three numbers, it's 1. Why? Why do those three numbers have to add up to one? Think about it. Now, if these are our genotype ratios for the next generation, then when you go back to the next generation, you would expect to see 610 red flowers. Since we have 0.61 homozygous red, 340 pink flowers and 50 white flowers, assuming the population stays the same size from generation to generation. So we're just multiplying the frequencies by the total size of the population to tell us how many individuals we have. So if this is what you find in the second generation, 610Red, 340Pink, and 50White. Calculate the allele frequencies. So plug these numbers into the table again and see what do you get for the allele frequencies in the second generation. Okay, so fill in the chart, do some math, see what happens. These notes were taken with Minutes AI (https://myminutes.ai) Now, this population may experience some changes over time. So what if a cow comes along and happens to eat all of the white flowers in this Meadow? Well, the 610 red flowers will survive. The 340 pink flowers will survive. Speaker 4 (51 29) But all 50 of those white flowers. Speaker 1 (51 31) Are going to die. They're going to disappear. Their alleles are going to get removed from the gene pool. So what happens now? If you calculate the allele frequencies, fill in the chart again with these new proportions. Okay. And be careful when you're calculating the aloe frequency, because notice that the overall number of low in the gene pool has changed. 50 individuals have died. How many alleles have disappeared? Okay, figure it out. Fill in the chart, see what you get. Now, what you will see is that the allele frequencies in the population will change. And when the allele frequencies change, it is the result of the fact that all those white alleles in the white flowers got removed from the gene pool. That kind of allele frequency change over generations is evolution. We define evolution as allele frequency change within a population over generations. So if allele frequencies are changing. Speaker 3 (56 19) These notes were taken with Minutes AI (https://myminutes.ai) : : : It. Speaker 1 (57 05) Then evolution is occurring. Okay, that's it for this video. Speaker 4 (57 11) How they actually experience evolutionary change. In particular, we're going to highlight two of the mechanisms that can cause populations to evolve. And those two for this video are mutation and non random B. Now, before we get into what those actually are and how they work, I just wanted to briefly introduce you to a tiny bit of the history of how scientific have thought about evolution within populations. Now, the first person to actually propose a way in which populations could evolve was a guy named Lamarck in 1795. And Lamarck came up with an idea called inheritance of acquired characteristics. The way that this works, that can cause population and those two for this video are mutation and non random P. Now, before we get into what those actually are and how they work, I just wanted to briefly introduce you to a tiny bit of the history of how Scientology have thought about evolution within populations. Now, the first person to actually propose a way in which populations could evolve was a guy named Lamarck in 1795. And Lamarck came up with an idea called inheritance of acquired characteristics. The way that this works, according to Lamarck, is that if an organism uses trait a lot during its lifetime, then that could actually change the trait and that could get passed down to the next generation. So if you have a giraffe who's eating some leaves off the tree and then he eats all the low leaves, and then he has to stretch a little bit to try to get to higher leaves, that's going to cause his neck to stretch, and then he eats those ones and he has to stretch a little more, stretch a little more, stretch a little more, and over time his neck gets longer and Longer within that giraffe's interest, individual lifespan. These notes were taken with Minutes AI (https://myminutes.ai) : : And then when that giraffe reproduces, he goes on to have offspring who also have that longer neck. So it is changes acquired during the organism's lifetime that would get passed down. Now this is wrong. Well, Mark was incorrect. You actually don't pass down things that you do during your lifetime. So if you think about if you lifted a bunch of weights, you wouldn't necessarily have a really muscular baby, right? Speaker 1 (01 00 37) If you dyed your hair purple, you're. Speaker 4 (01 00 39) Not necessarily going to have a purple haired baby. So Lamarck was wrong, but we still talk about him a lot because he was the first person to really come up with a proposal for how evolutionary change could occur. And also because it's really easy to fall into the trap of this kind of thinking. So I want you to watch out when you are thinking about these ideas and doing, writing about these ideas, all kinds of those kinds of things that you are not falling into the trap of speaking like Lamarck. Speaker 1 (01 01 11) Okay? Speaker 4 (01 01 11) And sometimes you'll see on your paper, sometimes I'll write Lamarck. That means you are implying individual change within the organism's lifetime. And that's not how evolution works. Speaker 1 (01 01 21) These notes were taken with Minutes AI (https://myminutes.ai) : : : : : : : : : : Okay? Speaker 4 (01 01 22) So this idea was then supplanted by descent with modification, which was proposed simultaneously by Darwin and another by one Wallace in the 1850s. And according to this theory of evolution, it is going to begin with pre existing variation among organisms in a population. So some giraffes already have shorter necks and other giraffes already have longer necks. As a result of the genes that they carry. Some of those individuals may reproduce. And when they reproduce, they're going to pass on whatever kind of neck they already had. So if they had a short neck, they have short neck babies, they have a long neck, they have long neck babies. But over time, some individuals may reproduce more than others. And if that is true, then certain phenotypes will appear more often than others in subsequent generations. So there will. The real difference here is that it is based on previously existing variation. And then some of that variation gets passed down and some of it doesn't. And that's the real foundation for evolutionary change. So our goal for today is to look at a couple of ways in which this could potentially occur. Now remember that we define evolution as change in allele frequencies within a population over time. And so we want to look at the allele frequencies in the population. So within the gene pool, you have some proportion of one allele, some proportion of another allele, and you want to see do those proportions change over generations? How does one allele become more common and another allele become less common within the gene pool of the population over generations? Now, our first mechanism of evolution is going to be mutation. These notes were taken with Minutes AI (https://myminutes.ai) : : Mutation is defined very simply as any kind of change in the sequence of the DNA. So the nucleotides, the as, Ts, Cs and GS, can get changed into different nucleotides. Normally, this occurs as a result of mistakes during DNA replication. So every time a cell becomes, it has to do DNA replication, and sometimes you don't put in quite the right nucleotide and you can get a mutated DNA sequence. That is the main way in which mutations occur. Just errors during DNA replication. There are some factors that can increase the likelihood of those errors occurring, such as very strong radiation, like X rays or UVs, or chemicals that can cause mechanical damage to the DNA, which then makes it harder to copy and more likely you will get errors. But it's really during DNA replication that the errors actually occur. Now, mutation is important for evolution because it is the ultimate origin of all genetic variation. It is the way in which new alleles are created. Because if you change the gene sequence, you might change the trait that the gene codes for. So all alleles ultimately can be traced back to mutation. Now, when we get those new alleles through mutation, they could be helpful, they could be harmful, or they could be neutral and not actually have any phenotypic effect at all. It's actually much more common for mutations to be neutral or harmful than it is for them to be beneficial. But sometimes you can get a beneficial mutation, and then that would be very, very good. Now, mutations are quite infrequent. DNA replication is a pretty accurate process. So, for example, in humans, you only get about one mutation per every hundred thousand cell divisions. So it's not something that's happening all that often. And that means it's going to be a fairly low, weak mechanism of evolutionary change. But it is inevitable. These notes were taken with Minutes AI (https://myminutes.ai) At some point, mutations are going to occur. It's just not a perfect process. So inevitably, within a population, as cells are being replicated and organisms are reproducing, there's going to be some mutations along the way. Those create new alleles, thereby changing the allele frequencies in the gene pool, in the population. So that's really the important thing about mutationary change divisions. So it's not something that's happening all that often. And that means it's going to be a fairly weak mechanism of evolutionary change. But it is inevitable at some point Mutations are going to occur. It's just not a perfect process. So inevitably within a population, as cells are being replicated and organisms are reproducing, there's going to be some mutations along the way. Those create new alleles, thereby changing the allele frequencies in the gene pool in the population. So that's really the important thing about mutations creating new alleles. Speaker 1 (01 06 42) Alright. Speaker 4 (01 06 43) Our second mechanism of evolution is non random mating. And this is pretty much what it sounds like. It occurs whenever individuals within the population mate non randomly with respect to their genotype. So individuals with certain genotypes might be more or less likely to mate with each other. One form of non random mating would be assortative mating. Assortative mating is when two genetically similar individuals are more likely to mate with each other. Like with like, in other words, inbreeding. These notes were taken with Minutes AI (https://myminutes.ai) : : : : And this is going to happen anytime that genetically similar individuals are especially likely to mate. Speaker 3 (01 07 34) It. Speaker 4 (01 08 15) The pinnacle of this would be something like self fertilization, which is quite common, for example, in plants where on a single flower you can have both male and female parts and pollen from the male part can land and fertilize the female part of that same individual. Can't get any more genetically similar than that. Speaker 1 (01 08 33) Right. Speaker 4 (01 08 34) Still sexual reproduction, but highly assortative. The opposite of assortative mating would be disassortative. And this is when genetically different individuals are more likely to mate with each other. So you can think of this as like, opposites attract. Okay, so like with like is attached and opposites attract is disassortative. Now for both of these forms of non random mating, assuming everyone in the population actually reproduces, you're not really going to see allele frequencies changing. However, something else about the gene pool could change. So if it's not the allele frequency changing, what else might change? What might become more common in the population if you see assortative mating, what might become more common if you see disassortive mating. I'll let you think about that one. So for non random mating, I did want to show you kind of an example of what this might look like. These notes were taken with Minutes AI (https://myminutes.ai) : : : : : : : : And here's a graph comparing educational levels of wife and husband in some human populations. On the x axis here on the graph, you can see the wife's education level. And these represent some high school, a high school diploma, some college, college diploma, or beyond a college degree like a master's or a PhD. And then the different color bars represent the husband's level of education. So for each level of the wife's education, you can take A look at the husband's level of education. Try to puzzle out this graph and think about what does this tell us? Is this graph suggesting that non random mating occurring in humans? What does it look like to you? See if you can figure it out. All right, so that is it for mutation and non random mating. We will continue with genetic drift and gene flow in the next video. The mechanisms of evolution within populations. In the last video we covered mutation and non random mating. And in this video we'll talk about genetic drift and gene flow. So remember, we're trying to understand how these mechanisms can cause allele frequency change within populations over generations. So next up is genetic drift. And genetic drift refers to any kind of change in allele frequency as a result of random chance. So you really want to start to associate in your mind drift with random. Okay, drift is due to random chance. So any kind of maybe question you might see that says something like randomly or by accident or by chance, that's talking about genetic drift. So how does this actually happen? The key behind genetic drift is that in pretty much every population, not every single allele is going to get passed down. These notes were taken with Minutes AI (https://myminutes.ai) If you think about the gene pool, as everybody's gaining in the entire population, not every single one of those gametes is actually going to get used. You're not going to use every single one of your eggs. You're not going to use every single. Speaker 1 (01 11 41) One of your sperm. Speaker 4 (01 11 42) And that means that we are taking a sampling of the genes that are actually available in the gene pool. You're not using all the gametes, you're only using a sample of the gamete. And that sampling can result in sampling error. Anytime you take a sample out of a larger pool, there's the possibility for that sample not to be representative of the whole. And that leads to genetic drift. So let's imagine our gene pool as a bag of marbles. And in this bag of marbles, there are two different alleles, the tan allele and the green allele. And initially, within the population, there's a 50, 50 ratio of those two alleles. So 50% frequency of the tan allele, 50% frequency of the green allele. Now remember, we're not going to use. Speaker 2 (01 12 35) Every allele in the gene pool. Speaker 4 (01 12 37) You're going to take a sampling of eggs and sperm. And by chance, when you sample from this gene pool, you're going to get six tan alleles and four green alleles. These notes were taken with Minutes AI (https://myminutes.ai) : : : : : : : : Those alleles will then go on to start the next generation of this population. And so in the second generation, you will have 60% frequency of the tame allele and only 40% frequency of the green allele. And then when this population reproduces again, we're going to take a sample out of the gene pool. And this time maybe you get seven of the tan alleles and only three. Speaker 2 (01 13 13) Of the green allele. Speaker 4 (01 13 14) And then in the next generation, the allele frequencies will change again. Now, the reason this occurs is because you aren't exactly getting the right frequency of alleles. When you take the sample, there is sampling error. Think of it as flipping a coin. When you flip a coin, you're not necessarily going to get exactly 50, 50 heads and tail because of sampling error. And that's the same thing that's going on now. This can happen in any population at any point. So to give you kind of a real world example, let's say you have a little population of beetles. And in this population of beetles, there's a tan allele and a green allele at some proportion. And then a guy comes along and accidentally, by random chance steps on some green beetle. He wasn't intentionally stepping on green beetles. He didn't select out the green beetles. He just accidentally, randomly happened to step on some green beetles. As a result, those beetles will not pass on their alleles to the next generation, and the tan beetles will pass on their alleles. These notes were taken with Minutes AI (https://myminutes.ai) : : : : So the tan allele will increase in frequency. This was a totally random, accidental change that resulted in allele frequency change over generations. Now, genetic drift has a much, much stronger effect in small populations as compared to large populations. And to see why this is so, you can think back again to flipping a coin. If you only flip a coin 10 times, it's pretty unlikely you're going to get extra exactly five heads and exactly five tails. But if you flip the coin a thousand times, you're probably going to get a lot closer to that 5050 ratio. So the sampling error is greater for smaller samples than for bigger samples. So as an example, we can look at some graphs showing the simulation of genetic drift on some populations. And this graph is showing you 10 different populations which are represented by these 10 different lines over a number of generations. Each of these populations starts out with a 50 50 ratio of the dominant allele to the recessive allele. You see, they all start out at 0.5 on the Y axis. But over generations, some populations will see the dominant allele becoming more common, other populations will see the recessive allele becoming more common. And you can see that there's a fair bit of like, random noise in there, right? Random fluctuations in allele frequencies due to chance. Now, in this population, These are only 25 individuals in the population. And so we see very large fluctuations. As a result, genetic drift has a big impact on small populations. If we run the same simulation with populations of 500 individuals, you can see that there still is some random fluctuation in allele frequencies, right? Some of them go a little up, some of them go a little down. But over time, those frequency changes are not as dramatic. They just kind of fluctuate a little bit up and down, but they stay pretty close to where they started. These notes were taken with Minutes AI (https://myminutes.ai) : So in a small population, genetic drift results in large changes. But in a big population, genetic drift still happens, but it's just not very important, not as powerful. Now, notice that this can happen completely independently of the relative fitness of those alleles. So regardless of whether the allele is beneficial or not, it can still fluctuate in frequency as a result of genetic drift. The other thing I want you to notice is that it is possible, as a result of genetic drift, for alleles to completely disappear from the population. In other words, for the population to lose genetic variation. And you can see this in particular in the smaller populations, where in half of the populations, they went to only the dominant allele in the gene pool, and in four of the populations, they went to only the recessive allele in the gene pool. Which way they went was totally random. So you can lose alleles from the gene pool at random, and that's going to be important because it means that even harmful alleles could end up going to 100% in the population. So in small populations, genetic drift lowers variation and can result in harmful alleles becoming really, really common. So genetic drift can have a big negative effect on small populations. I want to show you a couple of examples of genetic drift, but first, I have a kind of a thought question for you. We know that genetic drift tends to produce variation within one population, right? Within that population, we might lose alleles. But what about across two populations that have two different populations? Are they going to experience genetic drift in the same way and therefore become more similar to each other? Or is it more likely that they would experience genetic drift in different ways and therefore evolve differently from each other over time? So is genetic drift going to make these two populations of butterflies more similar to each other over time, or more different from each other over time? These notes were taken with Minutes AI (https://myminutes.ai) I'll let you think about that and we'll talk about it more together in class. So remember that genetic drift is especially powerful in small populations, and because of that, we see Two special kinds of genetic drift where we're taking a very small sample. And so genetic drift has a very strong effect. The first of these special kinds is called the founder effect. The founder effect refers to a scenario where you have a reasonably large parent population with some frequencies of alleles in that gene pool. And you're going to take a small sample out of that population and go and send that sample to a new location to foul a brand new population. So you are founding a new group of organisms. Now, when this happens by chance, the founding population may have slightly different allele frequencies from the parent population. And so the new population that they start may also experience different allele frequencies. Now, the other special kind of genetic drift is called a population battle. The difference between a founder effect and a bottleneck is really just what happens to the parent population. In the founder effect, the parent population is still there and we're just sending the sample elsewhere. In a bottleneck, the parent population is going to die. So there's some kind of large event that occurs that is going to kill off the vast majority of the population. That's the bottleneck. Who dies and who makes it through the bottleneck is completely random. It's not selection, it's random. So we're going to get some random number of individuals who survived the bottleneck. Those survivors will then go on to reestablish the population. And depending on the frequency of alleles in the survivors, there's a small sample out of the parent population. So the reestablished population may experience allele frequency change. These notes were taken with Minutes AI (https://myminutes.ai) You can see in this example, the blue allele became more common because by chance, most of the survivors had the blue allele, not because the blue allele helped them survive just by random chance. So these are two special cases where drift can be especially powerful. Speaker 2 (01 21 18) Let's take a look at some real. Speaker 4 (01 21 19) World examples of these. We'll start with the sounder effect. For a sounder effect, we you can take a look at a tiny little island in the Atlantic Ocean called Tristan da Cunha. This island was not colonized by humans until 1814, when 15 British settlers arrived from the UK. Now, by chance, one of these 15 British settlers happened to carry a recessive allele for a genetically determined form of blindness. Speaker 1 (01 21 49) He wasn't blind. Speaker 4 (01 21 50) He didn't know he had it. He was just a carrier of the recessive allele. But that was one allele out of 15 people. So out of 30 alleles. And today, if you look at Tristan da Puglia, the frequency of this recessive form of blindness is about 10 times higher than it is in the parent population back in Britain. So the founding population had a different allele frequency because it was just a sample from the parent population. For an example of population bottlenecks, we can look right here in California. These notes were taken with Minutes AI (https://myminutes.ai) : : : : : : : : And all up and down the coast of California and further north are elephant seals. Elephant seals are found pretty widespread today, but they actually were hunted extensively in the 1800s, mostly for oil. They were hunted to such a degree that we actually thought they went extinct for a while. But then we rediscovered a population of just 30 elephant seals. So the hunting was the bottleneck. The survivors were those just 30 elephant seals. And today, from those 30 elephant seals, they bounced back to a population of about 200,000. But since they went through that bottleneck, There is very low genetic diversity in today's elephant seals. In fact, it is thought that at the bottleneck of 30 individuals, there was only one dominant male. Elephant seals reproduce in a harem system where there is one male who kind of gets to mate with a whole group of females. And the less dominant males often don't mate with anybody at all. So it's thought that at this bottleneck of 30 individuals, there was one dominant male who got to mate with all the females. And that means that all 200,000 elephant seals that we have alive today are descended from that one dominant male. So you can see how a bottleneck would result in lowered genetic variation. Alright, so that's genetic drift. Next up, we have gene flow. And I'll tell you right off the bat that it's easy to get mixed up between genetic drift and gene flow. So remember, in your mind, drift is random, right? You don't know what's happening. It's random with gene flow. Think of it like a river, okay? These notes were taken with Minutes AI (https://myminutes.ai) Flowing river. Movement. Drift is random. Flow is movement. So gene flow refers to the physical movement of genes from one population to another. And this is going to happen anytime, individually, move from one population to another. So you can see we have a population of deer on one side of the mountain and a population of deer on the other side of the mountain. If some deer make it across that mountain, they're going to bring their genes with them and could potentially change the allele frequencies of the population that they are entering. Now, gene flow is interesting because it is one of the very few ways in which a population can get new alleles. Normally, the only way to get new alleles is to create them through mutation. But gene flow can also introduce new alleles that maybe previously didn't exist in A particular population. So in our deer example, the eastern population did not previously have any tan alleles. When those couple of tan deer groups come in, they bring their tan alleles with them. And now the population has blue alleles. That could potentially be helpful, But a lot of the time it isn't, because it tends to make two separate populations more genetically similar to each other. So they're sharing genes and they're going to have their gene pools starting to overlap a little bit. So as populations become more genetically similar, as the differences between them start to erode, if there is enough gene flow, that could actually prevent the populations from adapting to their local environments. So maybe, for example, in the eastern side of the mountain, it is way better to be brown than it is to be tan. But those tan deer keep coming in, keep bringing in their tan alleles, and that's going to eventually prevent the population from successfully adapting to the brown. These notes were taken with Minutes AI (https://myminutes.ai) So since gene flow requires the sharing of alleles between populations, it can actually prevent those populations from adapting to their specific local environments. And that's fairly likely since not every environment is the same. It's likely that the two sides of the mountain are a little bit different, and they're not going to see adaptation to those differences if there's a lot of gene flow across the mountain. Now, one thing to keep in mind with gene flow is that it doesn't necessarily have to be adult individuals moving from one population to another. It could be gametes moving from one population to another, for example, through pollination. Could also be juveniles moving from one population to another. So this little guy that we're looking at here is a juvenile sea urchin. Sea urchins don't really move, right? But the larvae are actually swimming and can float around in water. So it doesn't have to be the adult who accomplishes gene flow. It can be any stage of the life cycle. Speaker 2 (01 27 15) All right, so at this point, we. Speaker 4 (01 27 16) Have covered four of the mechanisms of evolution. There is patient non random meetings, genetic. Speaker 1 (01 27 22) Drift and gene flow. Speaker 2 (01 27 24) And today we're going to cover our final one, but arguably one of the most important ones, which is natural selection. Now, we can define natural selection as unequal survival and reproduction based on heritable variation in phenotypes. These notes were taken with Minutes AI (https://myminutes.ai) : : : : : : : : It's a long definition, right? So let's try to break this definition. Speaker 1 (01 27 46) Down into its key components. Speaker 2 (01 27 48) There are really three important parts of this definition. The first thing that you need in order for natural selection to work is going to be phenotypic variation. Speaker 1 (01 28 01) So in our example here, we've got Some green beetles and some brown beetles. Speaker 2 (01 28 05) There is variation in phenotypes within the population. Speaker 1 (01 28 09) The second thing we need is for. Speaker 2 (01 28 11) That variation to be heritable, meaning it is passed down from parent to offspring. So you can see in our beetles, they have little green pieces of DNA with green alleles, or little brown pieces of DNA with brown alleles. And then when they reproduce, they will pass on those traits to their offspring. So heritable variation in phenotypes. And then the third thing we need is for there to be unequal survival. Speaker 1 (01 28 37) And reproduction of individual in the population. These notes were taken with Minutes AI (https://myminutes.ai) : : : : : : : : : : : : : : Speaker 2 (01 28 40) Based on that phenotype. So depending on the phenotype you have, you are more likely to survive and reproduce or less likely to survive and. Speaker 1 (01 28 49) Reproduce and pass on your phenotype. Speaker 2 (01 28 52) So for our beetles, let's say they live on a brown tree trunk and so the brown ones camouflage really well and can survive and reproduce and pass on their brown alleles, whereas the green ones get picked out because they're easy to spot. So they don't survive, they don't reproduce, and they don't pass down that green phenotype. Anytime these three conditions are met, anytime you have variation that is inherited and affects survival and reproduction, natural selection will occur. So we can kind of define natural selection and understand how it works based on these three variation, heritage and unequal survival and reproduction. So to look at an example of how these criteria can lead to population evolution, we're going to start with our first criterion. The first thing we need is variation. That variation has to be present in. Speaker 1 (01 29 47) The population for these little guys. Speaker 2 (01 29 49) These are called old field mice. And you can see that they do have variable coat color. They can be dark coated or light colored, or kind of a range of brown in between. These notes were taken with Minutes AI (https://myminutes.ai) : : : : : : : : : : Speaker 1 (01 29 59) So we've got variation checked off. Speaker 2 (01 30 01) The next thing we need is for that variation to be inherited. And in fact, in old field mice, their coat color is controlled by genes. That's represented by the white and black bars. In this picture, the white bars would be the white alleles and the black. Speaker 1 (01 30 17) Bars would be the black alleles. Speaker 2 (01 30 19) And then the offspring can have various recombinations of those alleles. Speaker 1 (01 30 23) So we've got inherited phenotypic variation. The final thing we need is for. Speaker 2 (01 30 29) That variation to result in unequal survival and reproduction. Now, to see why that might be the case in old field mice, we gotta take a look at the environment where they live. And actually, these mice live in two different habitats. A beach habitat with nice white sand, or an inland habitat with soil and kind of shrubby plants. And grasses and things like that. So looking at these two habitats, you. These notes were taken with Minutes AI (https://myminutes.ai) : : : : : : : : : : : : Speaker 1 (01 30 56) Could kind of look at the colors. Speaker 2 (01 30 57) Of the mice and come up with a pretty obvious idea, a hypothesis that maybe their coat color affects the chances of surviving in these different habitats, since they might camouflage better or worse in any given. So we're going to try to test this hypothesis. Does coat color affect their chances of surviving in these two habitats? The simple experiment was done using painted models made out of clay to represent the mice. So they had brown clay mice and white clay mice, and they put these in the two different habitats, the inland habitats and the beach habitats, and then they left them there for 24 hours, and after the 24 hours, came back and looked for little bite marks in the clay and measured that, as did this, mice get eaten? So was there a predator that attempted to bite onto this mouse? Speaker 1 (01 31 55) Okay, so they're measuring predation rate. Speaker 2 (01 31 57) So given the information that I have. Speaker 1 (01 31 59) Just provided you, what would be the. Speaker 2 (01 32 01) Control treatments in this experiment? What would be the exact experimental treatments for these mice? Speaker 1 (01 32 07) These notes were taken with Minutes AI (https://myminutes.ai) : : : : : : : : : : : : : : And what's the independent variable? Speaker 2 (01 32 09) What's the dependent variable? What do you think? See if you can identify these different important aspects of this experiment, and I will go over the answers in class together. Okay, so we've got our four different treatment groups here, right? Four groups based on the color of the environment and the color of the mice. And here are the results for predation. And the bars represent the proportion of. Speaker 1 (01 32 34) Mice that got eaten closest to the picture. Speaker 2 (01 32 37) So do these results support the hypothesis? Is there unequal survival and reproduction based. Speaker 1 (01 32 43) On phenotype in these different environments? Speaker 2 (01 32 47) You can see that there is, right? So for the mice that are in an environment that's a really different color, they have a much higher predation rate. Like 75% of them get eaten. Whereas if they match their background environment, only about 25% of them get eaten. So there is unequal survival and reproduction. So in the inland habitat, the brown mice camouflage and get to survive and reproduce and pass on their brown alleles, Whereas the white mice do not camouflage. These notes were taken with Minutes AI (https://myminutes.ai) : : : : : : : : : : So they get eaten, and their alleles would get removed from the population. Since those alleles get removed in the next generation, you would see a lower frequency of white alleles and a higher frequency of brown alleles. And then the opposite thing would happen on the beach habitat. The brown mice would stand out. Speaker 1 (01 33 42) They would get eaten, Their alleles would. Speaker 2 (01 33 44) Disappear from the gene pool, and the frequency of the brown allele would decrease. Whereas the white mice get to survive, they get to reproduce, pass on their white alleles, and in the next generation, you'd see a higher frequency of white alleles. So there is allele frequency change as a result of unequal reproduction. That's how natural selection causes evolution. Certain phenotypes will have a better time surviving and reproducing. Because they are reproducing more often, they are going to pass on more of their alleles to the next generation, thereby causing allele frequency change. So remember always that the first thing we need is variation, and that variation generally comes from mutation. Mutation is the way you get new alleles, and then based on the alleles that are present in the gene pool, those create phenotypes which may reproduce and pass on those alleles or may not reproduce and will not pass on those alleles. And so favorable alleles get passed down more often, leading to allele frequency change. Now, we can describe this ability to reproduce as fitness. Fitness is defined in evolutionary biology a little bit differently from how we normally think of it. It's not like, oh, are you strong or fast? What it really is is one individual's relative genetic contribution to future generations. In other words, how many of your alleles are you passing on relative to everybody else in the population? These notes were taken with Minutes AI (https://myminutes.ai) : : : : Speaker 1 (01 35 27) Do you reproduce? Speaker 2 (01 35 28) Do your offspring reproduce? Do you, do your grand offspring reproduce? Speaker 1 (01 35 33) That's fitness. Speaker 2 (01 35 34) Amount of reproduction. Now, it is relative, meaning that it's relative to others in the population. So it's not just enough to reproduce. You have to reproduce more than everybody else. Now, fitness, if you have an allele that causes high fitness, that means that it causes a lot of reproduction. And so those alleles will tend to increase in frequency within the population from generation to generation because they are getting. Speaker 1 (01 36 03) Reproduced the most often. Speaker 2 (01 36 05) And so high fitness traits will tend to spread through the population, whereas low fitness traits will tend to disappear because they are not getting reproduced from generation to generation. And that's how natural selection works. These, those traits that increase your fitness, your ability to reproduce, are called adaptations. These notes were taken with Minutes AI (https://myminutes.ai) : : : : : : : : : : : : And so an adaptation is just any heritable trait that increases your ability to reproduce. Now, you may have heard the phrase survival of the fittest. Speaker 1 (01 36 38) I actually don't think that's a great. Speaker 2 (01 36 39) Phrase because as you can see from our discussion of fitness, it's really not just about survival. It's much more about reproduction. It's not enough just to survive. You have to actually reproduce alleles and. Speaker 1 (01 36 53) Contribute that to the gene pool of the next generation. Speaker 2 (01 36 56) So it should really be reproduction of the fittest. Right? Speaker 4 (01 36 59) Adaptations are traits that allow you to reproduce more. Speaker 2 (01 37 03) Now, what is an adaptation and what. Speaker 1 (01 37 05) Is not is going to be dependent. Speaker 2 (01 37 07) On the particular environment where an organism lives. These notes were taken with Minutes AI (https://myminutes.ai) : : : : : : : : : : : : : : : : You can see that this is the case in the mice. If you live on the beach, the brown allele is not adaptive. But if you live on the inland habitat, then the brown allele is adaptive. So whether a given trait or a given allele is adaptive or not is going to depend on the environment where you live. Now, the reason that natural selection is a very important mechanism of evolution is. Speaker 1 (01 37 35) Because it is the only one out. Speaker 2 (01 37 38) Of the five that will consistently lead to adaptation, meaning it will consistently increase the frequency of adaptive alleles. Mutation could create an adaptive allele, genetic drift could increase the frequency of the adaptive allele, but you could just as easily get a negative mutation or just as easily increase the frequency of a negative allele by genetic drift. So natural selection is going to be the only mechanism that consistently, reliably will increase the frequency of the adaptive allele. So let's look at one more example of how natural selection can work. Speaker 1 (01 38 21) And here we're going to look at. Speaker 2 (01 38 22) Darwin's finches from the Galapagos Islands. These are a great example of natural selection. And the first thing we need for natural selection. What is it? It's got to be variation. Right now, in the ground finches, we see variation in bill size and shape, both across species and within species. These notes were taken with Minutes AI (https://myminutes.ai) : : : : : : : : Speaker 1 (01 38 42) So if you look at different species. Speaker 2 (01 38 43) Of finches, their bills look a little bit different. But this data that we're looking at here is from just one species, the medium ground finch. And you can see that the beak size in the medium ground finch ranges from about 6 millimeters to 14 millimeters, with an average somewhere around 9 and a half millimeters. Most birds fall right around the average, right between 8 and 10 millimeters. So that's the average in our population. But there is variation around the average. So we've got phenotypic variation. The next thing we need is for that variation to be inherited. And in fact, in Darwin's finches, bill size is inherited. So let me help you figure out this graph here. Speaker 1 (01 39 24) On the X axis we have mid parent beak size. Speaker 2 (01 39 28) That's the average of the mother and the father. Okay, so the average of the parents. And then on the Y axis we have the offspring beak size. Putting aside the two different years for now, what you should be able to see is that there is a strong positive correlation between parent beak size and offspring beak size. The bigger the parrot's beaks, the bigger the offspring's beaks. Speaker 4 (01 39 51) And that means that it is in. These notes were taken with Minutes AI (https://myminutes.ai) : : : : : : : : : : Speaker 2 (01 39 53) Fact, an inherited trait. So we've got variation, we've got heritability. The last thing we need is unequal survival and reproduction. Or you can say unequal fitness, since fitness is survival and reproduction. Speaker 1 (01 40 07) Right. Speaker 2 (01 40 07) So we need unequal fitness, unequal reproduction based on the size of the bills. And for the brown finches, this was definitely the case in the late 1970s. So I'm going to show you some data and see if you can pick out some patterns from this data. Speaker 1 (01 40 22) So you can see we are looking. Speaker 2 (01 40 23) At the finch population from 1975 to 1978. The orange line shows you the size of the population. The green line shows you the abundance of seeds, which is their food source. And the purple line shows you the average body size within the population. A little bit more data here. This is showing you for 1976, the relationship between bill size and the percentage of birds that were able to survive. One more piece of information here is that in 1976, this population underwent an extremely severe drought, really, really bad drought for a couple of years. You can see that the population crashed, but who was able to survive was highly dependent on bill size. These notes were taken with Minutes AI (https://myminutes.ai) : : : : : : : : : : If you had an average bill, remember that was about nine and a half millimeters. If you had an average bill, you only had a 10% chance of survival. Very, very low. So the vast majority of the birds died. The ones that had a chance to make it were the ones with the very, very biggest bills. So I'd like you to think about why that was the case and what effect that would have on the population from one year to the next. But we do see clearly that the larger the bill, the higher the chance of survival and therefore reproduction for this population. Now, one thing to keep in mind about natural selection is that if the environmental pressures are really strong, it can actually occur quite quickly. We tend to think of evolution as something slow, right, like it takes hundreds. Speaker 1 (01 42 03) Of years to happen. Speaker 2 (01 42 04) That's not necessarily the case. If a population is experiencing very strong natural selection, it can evolve very, very quickly. We've just seen this in the finches. They evolved over the course of a couple of years. Sometimes it can even happen faster than that. So let's say, for imagine, for example, that you have a population of beetles, and in these beetles, first thing we need for natural selection is variation. So there is previously existing variation in a particular allele which is responsible for giving them resistance to pesticides. And this particular allele actually controls the number of receptor proteins on cell membranes. So if they have fewer of these uptake proteins, then they don't take up the pesticide and can survive it. These notes were taken with Minutes AI (https://myminutes.ai) : : : : Now, notice that initially in the population, there's only a very small number of alleles that are resistant. Most individuals are susceptible to the pesticide. So if you spray a field of crops with these pesticides, the vast majority of individuals are going to die. The only beetles who survive are the ones who have that resistant allele. And so those are the only beetles who are going to reproduce. And in the next generation, you're going to have 100% frequency of the resistant allele. So in just one generation, maybe just a month or so, you can go from having very little of this allele to having 100% of this allele straight. Strong natural selection means quick evolution. Now, this is exactly what has happened with insects and pesticides. We started using pesticides widely following World War II, and as we sprayed fields with pesticides, insects have been able to adapt because they reproduce a lot and have fairly high amounts of variation. So every time we sprayed them with a pestle, they've been able to evolve resistance to it quite quickly. And that has spread through the population until today you have species like the Colorado potato beetle. You can see the amount of damage that one of these beetles could do to a potato plant. And they are today resistant to over 65 different kinds of pesticide. There is no pesticide that we can spray them with that they haven't actually adapted to. So you can get really, really rapid evolution through adaptation, assuming variation is present. This also happens with antibiotic resistance in bacteria. Speaker 1 (01 44 41) Now, remember, in order for selection to. Speaker 2 (01 44 44) These notes were taken with Minutes AI (https://myminutes.ai) : : : : Occur, the first thing you need is always variation. There has to be variation present in the population, and that comes generally from mutation. Now, bacteria have really high rates of mutation and reproduce quite quickly. So there have a lot of different alleles present in any given population. It is not that you treated the bacteria with antibiotic and then they got a mutation. The mutation was already there before you ever put the antibiotic on them. Speaker 1 (01 45 14) Okay. Speaker 2 (01 45 14) Previously existing variation. So at some point in the past, some, some of these bacteria got mutations and now there's an allele that gives them resistance to an antibiotic. If you treat these bacteria with an antibiotic, then the susceptible ones will die and not reproduce, whereas the ones with the resistant allele will survive and reproduce. They will have higher fitness and pass on the resistance allele until it spreads through the population. So you know how when you wash your hands with antibiotic soap, you're killing off a lot of bacteria, Right, but you're not killing off all of them. And so then, you know, in the next generation of bacteria, they're all going to be resistant to whatever antibiotic you just treated them with. So we want to be careful with things like pesticides and antibiotics to make sure we're not overusing them, because quickly reproducing species like bacteria and insects do have the ability to evolve really, really quickly, assuming the variation is present. Okay, so that's how natural selection works. You have variation. It's heritable and unequal fitness as a result, and that can cause the population to evolve. In the next video, we'll look at some different special types of natural selection. These notes were taken with Minutes AI (https://myminutes.ai) : : : : Speaker 1 (01 46 29) And how those work. See you then. These notes were taken with Minutes AI (https://myminutes.ai) : :