Population Genetics: When Darwin Met Mendel - Crash Course Biology #18

Key Takeaways

Hey look it's our friend Gregor mendle the super monk who discovered the basic principles of genetics hopefully you remember all of this both parents contribute one version of each of their genes called an Al to their offspring and some of those alal are dominant or always expressed While others are recessive and only expressed when they're not paired with a dominant one oh and here's our old Fred Chucky D he lets me call him that all this information that mendal figured out would have been really quite interesting to him because Darwin spent his whole life defending his ideas of natural ction as the primary force for evolution but Darwin had no idea how traits were passed on to their offspring even though these two guys were living and working at the same time both mendle and Darwin died not knowing how their ideas fit together so today we're going to introduce them and their ideas to one another through the science of population genetics which demonstrates how genetics and evolution influence each other and I have good news it involves a lot of math [Music] population genetics on the surface is not a complicated idea it's the study of how populations of a species change genetically over time leading to a species evolving so let's start out by defining what a population is it's simply a group of individuals of a species that can interbreed and because we have a whole bunch of fancy genetic testing gadgets and because unlike Darwin we know a whole lot about heredity we can now study the genetic change in populations over just a couple of generations this is really exciting and really like fun because it's it's basically like scientific instant gratification I can now observe Evolution happening within my lifetime so just cross that off the old bucket list now part of population genetics or pop gen and know we've got fancy abbreviations for everything now involves the study of factors that cause changes in what's called Al frequency which is just how often certain alals turn up within a population and those changes are at the heart of how and why Evolution happens so there are several factors that a frequency within a population and just like Fast and Furious movies there are five of them and unlike Fast and Furious movies they're actually very very important and are the basic reason why all complex life on Earth exists the main selective pressure is simply natural selection itself Darwin's sweet little baby which he spent a lot of his career defending from haters obviously we know this natural selection makes the alals that make animals the strongest and most verile and least likely to die more frequent in the population now most selective pressures are environmental ones like food supply or Predators or parasites but at the population level one of the most important evolutionary forces is sexual selection and population genetics gets it special attention particularly when it comes to what's called non-random mating which is a lifestyle that I encourage in all of my students do not mate randomly sexual selection is the idea that certain individuals will be more attractive mates than others because of specific traits this means they'll be chosen to have more sex and therefore more offspring the pop gen spin on things is that sexual ual selection means mating isn't random there are specific traits that are preferred even though they may not make the animals technically more fit for survival so sexual selection changes a genetic makeup of a population because the alals of the most successful maters are going to show up more often in the gene pool Mater's going to mate another important factor here and another thing that Darwin wished he had understood is mutation sometimes when eggs and sperm are formed through meiosis a mistake happens in the copying process of DNA bad errors in the DNA could result in the death or deformation of Offspring but not all mutations are harmful sometimes these mistakes can create new alal that benefit the individual by making it better at finding food or avoiding Predators or finding a mate these good errors and the alals they made are then passed to the Next Generation and into the population fourth we have genetic drift which is when an Al's frequency changes due to random chance a chance that's greater if the population is small and thus happens much more quickly if the population gets knocked way back by a tornado or something genetic drift does not cause individuals to be more fit just different finally when it comes to ilal game changers you got to respect the gene flow which is when individuals with different genes find their way into a population and spread their alals all over the place immigration and immigration are good examples of this and as with genetic drift its effects are most easily seen in small populations again our factors natural selection alals for fitter organisms become more frequent sexual selection Al Al for more sexually attractive organisms become more frequent mutation new alals popping up due to mistakes in DNA genetic drift changes in a Le frequency due to random chance and gene flow changes in a Le frequency due to mixing with new genetically different populations now that you know all that in order to explain specifically how these processes influence populations we're going to have to completely forget about them this is what's called the Hardy Weinberg principle Godfrey Hardy and wiel Weinberg were two sign scientists in 1908 who independently at the same time came up with the exact same equation that describes how under the right circumstances mandelian genetics works at the scale of a whole population but those right circumstances assume that none of the factors I just mentioned are at play Hardy and weinberg's simple equation shows us the frequency with which you could expect to find different alals within a hypothetical population that's not evolving this weird hypothetical state is called the Hardy Weinberg equilibrium in which the fre quency of alals in a population remains constant from generation to generation and to make sure that happens no funny stuff is allowed to go on to it the Hardy Weinberg equilibrium requires no natural selection which means that no alals are more beneficial than any other so the better alals will not be selected within a population no sexual selection which means that mating within the population must be completely random no individual can have a better chance of getting it on than any other no mutations because mutations modify the gene pool Hardy Weinberg Demands a gigantic population size because the smaller the population the more likely you are to get genetic drift and finally no gene flow that means that nobody can bring over their hot cousin from the next Island over because that would significantly mess with the AL frequencies if you know what I mean so clearly no fun and lots of rules Hardy and Weinberg they figured this out at the exact same time so it can't be that complicated because it wasn't some kind of stroke of like einsteinian inspiration they just they just figured out a thing that it's pretty simple the question is can we do the same thing right now can we figure it out on our own what we're looking for is the relationship between the phenotype and the actual frequency of the genes in the population so how do we proceed from here alas ear wax the consistency of ear wax is a mandelian trait wet ear wax is a Big W cuz it's dominant and dry ear wax is recessive so it's a little W now let's call the frequency of the dominant wet alal in the population p and the frequency of the recessive dry alil Q which if you've never known noticed Q is kind of a backwards P since there are only two alals for this Gene in the entire population P plus Q is going to equal one so the frequency of p is 75% the only other thing it could be is Q so that's going to be 25% which is one so imagine we go to this hypothetical no fun Hardy Weinberg Island and there are 100 people and we poke every single one of them in the year and nine of them have dry ear wax so that's 9 over 100 or 9% or 0.09 you know math but this is not Q it's not the frequency of the Little W it's the frequency of WW uh homozygous ww so this is the expressed phenotype it's not the genotype we don't know that yet we know the frequency of WW but you know that there's going to be a bunch of other W alals hanging around in heterozygous pairs so how do we figure out where those are how many of those there are well I have no idea I now am stuck I do not know I am lost when I'm stuck in situations like this what I do is I go back to what I do know and what I know is that the frequency of Big W plus the frequency of little W equals 1 but that's the entire population in each individual we want to know their genotype so two different alals so what's happening is this is happening twice in every individual so what we need to do is square it and when we square that equation if you remember algebra at all you get p² plus 2 PQ I'm such I have excellent handwriting plus Q ^2 equals one and that my friends is what Hardy and Weinberg did and it is the Hardy Weinberg equation so p^ squ is the odds of it being a ww this 2pq here is the heterozygotes and the q^ squared is the homozygous recessive well good news we know WW we know the homozygous recessive is 09 so we already have that that information so we know what q^2 is it's 9 and in order to get what Q is we just take the square root of that that was a horrible square root symbol which is 3 0 or 30% 30% frequency of the Q in the population and then we just use the simplest equation in the world to figure out what p is just minus one and that's 70 now using our Hardy Weinberg equation we can go beyond the frequency of the alals and actually talk about the frequency of the genotype types so the frequency of the WW homozygous dominant is the p^ squ so we have P so we just have to square this and that equals 0.49 or 49% of the population is homozygous dominant and now the math gets even easier because we know p and Q so to figure out how many heterozygotes there are we just do 2 * P which is 7 * 3 which is q and that equals 0.42 which is math that I did beforehand no didn't just know that so 9% of the population homozygous recessive 49% homozygous dominant and 42% heterozygous displaying wet ear wax but with that little W in there as well what's awesome about all this is that we can see mle's ideas at work in a big population and when things aren't lining up with this equation we know that there are one of those five factors at work probably more than one like for example a bunch of hot Surfers move to the island they all happen to have dry ear wax and they start spreading their hot Surfer Gene all over the [Music] place non-random mating it always goes out the window whenever the hot Surfers get involved I don't know about you this stuff's pretty beautiful to me so don't don't give me too hard of a time in the comments where you can ask questions or on Facebook or Twitter thank you for watching this episode of Crash Course biology we will see you next time [Music]