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Population genetics: When Darwin met Mendel

Hank talks about population genetics, which helps to explain the evolution of populations over time by combing the principles of Mendel and Darwin, and by means of the Hardy-Weinberg equation. Created by EcoGeek.

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  • piceratops ultimate style avatar for user N. J. P.
    So because Darwin didn't know about alleles and such, did he know anything about mutation? Were mutations understood at all during that time?
    (21 votes)
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    • blobby green style avatar for user mhhartman219
      He understood that individuals inherit traits from their parents and that those traits were in the gametes produced by the parents: "this shows how unimportant the direct effects of the conditions of life are in comparison with the laws of reproduction, and of growth, and of inheritance". He also understood that "varieties" arise because of changes (mutations) in inherited material.So to say that he "just conceived the theory of natural selection" is false. Especially considering how many of his peers were preparing to put their own works forward on natural selection (Alfred Russel Wallace).
      (18 votes)
  • blobby green style avatar for user Nick Martain
    I had a quite specific question about insects which live in hierarchised communities (bees, ants, etc.) where the majority of the population does not reproduce : how come there is any genetical diversity whatsoever in their gene pool? Isn't having only one mother (the queen) a disadvantage when it comes to genetical diversity, since only she passes down her genes to her offspring?

    Why are these types of communities so successful, both at adapting and thriving, while they have this problematic way of reproduction?
    (14 votes)
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    • piceratops ultimate style avatar for user Just Keith
      First, you need to understand that in the vast majority of animal species, only a small minority of offspring actually wind up reproducing. So, that much is not unusual.

      With the insect species in which there are numerous infertile offspring, those offspring (though not reproducing themselves) work in service to their relatives that do reproduce. If you think about how this situation affect, not the colony as a whole, but just those that can reproduce (and, thus, are subject to natural selection), you'll see the advantage. The queen is surrounded by multiple slaves that protect her from predation, provide her with food, protect her young, etc. So, she doesn't have to put her life on the line to successfully reproduce.

      When she does have fertile offspring (how often that happens and how that happens varies by species) it is often the case that these young are very much protected by the infertile slaves. They don't have to compete directly the way that an insect that doesn't have these infertile slaves. While they are immature larvae, they are not having to find food and keep from being eaten to such an extent as is the case with most insects. The slaves keep them fed, protect them from harm, etc., until they are mature and ready to start their own colonies.

      Thus, the genetic diversity or lack thereof of the infertile slaves is irrelevant. If they die, the queen just makes more. They are expendable. It is only the fertile offspring whose genetic diversity matters. And since they seek mates in the normal way for insects, they are just as genetically diverse as any other insect. Furthermore, since it is easy for them to survive until adulthood, there is a larger amount of genetic diversity (though, of course, there is still natural selection, the fertile offspring do have the be healthy enough to find a mate and successfully start their own colonies).
      (19 votes)
  • aqualine ultimate style avatar for user Trinity Viscarra
    How did Charles Darwin reach the conclusion that Natural Selection is the mechanism of evolution?
    (10 votes)
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  • blobby green style avatar for user Rishabh saxena
    whats the difference between genetic drift and natural selection..??
    (5 votes)
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    • piceratops ultimate style avatar for user Just Keith
      Genetic drift is the buildup in genetic variety in a species that does not affect in any substantial way reproductive success. For example, in humans hair color is fairly irrelevant when it comes to reproductive success. Traits subject to genetic drift are neither favored nor disfavored, so all versions of these traits propagate through the species over the course of generations more or less freely. (Note: this applies to all versions that do NOT make a difference in reproductive success: if any version comes along that DOES make a difference, then it may become naturally selected.)

      Natural selection is the mechanism by which certain traits are significantly favored or disfavored for reproductive success. Individuals with a negatively selected traits have a significantly worse odd at being reproductively successful. Individuals with a positively selected trait have significantly better odds at being reproductively successful. As a result, over the course of generations, the negatively selected traits will become more and more rare in the species, whereas positively selected traits will become more and more common in the species.

      As environmental conditions change, a trait or collection of traits may move from being governed by genetic drift to being controlled by natural selection. Or, if a previously important trait ceases to be so, it can move from being naturally selected to being subject to genetic drift.

      Genetic drift can cause the loss of a trait in a species. For example, apes cannot make their own Vitamin C despite having the gene for doing so because the gene is defecting. This apparently happened due to genetic drift -- being fruit-eaters, apes get enough Vitamin C in their diet that they don't need to make their own. Thus, due to genetic drift mutations in the gene for making Vitamin C were irrelevant, and so defects built up in that gene to the point that none of the apes is now able to make Vitamin C.
      (8 votes)
  • aqualine ultimate style avatar for user Sdk Sdk
    At , can you give me an example of a good mutation?
    (5 votes)
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  • female robot amelia style avatar for user Fareeha Rahman
    When Hank mentions that not all mutations are harmful, how would a mutation be harmful?
    (2 votes)
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  • primosaur ultimate style avatar for user T-TAS
    Why does genetic drift need a small population ?
    (2 votes)
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    • piceratops ultimate style avatar for user Just Keith
      Genetic drift does not require a small population, but in a small population genetic drift has a much more pronounced effect.
      Here are a couple of links that cover genetic drift and small populations:

      Genetic drift also plays an important (but very long term) role in the evolution of diversity in large populations. This is sometimes not covered in introductory courses. So, allow me to present a very simplified explanation of how genetic drift affects large populations (this really doesn't work this way in small, inbreeding populations):

      Genetic drift is random variation and random changes in the frequency of alleles that is not due to selection. In alleles for which there is not a strong selection, genetic drift dominates how the species diversifies in amongst those alleles. The stronger the selection pressure, the less room genetic drift has to operate.

      Consider a population of wolves which lives in an environment where coloration makes essentially no difference in reproductive success. Under those conditions, natural selection has no effect on coloration. But the individuals must be some color. So the alleles for brown, gray, black, tan, white, and various mixtures of these colors just drift about in the population as the generations go by, neither favored nor disfavored. They are not in competition with each other so how common each one happens to be is completely random and changes randomly. Furthermore, mutations can allow new colors to enter the gene pool -- as long as the new trait experiences no selection pressure, it is neither favored nor disfavored.

      Now, in small populations it is much easier for genetic drift to erase alleles completely, resulting in loss of diversity. But in large populations, just the opposite happens: genetic drift enhances diversity in alleles that lead to equal reproductive success.
      (3 votes)
  • aqualine ultimate style avatar for user Ben Braun
    if Q squared =0.09 wouldn't Q = 0.3 OR -0.3?
    (1 vote)
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  • starky sapling style avatar for user Nurinski Mercury
    i don't understand genecic drift
    (1 vote)
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    • piceratops seed style avatar for user azam.m.awais
      Genetic drift is deviation from the genetic pool, ie the deviation from the already presrnt genotypes in the population. It occurs when a new genotype appears. So the new genotype is a "drift" away from the already present genotypes. It can occur due to many reasons, but it always diversifies the population.
      (2 votes)
  • aqualine ultimate style avatar for user Cozmo
    Is there a population that is in Hardy-Weinberg equilibrium?
    (2 votes)
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Video transcript

- Hey, look! It's our friend, Gregor Mendel, the super monk who discovered the basic principles of genetics. And hopefully, you remember all of this. Both parents contribute one version of each of their genes called an allele to their offspring, and some of those alleles 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 friend, Chucky D. He lets me call him that. All this information that Mendel figured out would have been really quite interesting to him because Darwin spent his whole life defending his ideas of natural selection 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 Mendel 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. (upbeat music) Population genetics, on the surface, is not a complicated idea. It's the study of how populations of a species changed genetically over time, leading to a species evolving. So I 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 fun because it's basically like scientific instant gratification. I can now observe evolution happening within my lifetime, so cross that off the old bucket list. Now part of population genetics or pop gen, I know, we've got fancy abbreviations for everything now, involves the study of factors that cause changes in what's called allele frequency, which is just how often certain alleles turn up within a population. And those changes are at the heart of how and why evolution happens. So there are several factors that change allele 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 alleles that make animals their strongest and most virile and least likely to die, more frequent in the population. Now most selective pressures are environmental ones, like food supply, predators, or parasites. But at the population level, one of the most important evolutionary forces is sexual selection. And population genetics gets its special attention, particularly when it comes to what's called nonrandom 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 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 the genetic makeup of a population because the alleles of the most successful maters are gonna show up more often in the gene pool. Maters gonna mate! Another important factor here, and another thing that Darwin wished he had understood, is mutation. Sometimes when eggs and sperm are formed through miosis, a mistake happens, and 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 alleles that benefit the individual by making it better at finding food, or avoiding predators, or finding a mate. These good errors and the alleles they made are then passed to the next generation and into the population. Fourth, we have genetic drift, which is when an allele'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 allele game changers, you gotta respect the gene flow, which is when individuals with different genes find their way into a population and spread their alleles all over the place. Immigration and emigration are good examples of this, and as with genetic drift, its effects are most easily seen in small populations. Again, our factors, natural selection, alleles for fitter organisms become more frequent. Sexual selection, alleles for more sexually attractive organisms become more frequent. Mutation, new alleles popping up, due to mistakes in DNA. Genetic drift, changes in allele frequency due to random chance. And gene flow, changes in allele 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 Wilhelm Weinberg were two scientists in 1908, who independently, at the same time, came up with the exact same equation that describes how under the right circumstances, Mendelian 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 alleles within a hypothetical population that's not evolving. This weird, hypothetical state is called the Hardy-Weinberg Equilibrium, in which the frequency of alleles in a population remains constant from generation to generation. And to make sure that happens, no funny stuff is allowed to go on! To wit, the Hardy-Weinberg Equilibrium requires no natural selection, which means that no alleles are more beneficial than any other. So the better alleles 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 allele 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 figured out a thing that was pretty simple. So 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 Mendelian trait. Wet ear wax is a big W because it's dominant, and dry ear wax is recessive, so it's a little w. Now let's go to the frequency of the dominant wet allele in the population, P, and the frequency of the recessive, dry allele, q, which if you've never noticed, q is kind of a backwards p. So since there are only two alleles 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 gonna be 25%, which is 1. So imagine we go this hypothetical, no fun Hardy-Weinberg island and there are 100 people and we poke every single one of them in the ear, and nine of them have dry ear wax. So that's nine over a hundred, 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, 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 alleles 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 one. But that's the entire population. In each individual, we want to know their genotype. So two different alleles. So what's happening, is that 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 p squared plus 2pq, I have excellent handwriting, plus q squared equals one. And that, my friends, is what Hardy and Weinberg did, and it is the Hardy-Weinberg equation. So p squared 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 0.09, so we already have that information. So we know what q squared is, it's .09, 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 .30 or 30%. 30% frequency of the q allele in the population. And then we just use the simplest equation in the world to figure out what p is, this minus one, and that's .70. Now using our Hardy-Weinberg equation, we can go beyond the frequency of the alleles, and actually talk about the frequency of the genotypes. So the frequency of the WW, homozygous dominant, is the p squared. 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 two times p, which is .7 times .3, which is q, and that equals 0.42, which is math that I did beforehand, no, I 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 of this is that we can see Mendel's ideas at work in a big population. And when things aren't lining up with this equation, we know that they 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 genes all over the place. (surfing music) Nonrandom mating, it always goes out the window whenever the hot surfers get involved!