Hello. And welcome back to Introduction to Genetics and Evolution. Now, in the past, we've talked about Recombination. We've talked about, the Process of Recombination. Primarily, in the Context of Transmission Genetics and Genetic Mapping. Now, the question I wanted to ask you today, is what are the Evolutionary Advantages of Sex and Recombination? Those two things are intrinsically tied together. Basically, why do we see so much Recombination within a species? Well, I have a cute subtitle for today's lecture. That is,still more reasons that Recombination rocks! Now, Sex and Recombination are intrinsically tied together. So let's first explore, Sexual Reproduction versus Asexual Reproduction. Now, there are several Forms of Asexual Reproduction, some of which you may have heard of. Asexual Reproduction can be through Binary Fission. Where you have basically, a single cell that splits into two cells, that are essentially complete organisms in and of themselves. You can have budding as depicted here on this slide with the hydra. You can see the little baby hydras coming off the mom hydra. We obviously, don't do that. My arm can't grow off into a kid who grows up. You can have Parthenogenesis, which is offspring that come out from unfertilized eggs. That's actually observed in various insect species. All of these processes produce exact genetic replicas of the parents. Basically, they produce clone. Sexual Reproduction, in contrast, involves the union of genetic material from a mom and a dad, or two parents. It doesn't have to be a mom and a dad. And they're put together. And then ultimately, before that new organism passes on its offspring, its genetic material can be shuffled in its gamete. Now, what process shuffles this genetic material in the gametes? It's our favorite process, Recombination. You may not know this, but there are several benefits to asexual reproduction. First, with asexual reproduction, every individual can make babies directly. In contrast with sexual reproduction as we humans do, you actually need to find, if you are a male, you need to find a female in order to produce offspring. With asexual reproduction, anybody can just bud off a baby. [LAUGH] So this is potentially a good thing. Similarly, with asexual reproduction, you don't need to go out and find mates. So this is something that sometimes involves a lot of energy. You have to sometimes have these characteristics that may actually be maladaptive, in general, but are good for attracting mates. So this is actually a big advantage to asexual reproduction. In contrast, sexual reproduction requires, that you actually go and woo your potential mate. The problem with asexual reproduction, offspring get all your genes. In contrast with sexual reproduction, offspring get half your genes. So, it seems like overall, there is a lot of benefits to asexual reproduction. You can pass on more of your genes. You don't need the intermediary. You don't need to go hunting and wooing mates. Nonetheless, we find that sexual reproduction is very, very common. It is particularly true in the animal, plant, and fungal kingdoms. We tend to see for many of the species, they only have sexual reproduction. And those few that do have asexual reproduction, it's often not complete. That basically, they'll go through a few generations of asexual reproduction. And then, they'll have a sexual phase. So we see all these benefits of asexuality, on the one hand. But on the other hand, sexual reproduction is very, very common. Why is this? Well, the benefits come from our friend recombination. So, I'm going to give you two major reasons but there's actually a slew of reasons that contribute to why sexual reproduction and recombination are beneficial. The first one we'll talk about is, that recombination makes combinations of alleles across two or more loci that may be advantageous. So imagine that, for example, to have maximal fitness, you need to be, you need to have the big A allele at one of your loci. And you'd have a big B allele at another loci. How do you put these things together? Recombination facilitates that and I'll show you that in just a second. But this is especially important when you have epistasis or interactions between loci that favor particular combinations. So let's use an example, let's imagine just as I was saying before, that you have a starting population where everybody is aabbb. Your optimal genotype has a A & B, but mutations from a to A are very rare. And mutations from b to B are very rare. So you have this population, are mostly little a, little a, little b, little b. And every now and then a mutation crops up. Well there's a big A, well that's good, but it's still not the optimal. Well there's a big B, that's good, but it's still not the optimal. There's a big B. There's a big A. If you only have asexual reproduction, this big A and this big B will never come together. Even if they have kids together, well obviously, they're not having kids together, if it's asexual reproduction. But if in some way, they interacted, you would not put this big A with this big B into one individual. So the question is, how could you put this genotype, big A, little a, big B, little b together? If you have the only asexual reproduction. You basically have to wait for that mutation to arise. So here this individual has a big A, you have to wait for a big B mutation to come up in either this individual or one of it's offspring in the future. And this individual here, you'd have to wait for a big A to rise either in this individual or in one of it's offspring. You'd have to wait for mutations. With outright combination, you need to wait a very long time to get the optimal genotype. In contrast, if you have recombination, you can accelerate adaptation. That basically, this individual could have a baby. This individual and some of the offspring would have that optimal genotype. Recombination can make advantageous mutations, get together much more rapidly. This allows for more efficient adaptation, second reason is related. Recombination basically helps you get rid of bad mutations. And in this regard, it helps you create or restore. And you'll see what I mean in just a second, mutation-free offspring. So let me illustrate this. Let's imagine a case of several independent lineages that are asexual. So here are your four lineages here. The first individual, second, third, and fourth. This right here on the far right, is depicting the number of detrimental mutations that we have per individual in that generation. In this case, nobody has any bad mutations. Everybody's got a 0, and the range that we see here is a 0. So, let's wait 10 generations. Oop! Another mutation arrives here. This individual is a little less fit than the rest of these. Well, guess what? Every offspring that comes from this individual is gonna have that same bad mutation. There's no way to get rid of it. There's no way for him to eliminate that, so all his offspring are doomed to carry this bad mutation. These ones are still all okay. But what's gonna happen after more time? Well, we'll have more mutations arising here, here, here, here. Eventually as you see, there's a range of 0-1 bad mutations, 0-1, a couple more have 1, a couple more have 1. Down here, the range is one to two. We actually have no individuals left in the population after a very long period of time, that are lacking bad mutations. Everybody has bad mutations. So in this regard, the population, as a whole, is getting more sickly. And in fact, the population gets worse, or sicker, and worse every single generations. A few generations later, you might see this distribution of number of bad mutations, again, nobody has zero bad mutations. The previous example you saw, it was just one to two but maybe after a while, it's a hump around three. Very few individuals have one, more have three, more than four bad mutations. You wait a little bit longer, you lose the zero and one. And now, everybody has two or more bad mutations. Essentially, again, the population gets worse and worse because individuals tend to accumulate these mutations. And they have no way to shed them. Right, there's no way to get this good version of this gene and this good version of that gene. And put them together. This process is called Muller's ratchet. If any of you are familiar with a ratchet, it's something that actually will only turn things in one direction. In this case, the analogy here, is that the population is getting worse. You turn it one direction. There's no way to move the, let me move the socket the other way. Now, if you had a recombination. You actually can get some offspring that don't have bad mutations. So here are two individuals that are both curing two bad mutations. This one has, in this case, the red is indicated a bad mutation rather than a good one. So this one has bad allele at A, bad allele at B. This one has bad allele at C, a bad allele at D. Well, we cross them together. Some individuals, some of the off spring from this will actually, will be worse. They'll have four bad mutations. Those will be very bad. Some will be comparable. This one has two bad mutations, but it has a different combination of bad mutations than either of the parents. But some will be better. This is the critical element. You can actually recreate the zero-mutation class. You can recreate some individuals that have no bad mutations. This one will reproduce more, this will reproduce some and this will probably be the most poor reproduction. So eventually, this is the type that will spread within the population. So overall, I'd have to say that recombination is good despite the many costs of sex. Because recombination can produce these advantageous combinations of alleles as I showed you. And in that regard, it can accelerate adaptation. Recombination also allows the population to get rid off, or unload itself, from these bad mutations. Essentially, it stops the ratchet. Now this will especially be true, if you're living in a variable environment. If you're in a very constant environment, then maybe not having a recombination isn't so bad. But if you're in a variable environment, you're essentially hedging your bets by producing more buried offspring. So this is one of the many ways that recombination is really good for our species. And this is why, we tend to think that recombination is very, very common when we work across species, particularly animals, plants, and fungi. And now, what does recombination do, in terms of, molecular evolution? Now, we've shown that it can combine good mutations. We've shown that it can get rid of bad mutations. But how does it affect neutral sequence. Sequences that really don't matter. How much does it affect? How much variation there is present in nucleotide sequence? This variation that has no particular effect out of it. But we'll come back to that, the next set of videos. Thank you.
