WEBVTT - Mutational Meltdown 0:00:03.040 --> 0:00:06.840 Welcome to Stuff to Blow Your Mind, a production of iHeartRadio. 0:00:13.240 --> 0:00:15.440 Hey you welcome to Stuff to Blow Your Mind. My 0:00:15.520 --> 0:00:16.759 name is Robert Lamb. 0:00:16.920 --> 0:00:19.639 And I'm Joe McCormick, and today we're going to be 0:00:19.680 --> 0:00:26.160 talking about a concept in the realm of genetics and reproduction, 0:00:27.000 --> 0:00:31.880 a concept known as mutational meltdown. Very enticing name, Rob 0:00:32.000 --> 0:00:37.040 I understand you became interested in mutational meltdown earlier this week. 0:00:37.080 --> 0:00:38.520 What got you going on this. 0:00:39.280 --> 0:00:41.920 Well, it actually didn't have anything to do directly with 0:00:42.680 --> 0:00:44.640 any melt movies we might have been talking about on 0:00:44.680 --> 0:00:48.000 Weird House Cinema. I actually, I think I was on 0:00:48.000 --> 0:00:50.400 a walk with my family and I said, hey, I 0:00:50.400 --> 0:00:52.159 think we're going to need an episode for Thursday. What 0:00:52.159 --> 0:00:55.440 should we do it on? And there my wife and 0:00:55.440 --> 0:00:56.760 my son are like, oh, you should do it on 0:00:57.120 --> 0:01:01.720 asexual reproduction. So okay, let's just started looking around a 0:01:01.760 --> 0:01:04.840 little bit. And Yeah, this particular term kind of jumped 0:01:04.880 --> 0:01:07.160 out at me. I wasn't familiar with it, and it 0:01:08.319 --> 0:01:10.920 basically gets down into and I think for our purposes 0:01:10.959 --> 0:01:12.360 here on the show, you know, it's a reason to 0:01:12.440 --> 0:01:15.440 sort of provide an overview of sort of asexual reproduction. 0:01:15.560 --> 0:01:21.680 Versus sexual reproduction as sort of competing ways of going 0:01:21.720 --> 0:01:25.360 about sort of the same thing for an organism, but 0:01:25.560 --> 0:01:29.840 one with more short term benefits versus long term benefits. 0:01:29.880 --> 0:01:31.399 And I don't know, I just found it to be 0:01:31.520 --> 0:01:36.640 kind of a neat way to re examine and think 0:01:36.680 --> 0:01:40.920 about these concepts that I imagine we've covered on the show before, 0:01:41.080 --> 0:01:46.360 and many of you out there have encountered in varying formats. 0:01:47.040 --> 0:01:49.560 Sure well, I know over the years we have alluded 0:01:49.600 --> 0:01:53.240 to the big question in biology of like where sex 0:01:53.280 --> 0:01:56.560 comes from, the where, when and why of sexual reproduction 0:01:57.200 --> 0:02:00.640 as a part of the history of organisms on planet Earth. 0:02:00.720 --> 0:02:04.400 Not going to solve that problem today, but yeah, I 0:02:04.480 --> 0:02:08.160 think maybe this little subtopic could help shed a little 0:02:08.200 --> 0:02:08.920 bit of light there. 0:02:09.360 --> 0:02:13.760 Yeah, so let's start with the basics, though, we're going 0:02:13.800 --> 0:02:17.000 to just approach it as if you're not really familiar 0:02:17.080 --> 0:02:20.480 with any of the topics that we're discussing here. So 0:02:20.800 --> 0:02:25.520 asexual reproduction versus sexual reproduction on a very basic level, 0:02:25.680 --> 0:02:28.679 here's how it all goes down. So, with sexual reproduction, 0:02:28.919 --> 0:02:32.920 you have the offspring of two genetic parents inheriting a 0:02:33.000 --> 0:02:36.880 mix of genes from those parents, genetically distinguishing itself from 0:02:36.960 --> 0:02:40.840 either parent. The resulting genetic variation is highly adaptive because 0:02:40.880 --> 0:02:44.720 it provides individuals with varying traits that may prove necessary 0:02:44.760 --> 0:02:49.079 for survival in an ever changing environment. The resulting genetic 0:02:49.080 --> 0:02:52.639 diversity makes the population more resistant to disease as well. 0:02:52.840 --> 0:02:55.040 I think one of the theories we've talked about before 0:02:55.240 --> 0:02:59.880 is that an advantage of sexual reproduction is that it 0:03:00.160 --> 0:03:04.280 helps protect the host organism against various types of parasites 0:03:04.320 --> 0:03:08.600 by introducing genetic variability that makes it harder for the 0:03:08.639 --> 0:03:13.400 parasite to target each successive generation of the host. 0:03:13.720 --> 0:03:17.840 Yeah, this is a clumsy analogy at best, but I 0:03:17.880 --> 0:03:19.560 can't help but think too about like to say that, 0:03:20.360 --> 0:03:23.400 because essentially, when you're talking about asexual reproduction, you're talking 0:03:23.919 --> 0:03:27.080 essentially about making a clone of oneself. And so the 0:03:27.160 --> 0:03:32.160 clone army in the Star Wars prequels highly susceptible, to say, 0:03:33.320 --> 0:03:35.640 a single order coming out and telling them to turn 0:03:35.640 --> 0:03:38.480 on the Jedi, that sort of thing. But that's just 0:03:38.520 --> 0:03:40.880 a very very rough idea of how to think about it. 0:03:42.160 --> 0:03:46.040 But more specifically for our purposes here, another key benefit 0:03:46.440 --> 0:03:48.840 that comes up in the literature is looking at is 0:03:48.840 --> 0:03:53.440 that you can think of sex, and genetic recombination is 0:03:53.520 --> 0:03:59.000 ultimately a means of purging deletarious mutations. 0:03:58.760 --> 0:04:01.560 Right, So the impact of mutations that might be harmful 0:04:01.600 --> 0:04:06.360 to the organism can be blunted by sexual recombination. 0:04:06.960 --> 0:04:08.640 Yeah. Yeah, So you end up with this, I mean, 0:04:09.280 --> 0:04:11.040 roughly speaking, you know, you have kind of like a 0:04:11.160 --> 0:04:14.760 randomization of these different traits, and the individuals that end 0:04:14.800 --> 0:04:19.120 up the offspring with that end up with the negative traits, 0:04:19.160 --> 0:04:23.440 the harmful traits, they don't survive. The ones that have 0:04:23.520 --> 0:04:26.839 been purged of those mutations do survive, and therefore it 0:04:26.880 --> 0:04:30.520 can purge the mutation from a particular lineage. Okay, all right, 0:04:30.640 --> 0:04:34.680 So moving on to asexual reproduction. This is a case 0:04:34.680 --> 0:04:36.800 in which you have the offspring of a single genetic 0:04:36.920 --> 0:04:40.000 parent inheriting the genes of the parent, making it a 0:04:40.040 --> 0:04:43.040 clone identical to the parent. The advantage here is that 0:04:43.080 --> 0:04:47.599 you can reproduce rapidly without all of the energy expenditure 0:04:47.720 --> 0:04:50.719 of mating. And I mean that's a pretty big statement 0:04:50.800 --> 0:04:54.480 to think about, because so many organisms we end up 0:04:54.480 --> 0:04:57.360 discussing on the podcast. You know, what is the key 0:04:57.400 --> 0:04:59.840 thing that makes them interesting? Well, in some cases many 0:04:59.839 --> 0:05:02.400 ca is it's how they acquire their food. But in 0:05:02.480 --> 0:05:04.560 other cases it's how do they get a mate? How 0:05:04.600 --> 0:05:07.440 do they attract a mate or pursue a mate, and 0:05:07.520 --> 0:05:10.080 it ends up taking up a whole lot of time, 0:05:10.240 --> 0:05:12.680 a whole lot of energy. And what if you didn't 0:05:12.720 --> 0:05:15.200 have to do that? What if instead you could just 0:05:15.360 --> 0:05:16.520 essentially clone yourself. 0:05:16.800 --> 0:05:19.160 It would be very convenient and safer in a lot 0:05:19.200 --> 0:05:22.080 of cases, because I mean it varies by organism, but 0:05:22.360 --> 0:05:24.520 in many cases, yeah, if you have to go seeking 0:05:24.600 --> 0:05:28.200 out a mate, it is not only, you know, an 0:05:28.320 --> 0:05:31.360 energy expense to go looking around, but you're also often 0:05:31.600 --> 0:05:35.760 removing yourself from safe locations and going into dangerous ones. 0:05:36.080 --> 0:05:37.719 Yeah. I mean it's kind of like when you get 0:05:38.520 --> 0:05:41.440 some sort of new kit to a symbols of Ikia furniture, right, 0:05:41.839 --> 0:05:44.520 and the first thing you notice is that on the 0:05:44.560 --> 0:05:46.719 instructions it says, oh, you have to have two people 0:05:46.760 --> 0:05:49.440 to do this, and you're like, oh, that totally wrecks 0:05:49.480 --> 0:05:52.120 my day. Now I've got to get my significant other 0:05:52.360 --> 0:05:54.279 or a friend to help with this. We've got to 0:05:54.279 --> 0:05:56.880 align our schedules and we have to both work together 0:05:57.040 --> 0:06:00.440 to build this thing, as opposed to one where I 0:06:00.440 --> 0:06:03.000 can just build it myself. And put it where it 0:06:03.040 --> 0:06:05.719 needs to go in the house. Now, there are multiple 0:06:05.720 --> 0:06:07.880 types of asexual reproduction, and we're not going to go 0:06:07.920 --> 0:06:09.560 into all of them, but you have all sorts of 0:06:09.560 --> 0:06:13.880 things like asexual budding and so forth. The sources I 0:06:13.920 --> 0:06:18.240 was looking at dealt a lot with parthenogenesis, which occurs 0:06:18.279 --> 0:06:21.880 widely and invertebrates. This word stems from the Greek for 0:06:22.000 --> 0:06:25.840 virgin creation parthenos plus genesis. 0:06:25.960 --> 0:06:29.039 Okay, so this would describe, for example, a lot of vertebrates, 0:06:29.120 --> 0:06:33.360 like maybe some lizards or fish that can give birth 0:06:33.440 --> 0:06:36.880 without ever having without ever having their game meets fertilized 0:06:36.920 --> 0:06:38.320 by a member of the opposite sex. 0:06:38.800 --> 0:06:44.200 Yeah. Yeah, we're talking about lizards, geckos, various insects, particularly 0:06:44.200 --> 0:06:48.840 some sharks. And it's of course very important to note 0:06:48.920 --> 0:06:54.160 that there are obligate sexual reproducers and then they're obligate 0:06:54.240 --> 0:06:57.560 asexual reproducers. But then there are also organisms that can 0:06:57.600 --> 0:07:01.760 do either depending on environmental pressure. So a classic example 0:07:01.880 --> 0:07:05.760 of a sexually reproducing organism engaging in asexual reproduction is, 0:07:05.800 --> 0:07:09.120 of course, when an individual cannot find a mate. It's 0:07:09.240 --> 0:07:11.040 kind of there is I guess you could think of 0:07:11.080 --> 0:07:16.280 it as kind of a backup plan or some sort 0:07:16.280 --> 0:07:18.880 of a you know, an emergency button that can be pushed. 0:07:19.200 --> 0:07:20.840 And this has been the case with some of the 0:07:20.880 --> 0:07:23.640 famous examples of say sharks or lizards such as the 0:07:23.720 --> 0:07:29.480 Komodo dragon, reproducing in captivity, These so called virgin births 0:07:30.080 --> 0:07:32.280 that will suddenly occur in shock zoo keepers. 0:07:32.800 --> 0:07:37.200 So the ideal is to mix and match your genetic 0:07:37.240 --> 0:07:40.840 material with somebody else's. But in a pinch, you could 0:07:40.880 --> 0:07:43.800 just make a copy of yourself if you're the. 0:07:43.840 --> 0:07:46.960 Right species correct, Yeah, and if I'm remembering correctly. This 0:07:47.000 --> 0:07:49.200 also pops up in the plot of Jurassic Park, right, 0:07:49.240 --> 0:07:52.120 something to do with the way that they're recreating dinosaur 0:07:52.200 --> 0:07:53.880 DNA using amphibian DNA. 0:07:54.080 --> 0:07:56.760 Well, I don't know if this is parthenogenesis or if 0:07:56.800 --> 0:07:58.240 it would be different. I think what they say, at 0:07:58.280 --> 0:08:00.120 least in the movie, I don't remember what happens in 0:08:00.120 --> 0:08:02.400 the book. In the movie they say that because they 0:08:02.480 --> 0:08:07.240 use some frog DNA to cover up patches in the 0:08:07.360 --> 0:08:10.440 DNA sequence. This I'm just recalling from memory what mister 0:08:10.520 --> 0:08:15.360 DNA tells us, that that some frogs are able to 0:08:15.480 --> 0:08:19.280 spontaneously change sex in a single sex environment. And thus 0:08:19.360 --> 0:08:21.960 even though all of the dinosaurs in the park were 0:08:22.000 --> 0:08:25.080 supposed to be female, some changed into males and thus 0:08:25.080 --> 0:08:26.600 were sexually reproducing. 0:08:27.160 --> 0:08:29.480 Ah okay, I think that's the main thing. I'm either 0:08:29.520 --> 0:08:32.800 misremembering that or maybe there's something from one of the 0:08:32.880 --> 0:08:36.280 later like Jurassic World films that I'm only like half 0:08:36.320 --> 0:08:39.400 processing here. All right, So you have these two basic 0:08:39.440 --> 0:08:42.120 ways of reproducing. Then this of course means that there 0:08:42.160 --> 0:08:45.480 are drawbacks to either one. So in sexual reproduction, again, 0:08:45.920 --> 0:08:47.440 you got to put a whole lot of energy and 0:08:47.520 --> 0:08:52.800 time into mating behaviors. It necessitates the existence of males, 0:08:53.280 --> 0:08:56.400 which in some cases like do little or nothing else, 0:08:56.800 --> 0:09:00.600 Like you know, an entire division of the species just 0:09:00.720 --> 0:09:05.480 for reproduction. Mating can prove fatal in and of itself, 0:09:06.520 --> 0:09:09.760 not necessarily in a way that actually has any impact 0:09:09.800 --> 0:09:12.840 on the species. But still it's like the again, you 0:09:12.880 --> 0:09:15.400 get into these situations where the male's whole role is 0:09:15.679 --> 0:09:20.360 reproduction and then afterwards has no purpose except maybe death. 0:09:21.000 --> 0:09:24.559 And it can of course also be nutrition, could be nutrition. Yeah, 0:09:24.600 --> 0:09:27.679 so it's not a complete ways. But also just mating 0:09:27.800 --> 0:09:31.320 in general creates opportunities for predators in a number of ways. 0:09:31.920 --> 0:09:34.960 It could be something very specific like well, while you're mating, 0:09:35.040 --> 0:09:37.520 it's possible that something could could prey on you. But 0:09:37.600 --> 0:09:41.959 also again, just think of all the links that creatures 0:09:42.040 --> 0:09:47.720 end up going to inmate selection and so forth. Various 0:09:47.760 --> 0:09:51.040 examples of this, even if it's just say sexual dimorphism, 0:09:51.679 --> 0:09:53.880 could mean that one member of the species is more 0:09:53.920 --> 0:09:55.080 likely to be consumed than the other. 0:09:55.559 --> 0:09:57.760 Yeah, it makes me think about all of the I 0:09:57.760 --> 0:10:02.480 don't know, like birds that essentially where male birds are 0:10:02.480 --> 0:10:08.600 trying to attract mates, specifically by being conspicuous. Yeah, you 0:10:08.679 --> 0:10:10.840 got to think that that also that comes with some 0:10:11.040 --> 0:10:13.480 amount of predation risk, at least in many cases. 0:10:13.920 --> 0:10:17.160 Yeah. Another thing could be, yes, particular places you know 0:10:17.240 --> 0:10:19.839 they have to travel to in order to engage in 0:10:19.880 --> 0:10:24.880 the mating, et cetera. But another drawback to sexual reproduction 0:10:25.160 --> 0:10:27.520 is that if it's your only option, it means that 0:10:28.040 --> 0:10:33.319 isolated members of a particular species or population just cannot reproduce, 0:10:33.800 --> 0:10:36.880 and it also means that sufficiently reduced populations are just 0:10:37.000 --> 0:10:40.560 already at a dead end. Now in asexual reproduction, there's 0:10:40.600 --> 0:10:44.240 also a potential dead end there as well, because if 0:10:44.280 --> 0:10:48.360 you don't have genetic variation occurring, if you're basically just 0:10:48.400 --> 0:10:51.200 putting out the same model after the same model, after 0:10:51.200 --> 0:10:54.959 the same model. It may well improve, it may well 0:10:55.000 --> 0:10:59.319 prove impossible for the species to adapt or to change. 0:11:00.240 --> 0:11:01.880 So it's you know, if you're just putting out the 0:11:01.920 --> 0:11:05.880 same model after the same model, and like the market 0:11:06.000 --> 0:11:08.160 is the same for that product, then I guess you 0:11:08.160 --> 0:11:10.160 don't have anything to worry about so long as the 0:11:10.200 --> 0:11:13.400 market doesn't change. Is Suddenly, if the demand for a 0:11:13.440 --> 0:11:17.679 particular you know, toy or item, we were to alter 0:11:17.800 --> 0:11:21.000 in some way and you couldn't alter the product, then 0:11:21.040 --> 0:11:24.319 you'd be in trouble. And the same goes for any 0:11:24.400 --> 0:11:27.880 kind of biological form. What happens when say, things begin 0:11:27.920 --> 0:11:31.560 to dry up or there's warming or cooling, or whatever 0:11:31.600 --> 0:11:35.240 the case may be. Sexual reproduction is what gives you 0:11:35.320 --> 0:11:40.200 the ability to bust out these these different variations on 0:11:40.320 --> 0:11:44.240 the genetic code that could prove adaptive to change. 0:11:44.600 --> 0:11:46.840 Yeah, it gives you options, diversity. 0:11:46.720 --> 0:11:53.160 Yeah, yeah, diversifies your portfolio. Now, we mentioned disease and 0:11:53.200 --> 0:11:55.920 parasites already, so that's very much the case. If you 0:11:56.040 --> 0:11:59.040 just have a whole bunch of clones, then they all 0:11:59.080 --> 0:12:04.120 have the same susceptibility to illness or parasites. Overall, the 0:12:04.160 --> 0:12:07.600 big drawback is just a lack of genetic diversity, which 0:12:07.600 --> 0:12:12.160 can also result in the accumulation of harmful mutations. And 0:12:12.200 --> 0:12:14.680 another thing about the difference between the two though, that 0:12:15.240 --> 0:12:17.040 I guess I hadn't really thought about too much, is 0:12:17.080 --> 0:12:19.640 that it being a difference between short term and long 0:12:19.720 --> 0:12:24.280 term benefits. So, asexual reproduction is great for rapidly growing 0:12:24.280 --> 0:12:27.440 a population during a time of plenty, but the resulting 0:12:27.480 --> 0:12:31.320 population can run into problems long term. Meanwhile, sexual reproduction 0:12:31.440 --> 0:12:35.720 requires more energy and time, but generates diversity that may 0:12:35.720 --> 0:12:38.400 come in handy in the long term again when there 0:12:38.400 --> 0:12:42.440 are changes and obstacles that arise. Anyway, coming back to 0:12:42.480 --> 0:12:46.880 this idea that via asexual reproduction you can have this 0:12:46.960 --> 0:12:52.920 accumulation of harmful genetic changes. This brings us to the 0:12:52.920 --> 0:12:56.320 topic of Muller's ratchet, which is not something I was 0:12:56.320 --> 0:13:01.199 familiar with previously. The basic theory here is that long 0:13:01.280 --> 0:13:06.960 term reproduction, particularly a sexual reproduction, some of the studies 0:13:07.000 --> 0:13:11.760 we're looking at they're also looking at it with sexual reproduction. Basically, 0:13:11.800 --> 0:13:15.240 you see this accumulation of harmful genetic mutations, and after 0:13:15.520 --> 0:13:19.840 thousands of generations pass by, you can eventually reach a 0:13:19.920 --> 0:13:25.200 tipping point, which we refer to as mutational meltdown. And 0:13:25.240 --> 0:13:27.600 we'll get back to mutational meltdown in just a second. 0:13:28.000 --> 0:13:33.440 But interestingly, the namesake for Muller's ratchet is Hermann Joseph Muller, 0:13:33.480 --> 0:13:37.480 who lived eighteen ninety through nineteen sixty seven an American geneticist, 0:13:38.120 --> 0:13:42.640 mostly known for his work on mooda genesis, and for 0:13:42.679 --> 0:13:48.040 being like an outspoken critic and just sort of communicator 0:13:48.040 --> 0:13:51.240 on the dangers of radioactive fallout. He won the nineteen 0:13:51.280 --> 0:13:54.720 forty six Nobel Prize in Physiology or Medicine, and he 0:13:54.800 --> 0:13:59.480 was also the father of mathematician and computer scientist David E. Muller, 0:14:00.320 --> 0:14:04.080 who also has various things named after him. So you'll 0:14:04.080 --> 0:14:08.400 find a number of things in both genetic concepts and 0:14:08.440 --> 0:14:12.680 what have you, in genetics and mathematics that have the 0:14:12.720 --> 0:14:14.080 Mueller name attached to them. 0:14:14.320 --> 0:14:17.200 Now, Rob, before you suggested this, I had never heard 0:14:17.200 --> 0:14:20.240 of mutational meltdown or Mueller's ratchet, at least as far 0:14:20.280 --> 0:14:22.440 as I know. But one of the things that I 0:14:22.480 --> 0:14:26.880 got really interested in here is how it violates sort 0:14:26.880 --> 0:14:30.720 of the simple assumptions that you make when you think 0:14:30.720 --> 0:14:33.400 about evolution on a surface level, because of course, it 0:14:33.480 --> 0:14:37.680 makes this reference to the idea of harmful genetic mutations 0:14:37.760 --> 0:14:42.560 accumulating over time in the species, and at a surface level, 0:14:42.560 --> 0:14:45.600 you might think, well, wait a minute, why would harmful 0:14:45.720 --> 0:14:50.240 genetic mutations accumulate. Isn't natural selections supposed to get rid 0:14:50.280 --> 0:14:54.400 of those? And so over time, with enough enough opportunities, yes, 0:14:54.560 --> 0:14:58.560 mutations that bring more harm than benefit to an organism's 0:14:58.760 --> 0:15:02.680 ability to survive and rep produce will tend to disappear. 0:15:03.000 --> 0:15:07.080 But under certain circumstances, bad genes can accumulate. And one 0:15:07.080 --> 0:15:10.280 of the key concepts to understand here is what's known 0:15:10.360 --> 0:15:15.720 as genetic drift. So genetic drift is a change in 0:15:15.800 --> 0:15:19.880 the frequency of a particular gene variant also known as 0:15:19.880 --> 0:15:25.240 an allele in a population due to random chance rather 0:15:25.400 --> 0:15:30.520 than to natural selection. So random genetic drift is always happening. 0:15:30.560 --> 0:15:33.240 It's always going on in the background in the evolution 0:15:33.360 --> 0:15:36.440 of species. While you might think of natural selection as 0:15:36.480 --> 0:15:41.640 sort of acting in the foreground amplifying or diminishing alleles 0:15:42.040 --> 0:15:46.400 because they are helpful or harmful. So you might think of, 0:15:46.480 --> 0:15:50.720 say a gene for blue feathers in some kind of bird. 0:15:51.040 --> 0:15:56.240 That gene might increase in the population, not for any 0:15:56.320 --> 0:16:00.000 reason having to do with blue feathers making the birds 0:16:00.040 --> 0:16:04.520 survive or reproduce more. Maybe it's just you know, sheer 0:16:04.600 --> 0:16:07.960 luck one season. Or maybe there might be some kind 0:16:07.960 --> 0:16:11.160 of random thing that happens in the popular lakes. Maybe 0:16:11.160 --> 0:16:13.960 a big population of blue feathered individuals come across a 0:16:13.960 --> 0:16:16.760 big cache of food or something, or there is just 0:16:17.160 --> 0:16:21.520 the standard fluctuations in the sampling rate of the different 0:16:21.520 --> 0:16:25.840 alleles that get recombined in sexual reproduction. The smaller a 0:16:25.920 --> 0:16:30.480 population is, the more likely it is to be irreversibly 0:16:30.680 --> 0:16:36.120 changed by random trends in genetic drift. Now you might wonder, 0:16:36.160 --> 0:16:39.040 how would that work if the trends in genetic drift 0:16:39.160 --> 0:16:42.120 or just random, it's just chance, how would that cause 0:16:42.240 --> 0:16:46.000 irreversible changes? I think one way you might be able 0:16:46.000 --> 0:16:49.320 to compare this is if you think about gambling. Okay, 0:16:49.360 --> 0:16:54.120 imagine you're making bets on somebody flipping a coin. If 0:16:54.160 --> 0:16:57.240 you have an infinite pot of money to bet with, 0:16:57.480 --> 0:16:59.680 you could just keep doing this forever, right Like you 0:16:59.760 --> 0:17:01.440 might get a run of good luck, you might get 0:17:01.440 --> 0:17:03.880 a run of bad luck. You might call the coin wrong. 0:17:04.200 --> 0:17:06.120 You know, I don't know how many times it would 0:17:06.119 --> 0:17:08.399 be plausible eight times in a row and lose a 0:17:08.400 --> 0:17:12.080 lot of money, But eventually, on average, you'd have a 0:17:12.080 --> 0:17:14.440 winning streak again and you'd win your money back as 0:17:14.480 --> 0:17:17.560 long as you can keep gambling, as long as you've 0:17:17.600 --> 0:17:20.360 got like an infinite pot to play from. But if 0:17:20.400 --> 0:17:23.800 you are gambling with a fixed amount of money, you 0:17:24.000 --> 0:17:27.399 eventually will hit a random run of bad luck and 0:17:27.480 --> 0:17:30.080 lose it all. You will play to extinction. 0:17:31.000 --> 0:17:32.119 Very fitting, very fitting. 0:17:32.480 --> 0:17:36.720 So for my analogy here, you could compare the size 0:17:36.760 --> 0:17:39.080 of your purse you're going into gamble with with the 0:17:39.119 --> 0:17:42.600 size of the population where the random genetic drift is happening. 0:17:43.119 --> 0:17:47.879 Genetic drift in a small population can easily drive certain 0:17:47.960 --> 0:17:52.360 alleeles extinct, even though those alleles had no negative effect 0:17:52.440 --> 0:17:56.159 on survival. The other side of the other side of 0:17:56.200 --> 0:17:59.680 that is that in small populations, random genetic drift can 0:17:59.720 --> 0:18:02.840 also so do the inverse. It can take an allele 0:18:02.960 --> 0:18:06.760 and make it the only version of that gene left 0:18:06.800 --> 0:18:10.560 in the population, present in one hundred percent of individuals. 0:18:10.960 --> 0:18:14.520 And there's a term for this. The population genetics term 0:18:15.280 --> 0:18:19.040 for when an allele becomes present in the entire population 0:18:19.240 --> 0:18:23.439 is fixation. When that allele is the only version of 0:18:23.440 --> 0:18:26.520 that gene left. It is said to be fixed in 0:18:26.560 --> 0:18:29.760 the population. Everybody's got it. And of course, once a 0:18:29.840 --> 0:18:33.000 gene variant is fixed in a population, of course that 0:18:33.119 --> 0:18:35.800 means the individuals in that population are stuck with it, 0:18:35.880 --> 0:18:40.000 you know, unless there is new information introduced. Now that 0:18:40.040 --> 0:18:43.160 could be maybe a random mutation causes a new version 0:18:43.200 --> 0:18:45.720 of that gene to appear, and then it can maybe compete, 0:18:46.240 --> 0:18:50.040 or there is inflow of new alleles of that gene, 0:18:50.080 --> 0:18:54.439 maybe by interbreeding with another population or something like that. 0:18:54.680 --> 0:18:59.159 But for a closed population, once a gene variant is 0:18:59.200 --> 0:19:11.439 fixed with it. Now the important thing to realize is 0:19:11.440 --> 0:19:15.840 that alleles don't have to be the best version of 0:19:15.880 --> 0:19:18.639 that gene. They don't have to be helpful to survival 0:19:18.720 --> 0:19:22.960 or reproduction in order to become fixed in a population. 0:19:24.040 --> 0:19:28.840 In big populations, harmful versions of genes will not tend 0:19:28.920 --> 0:19:31.520 to dominate over time, they will tend to get removed 0:19:31.600 --> 0:19:35.480 or remain in the background. But in small populations, because 0:19:35.600 --> 0:19:39.960 you're essentially gambling with a small purse, those deleterious alleles 0:19:40.000 --> 0:19:44.320 can become fixed just through bad luck. So you imagine 0:19:44.320 --> 0:19:48.919 maybe every season within a population, you pick a randomly 0:19:48.960 --> 0:19:52.280 assorted number of the individuals in that population, you say, 0:19:52.320 --> 0:19:56.080 whichever allele they've got, make another copy of that one, 0:19:56.560 --> 0:19:58.840 and then you just keep doing that over and over. 0:19:59.400 --> 0:20:02.920 You can at random results where suddenly a gene that's 0:20:03.000 --> 0:20:06.480 not very good for the population is suddenly the only 0:20:06.520 --> 0:20:12.840 one left. So that's how genetic drift can cause deleterious, 0:20:13.160 --> 0:20:17.520 harmful genes to become fixed in a population. But I 0:20:17.520 --> 0:20:19.800 was wondering, Okay, so what's the deal with this idea 0:20:20.000 --> 0:20:24.200 of mutational meltdown? What's happening there? Well, I was reading 0:20:24.240 --> 0:20:27.359 about this in a in a biology textbook I found 0:20:27.359 --> 0:20:33.000 called Practical Conservation Biology, edited by David Lindenmeyer and Mark Bergmann. 0:20:33.240 --> 0:20:36.680 And you know, one of the things that the authors 0:20:36.680 --> 0:20:39.960 mention is that every population carries some load in the 0:20:39.960 --> 0:20:46.119 background of deleterious, recessive genes. But the core theory of 0:20:47.080 --> 0:20:51.240 mutational meltdown again, it's something that really applies in particular 0:20:51.320 --> 0:20:55.480 to small populations. That's where it's really dangerous. The author's 0:20:55.560 --> 0:20:59.840 rite quote. In small populations, the dominant genetic process is 0:21:00.840 --> 0:21:03.840 if the size of the breeding population is very small, 0:21:03.960 --> 0:21:08.320 then random drift can overwhelm natural selection and a population 0:21:08.480 --> 0:21:13.120 can accumulate and become fixed for quite deleterious mutations. If 0:21:13.160 --> 0:21:16.600 the decline in fitness that results from the accumulation of 0:21:17.200 --> 0:21:22.440 new mutations reduces fecundity, so reduces birth rates and reduces 0:21:22.480 --> 0:21:28.520 survival to the extent that the population declines, feedback between 0:21:28.720 --> 0:21:33.320 random genetic drift and mutation is set in motion. As 0:21:33.400 --> 0:21:38.520 the population size decreases, random genetic drift becomes a more 0:21:38.560 --> 0:21:43.840 significant force, and the rate of fixation of deleterious mutations increases, 0:21:44.280 --> 0:21:48.639 further reducing population size. So it is this feedback loop 0:21:48.760 --> 0:21:54.400 between the harmful mutations making the population smaller and thus 0:21:54.600 --> 0:21:58.760 increasing the effects of genetic drift compared to the effects 0:21:58.800 --> 0:22:00.200 of selection forces. 0:22:00.560 --> 0:22:03.880 Yeah, so at first you just have one wrong turn movie, 0:22:03.920 --> 0:22:06.160 and then you have two wrong turn movies, and before 0:22:06.200 --> 0:22:08.000 you know it, there's like twenty of them and you 0:22:08.200 --> 0:22:10.600 haven't seen a single one, but you know that they 0:22:10.640 --> 0:22:14.240 all have something to do with mutated hillbillies. 0:22:14.800 --> 0:22:18.080 Yes, it's a vicious cycle of some kind. And as 0:22:18.119 --> 0:22:19.600 a side note, by the way, that this is not 0:22:19.680 --> 0:22:22.120 relevant to most of the species we'd be talking about, 0:22:22.160 --> 0:22:24.600 but just because I thought it was interesting. The authors 0:22:24.640 --> 0:22:27.960 in the context of this conservation biology book also mention 0:22:28.119 --> 0:22:33.000 how this applies in captive populations in a conservation context. 0:22:33.200 --> 0:22:39.200 So because captive populations of animals where you know, there's 0:22:39.240 --> 0:22:44.160 concern for the species level survival because those might those 0:22:44.200 --> 0:22:48.240 animals are not really competing in the wild to survive. 0:22:48.920 --> 0:22:52.560 It is very easy, in fact, for them to accumulate 0:22:52.640 --> 0:22:57.080 deleterious mutations in their genome because you have this genetic 0:22:57.119 --> 0:23:01.640 drift factor. But then also the normal selection pressures are 0:23:01.640 --> 0:23:05.320 not really applying at all. So once the population is 0:23:05.400 --> 0:23:09.120 reintroduced into the wild, the build up of all these 0:23:09.160 --> 0:23:13.640 deleterious mutations acquired through genetic drift can be quite harsh, 0:23:13.680 --> 0:23:17.320 and they say that this could explain some examples of 0:23:17.960 --> 0:23:22.920 basically poor performance of captive bread individuals of endangered species 0:23:22.960 --> 0:23:24.760 after being released into the wild. 0:23:26.000 --> 0:23:28.720 Yeah, there are so many factors to take into account 0:23:28.760 --> 0:23:31.879 with captive populations, because on top of everything you just 0:23:31.920 --> 0:23:35.359 talked about, there's also the idea that some species will 0:23:35.400 --> 0:23:40.679 just then spontaneously asexually produce offspring, which of course is 0:23:40.720 --> 0:23:42.639 not going to that particular offspring is not going to 0:23:42.640 --> 0:23:48.440 be genetically diversified either, So yeah, you have this huge 0:23:48.480 --> 0:23:49.840 bottleneck potential there. 0:23:50.080 --> 0:23:52.600 One last thing from that book. The most common citations 0:23:52.640 --> 0:23:56.760 I see for the theoretical work on mutational meltdown are 0:23:56.920 --> 0:24:01.280 attributed to papers by Lynch published in the nineties, in 0:24:01.320 --> 0:24:05.560 the nineteen nineties, but they do note also in this 0:24:05.600 --> 0:24:09.119 book chapter that there have been some studies that looked 0:24:09.160 --> 0:24:11.719 for so that's the theoretical work by Lynch, but there 0:24:11.720 --> 0:24:14.040 were some studies that looked to try to find evidence 0:24:14.760 --> 0:24:18.840 of what they call greater genetic loads, these accumulations of 0:24:18.960 --> 0:24:25.920 mutations in small fruitfly populations. This was cited to Giligan 0:24:26.000 --> 0:24:28.760 at all in two thousand and five, and they didn't 0:24:28.760 --> 0:24:32.200 find it. They didn't find evidence of these of these 0:24:32.280 --> 0:24:36.600 loads they expected. So I guess some questions about how 0:24:36.640 --> 0:24:39.879 the theory of mutational meltdown actually applies to populations in 0:24:39.920 --> 0:24:40.399 the wild. 0:24:40.760 --> 0:24:44.159