Speaker 1: Hey, you welcome to Stuff to Blow Your Mind. My
Speaker 1: name is Robert.
Speaker 2: Lamb and I am Joe McCormick, and today we are
Speaker 2: bringing you an episode from the vault. This one originally
Speaker 2: published April twentieth, twenty twenty three, and it is called
Speaker 2: Mutational Meltdown.
Speaker 1: Yeah, this is one that I was drawn to initially
Speaker 1: solely because of the title Mutational Meltdown, How can you resist?
Speaker 1: But there's a lot of fascinating content in there as well,
Speaker 1: beyond just sort of the knee jerk feel of imagining
Speaker 1: some sort of mutant melting in some sort of a
Speaker 1: sci fi context. Let's jump right in.
Speaker 3: Welcome to Stuff to Blow Your Mind, production of iHeartRadio.
Speaker 1: Hey you welcome to Stuff to Blow your Mind. My
Speaker 1: name is Robert.
Speaker 2: Lamb and I'm Joe McCormick, and today we're going to
Speaker 2: be talking about a concept in the realm of genetics
Speaker 2: and reproduction, a concept known as mutational meltdown. Very enticing name, Rob.
Speaker 2: I understand you became interested in mutational meltdown earlier this week.
Speaker 2: What got you going on this?
Speaker 1: Well, it actually didn't have anything to do directly with
Speaker 1: any melt movies. We might have been talking about on
Speaker 1: Weird House Cinema. I actually, I think I was on
Speaker 1: a walk with my family and I said, Hey, I
Speaker 1: think we're going to need an episode for Thursday. What
Speaker 1: should we do it on? And there my wife and
Speaker 1: my son are like, Oh, you should do it on
Speaker 1: asexual reproduction. So okay, let's just started looking around a
Speaker 1: little bit. And yeah, this particular term kind of jumped
Speaker 1: out at me. I wasn't familiar with it, and it
Speaker 1: basically gets down into and I think for our purposes
Speaker 1: here on the show, you know, it's a reason to
Speaker 1: sort of provide an overview of sort of asexual reproduction
Speaker 1: versus sexual reproduction as sort of competing ways of going
Speaker 1: about sort of the same thing for an organism, but
Speaker 1: one with more short term benefits versus long term benefits.
Speaker 1: And I don't know, I just found it to be
Speaker 1: kind of a neat way to re examine and think
Speaker 1: about these these concepts that I imagine we've covered on
Speaker 1: the show before, and many of you out there have
Speaker 1: have encountered in varying formats.
Speaker 2: Sure well, I know over the years we have alluded
Speaker 2: to the big question in biology of like where sex
Speaker 2: comes from, the where when and why of sexual reproductions
Speaker 2: as a part of the history of organisms on planet Earth.
Speaker 2: Not going to solve that problem today, but yeah, I
Speaker 2: think maybe this little subtopic could help shed a little
Speaker 2: bit of light there.
Speaker 1: Yeah, so let's let's start with the basics. Though, we're
Speaker 1: going to just approach it as if you know, you're
Speaker 1: not really familiar with any of the topics that we're
Speaker 1: discussing here, So asexual reproduction versus sexual reproduction on a
Speaker 1: very basic level, here's how it all goes down. So,
Speaker 1: with sexual reproduction, you have the offspring of two genetic
Speaker 1: parents inheriting a mix of genes from those parents, genetically
Speaker 1: distinguishing itself from either parent. The resulting genetic variation is
Speaker 1: highly adaptive because it provides individuals with varying traits that
Speaker 1: may prove necessary for survival in an ever changing environment.
Speaker 1: The resulting genetic diversity makes the population more resistant to
Speaker 1: disease as well.
Speaker 2: I think one of the theories we've talked about before
Speaker 2: is that an advantage of sexual reproduction is that it
Speaker 2: helps protect the host organism against various types of parasites
Speaker 2: by introducing genetic variability that makes it harder for the
Speaker 2: parasite to target each successive generation of the host.
Speaker 1: Yeah, this is a clumsy analogy at best, but I
Speaker 1: can't help but think too about like to say that,
Speaker 1: because essentially, when you're talking about asexual reproduction, you're talking
Speaker 1: essentially about making a clone of oneself. And so the
Speaker 1: clone army in the Star Wars prequels highly susceptible to say,
Speaker 1: a single order coming out and telling them to turn
Speaker 1: on the Jedi, that sort of thing. But that's just
Speaker 1: a very very rough idea of how to think about it.
Speaker 1: But more specifically for our purposes here, another key benefit
Speaker 1: that comes up in the literature is looking at is
Speaker 1: that you can think of sex and genetic recombination is
Speaker 1: ultimately a means of purging deletarious mutations.
Speaker 2: Right, so the impact of mutations that might be harmful
Speaker 2: to the organism can be blunted by sexual recombination. Yeah.
Speaker 1: Yeah, So you end up with this, I mean, roughly speaking,
Speaker 1: you know, you have kind of like a randomization of
Speaker 1: these different traits, and the individuals that end up the
Speaker 1: offspring with that end up with the the negative traits,
Speaker 1: the harmful traits. They don't survive the ones that have
Speaker 1: been purged of those mutations do survive, and therefore it
Speaker 1: can purge the mutation from a particular lineage. Okay, all right,
Speaker 1: So moving on to asexual reproduction. This is a case
Speaker 1: in which you have the offspring of a single genetic
Speaker 1: parent inheriting the genes of the parent, making it a
Speaker 1: clone identical to the parent. The advantage here is that
Speaker 1: you can reproduce rapidly without all of the energy expenditure
Speaker 1: of mating. And I mean that's a pretty big statement
Speaker 1: to think about, because so many organisms we end up
Speaker 1: discussing on the podcast. You know, what is the key
Speaker 1: thing that makes them interesting? Well, in some cases, many cases,
Speaker 1: it's how they acquire their food, But in other cases
Speaker 1: it's how do they get a mate? How do they
Speaker 1: attract a maid or pursue a mate, And it ends
Speaker 1: up taking up a whole lot of time, a whole
Speaker 1: lot of energy. And what if you didn't have to
Speaker 1: do that? What if instead you could just essentially clone yourself.
Speaker 2: Convenient and safer in a lot of cases, because I
Speaker 2: mean it varies by organism, but in many cases, yeah,
Speaker 2: if you have to go seeking out a mate, it
Speaker 2: is not only you know, an energy expense to go
Speaker 2: looking around, but you're also often removing yourself from safe
Speaker 2: locations and going into dangerous ones. Yeah.
Speaker 1: I mean it's kind of like when you get some
Speaker 1: sort of new kit to a symbols of Ikia furniture, right,
Speaker 1: and the first thing you notice is that on the
Speaker 1: instructions it says, oh, you have to have two people
Speaker 1: to do this, and you're like, oh, that totally recks
Speaker 1: my day. Now, I've got to get my significant other
Speaker 1: or a friend to help with this. We've got to
Speaker 1: align our schedules, and we have to both work together
Speaker 1: to build this thing, as opposed to one where I
Speaker 1: can just build it myself and put it where it
Speaker 1: needs to go in the house. Now, there are multiple
Speaker 1: types of asexual reproduction, and we're not going to go
Speaker 1: into all of them, but you have all sorts of
Speaker 1: things like asexual budding and so forth. The sources I
Speaker 1: was looking at dealt a lot with parthenogenis, which occurs
Speaker 1: widely and invertebrates. This word stems from the Greek for
Speaker 1: virgin creation parthenos plus genesis.
Speaker 2: Okay, so this would describe, for example, a lot of
Speaker 2: vertebrates like maybe some lizards or fish that can give
Speaker 2: birth without ever having without ever having their game meets
Speaker 2: fertilized by a member of the opposite sex.
Speaker 1: Yeah, yeah, we're talking about lizards, geckos, various insects, particularly
Speaker 1: some sharks. And it's of course very important to note
Speaker 1: that there are obligates sexual reproducers and then they're obligate
Speaker 1: asexual reproducers. But then there are also organisms that can
Speaker 1: do either depending on environmental pressure. So a classic example
Speaker 1: of a sexually reproducing organism engaging in asexual reproduction is,
Speaker 1: of course, when an individual cannot find a mate. It's
Speaker 1: kind of there as I guess you could think of
Speaker 1: it as kind of a backup plan that or some
Speaker 1: sort of a you know, an emergency button that can
Speaker 1: be pushed. And this has been the case with some
Speaker 1: of the famous examples of say sharks or lizards such
Speaker 1: as the Komodo dragon reproducing in captivity, these so called
Speaker 1: virgin births that will suddenly occur in shock zoo keepers.
Speaker 2: So the ideal is to mix and match your genetic
Speaker 2: material with somebody else's, but in a pinch, you could
Speaker 2: just make a copy of yourself if you're.
Speaker 1: The right species correct, Yeah, and if I'm remembering correctly,
Speaker 1: This also pops up in the plot of Jurassic Park, right,
Speaker 1: something to do with the way that they're recreating dinosaur
Speaker 1: DNA using amphibian DNA.
Speaker 2: Well, I don't know if this is parthenogenesis or if
Speaker 2: it would be different. I think what they say, at
Speaker 2: least in the movie, I don't remember what happens in
Speaker 2: the book. In the movie they say that because they
Speaker 2: use some frog DNA to cover up patches in the
Speaker 2: DNA sequence. I'm just recalling from memory what mister DNA
Speaker 2: tells us that some frogs are able to spontaneously change
Speaker 2: sex in a single sex environment, and thus, even though
Speaker 2: all of the dinosaurs in the park were supposed to
Speaker 2: be female, some changed into males, and thus we're sexually reproducing.
Speaker 1: Ah okay, I think that's the main thing. I'm either
Speaker 1: misremembering that, or maybe there's something from one of the
Speaker 1: later like Jurassic World films that I'm only like half
Speaker 1: processing here. All Right, So you have these two basic
Speaker 1: ways of reproducing, then this of course means that there
Speaker 1: are drawbacks to either one. So in sexual reproduction again,
Speaker 1: you got to put a whole lot of energy and
Speaker 1: time into mating behaviors. It necessitates the existence of males,
Speaker 1: which in some cases like do little or nothing else, Like,
Speaker 1: you know, an entire division of the species just for reproduction.
Speaker 1: Mating can prove fatal in and of itself, not necessarily
Speaker 1: in a way that actually has any impact on the species.
Speaker 1: But still it's like the again, you get into these
Speaker 1: situations where the male's whole role is reproduction and then
Speaker 1: afterwards it has no purpose except maybe death. And it
Speaker 1: can of course also can be nutrition, could be nutrition. Yeah,
Speaker 1: so it's not a complete ways. But also just mating
Speaker 1: in general creates opportunities for predators in a number of ways.
Speaker 1: You could it could be something very specific, like well,
Speaker 1: while you're mating, it's possible that something could could prey
Speaker 1: on you. But also again, just think of all the
Speaker 1: the links that creatures end up going to inmate selection
Speaker 1: and so forth. Various examples of this, even if it's
Speaker 1: just say sexual dimorphism could mean that one member of
Speaker 1: the species is more likely to be consumed than the other.
Speaker 2: Yeah, it makes me think about all of the I
Speaker 2: don't know, like birds that essentially where male birds are
Speaker 2: trying to attract mates, specifically by being conspicuous. Yeah, you
Speaker 2: got to think that that also that comes with some
Speaker 2: amount of predation risk, at least in many cases.
Speaker 1: Yeah, another thing could be a particular places you have
Speaker 1: to travel to in order to engage in the mating,
Speaker 1: et cetera. But another drawback to sexual reproduction is that
Speaker 1: if it's your only option, it means that isolated members
Speaker 1: just of a particular species or population just cannot reproduce.
Speaker 1: And it also means that sufficiently reduced populations are just
Speaker 1: already at a dead end. Now in asexual reproduction, there's
Speaker 1: also a potential dead end there as well, because if
Speaker 1: you don't have genetic variation occurring, if you're basically just
Speaker 1: putting out the same model after the same model, after
Speaker 1: the same model, it may well improve, it may well
Speaker 1: prove impossible for the species to adapt or to change.
Speaker 1: So it's you know, if you're just putting out the
Speaker 1: same model after the same model, and like the market
Speaker 1: is the same for that product, then I guess you
Speaker 1: don't have anything to worry about so long as the
Speaker 1: market doesn't change. It's suddenly, if the demand for a
Speaker 1: particular you know, toy or item we were to alter
Speaker 1: in some way and you couldn't alter the product, then
Speaker 1: you'd be in trouble. And the same goes for any
Speaker 1: kind of biological form. What happens when say, things begin
Speaker 1: to dry up, or there's warming or cooling, or whatever
Speaker 1: the case may be. Sexual reproduction is what gives you
Speaker 1: the ability to bust out these different variations on the
Speaker 1: genetic code that could prove adaptive to change. Yeah, it
Speaker 1: gives you options, diversity, Yeah, yeah, diversifies your portfolio. Now,
Speaker 1: we mentioned disease and parasites already, so that's very much
Speaker 1: the case. If you just have a whole bunch of clones,
Speaker 1: then they all have the same susceptibility to illness or parasites. Overall,
Speaker 1: the big drawback is just a lack of genetic diversity,
Speaker 1: which can also result in the accumulation of harmful mutations.
Speaker 1: And another thing about the difference between the two though,
Speaker 1: that I guess I hadn't really thought about too much,
Speaker 1: is that it being a difference between short term and
Speaker 1: long term benefits. So, asexual reproduction is great for rapidly
Speaker 1: growing a population during a time of plenty, but the
Speaker 1: resulting population can run into problems long term. Meanwhile, sexual
Speaker 1: reproduction requires more energy and time, but generates diversity that
Speaker 1: may come in handy in the long term again when
Speaker 1: there are changes and obstacles that arise. Anyway, coming back
Speaker 1: to this idea that via asexual reproduction you can have
Speaker 1: this accumulation of harmful genetic changes. This brings us to
Speaker 1: the topic of Mueller's ratchet, which is not something I
Speaker 1: was familiar with previously. The basic theory here is that
Speaker 1: long term reproduction, particularly a sexual reproduction, but some of
Speaker 1: the studies we're looking at they're also looking at it
Speaker 1: with sexual reproduction. Basically, you see this accumulation of harmful
Speaker 1: genetic mutations, and after thousands of generations pass by, you
Speaker 1: can eventually reach a tipping point, which we refer to
Speaker 1: as mutational meltdown. And we'll get back to mutational meltdown
Speaker 1: in just a second. But interestingly, the namesake for Muller's
Speaker 1: ratchet is Hermann. Joseph Muller, who lived eighteen ninety through
Speaker 1: nineteen sixty seven, an American geneticis mostly known for his
Speaker 1: work on mooda genesis and for being like an outspoken
Speaker 1: critic and just sort of communicator on the dangers of
Speaker 1: radioactive fallout. He won the nineteen forty six Nobel Prize
Speaker 1: in physiology or medicine. And he was also the father
Speaker 1: of mathematician and computer scientist David E. Muller, who also
Speaker 1: has various things named after him. So you'll find a
Speaker 1: number of things in both genetic concepts and what have you,
Speaker 1: in genetics and mathematics that have the Muller name attached
Speaker 1: to them.
Speaker 2: Now, Rob, before you suggested this, I had never heard
Speaker 2: of mutational meltdown or Mueller's ratchet, at least as far
Speaker 2: as I know. But one of the things that I
Speaker 2: got really interested in here is how it violates sort
Speaker 2: of the simple assumptions that you make when you think
Speaker 2: about evolution on a surface level, because, of course it
Speaker 2: makes this reference to the idea of harmful genetic mutations
Speaker 2: accumulating over time in a species, and at a surface level,
Speaker 2: you might think, well, wait a minute, why would harmful
Speaker 2: genetic mutations accumulate? Isn't natural selection supposed to get rid
Speaker 2: of those? And so over time, with enough enough opportunities, yes,
Speaker 2: mutations that bring more harm than benefit to an organism's
Speaker 2: ability to survive and reproduce will tend to disappear. But
Speaker 2: under certain circumstances, bad genes can accumulate. And one of
Speaker 2: the key concepts to understand here is what's known as
Speaker 2: genetic drift. So genetic drift is a change in the
Speaker 2: frequency of a particular gene variant also known as an allele,
Speaker 2: in a population due to random chance rather than to
Speaker 2: natural selection. So random genetic drift is always happening. It's
Speaker 2: always going on in the background in the evolution of species.
Speaker 2: While you might think of natural selection as sort of
Speaker 2: acting in the foreground, amplifying or diminishing alleles because they
Speaker 2: are helpful or harmful. So you might think of, say
Speaker 2: a gene for blue feathers in some kind of bird,
Speaker 2: that gene might increase in the population, not for any
Speaker 2: reason having to do with blue feathers making the bird
Speaker 2: survive or reproduce more. Maybe it's just you know, sheer
Speaker 2: luck one season. Or maybe there might be some kind
Speaker 2: of random thing that happens in the popular lakes, maybe
Speaker 2: a big population of blue feathered individuals come across a
Speaker 2: big cache of food or something, or there is just
Speaker 2: the the standard fluctuations in the sampling rate of the
Speaker 2: different alleles that get recombined in sexual reproduction. The smaller
Speaker 2: a population is, the more likely it is to be
Speaker 2: irreversibly changed by random trends in genetic drift. Now you
Speaker 2: might wonder, how would that work. If the trends in
Speaker 2: genetic drift are just random, it's just chance, how would
Speaker 2: that cause irreversible changes. I think one way you might
Speaker 2: be able to compare this is if you think about gambling. Okay,
Speaker 2: imagine you're making bets on somebody flipping a coin. If
Speaker 2: you have an infinite pot of money to bet with,
Speaker 2: you could just keep doing this forever, right Like, you
Speaker 2: might get a run of good luck. You might get
Speaker 2: a run of bad luck. You might call the coin wrong.
Speaker 2: You know, I don't know how many times it would
Speaker 2: be plausible eight times in a row and lose a
Speaker 2: lot of money. But eventually, on average, you'd have a
Speaker 2: winning streak again, and you'd win your money back as
Speaker 2: long as you can keep gambling, as long as you've
Speaker 2: got like an infinite pot to play from. But if
Speaker 2: you are gambling with a fixed amount of money, you
Speaker 2: eventually will hit a random run of bad luck and
Speaker 2: lose it all. You will play to extinction.
Speaker 1: Very fitting, very fitting.
Speaker 2: So for my analogy here, you could compare the size
Speaker 2: of your purse you're going in to gamble with with
Speaker 2: the size of the population where the random genetic drift
Speaker 2: is happening. Genetic drift in a small population can easily
Speaker 2: drive certain alleles extinct, even though those alleles had no
Speaker 2: negative effect on survival. The other side of the other
Speaker 2: side of that is that in small populations, random genetic
Speaker 2: drift can also do the inverse. It can take an
Speaker 2: allele and make it the only version of that gene
Speaker 2: left in the population, present in one hundred percent of individuals.
Speaker 2: And there's a term for this, The population genetics term
Speaker 2: for when an allele becomes present in the entire population
Speaker 2: is fixation. When that allele is the only version of
Speaker 2: that gene left, it is said to be fixed in
Speaker 2: the population. Everybody's got it. And of course, once a
Speaker 2: gene variant is fixed in a population, of course, that
Speaker 2: means the individuals in that population are stuck with it,
Speaker 2: you know, unless there is new information introduced. Now that
Speaker 2: could be maybe a random mutation causes a new version
Speaker 2: of that gene to appear and then it can maybe compete,
Speaker 2: or there is inflow of new alleles of that gene,
Speaker 2: maybe by interbreeding with another population or something like that.
Speaker 2: But for a closed population, once a gene variant is fixed,
Speaker 2: they're stuck with it. Now, the important thing to realize
Speaker 2: is that alleles don't have to be the best version
Speaker 2: of that gene. They don't have to be helpful to
Speaker 2: survival or reproduction in order to become fixed in a population.
Speaker 2: In big populations, harmful versions of genes will not tend
Speaker 2: to dominate over time. They will tend to get removed
Speaker 2: or remain in the background. But in small populations, because
Speaker 2: you're essentially gambling with a small purse, those deleterious alleles
Speaker 2: can become fixed just through bad luck. So you imagine,
Speaker 2: maybe every season within a population, you pick a randomly
Speaker 2: assorted number of the individuals in that population, You say,
Speaker 2: whichever allele they've got, make another copy of that one,
Speaker 2: and then you just keep doing that over and over.
Speaker 2: You can get random results where suddenly a gene that's
Speaker 2: not very good for the population is suddenly the only
Speaker 2: one left. So that's how genetic drift can cause deleterious,
Speaker 2: harmful genes to become fixed in a population. But I
Speaker 2: was wondering, Okay, so what's the deal with this idea
Speaker 2: of mutational meltdown. What's happening there? Well, I was reading
Speaker 2: about this in a in a biology textbook I found
Speaker 2: called Practical Conservation Biology edited by David Lindenmeyer and Mark Bergmann.
Speaker 2: And you know, one of the things that the authors
Speaker 2: mention is that every population carries some load in the
Speaker 2: background of deleterious recessive genes. But the core theory of
Speaker 2: mutational meltdown again, it's something that really applies in particular
Speaker 2: to small populations. That's where it's really dangerous. The author's
Speaker 2: write quote. In small populations, the dominant genetic process is drift.
Speaker 2: If the size of the breeding population is very small,
Speaker 2: then random drift can overwhelm natural selection and a population
Speaker 2: can accumulate and become fixed for quite deleterious mutations. If
Speaker 2: the decline in fitness that results from the accumulation of
Speaker 2: new mutations reduces fecundity, so it reduces birth rates and
Speaker 2: reduces survival to the extent that the population declines, feedback
Speaker 2: between random genetic drift and mutation is set in motion.
Speaker 2: As the population size decreases, random genetic drift becomes a
Speaker 2: more significant force, and the rate of fixation of deleterious
Speaker 2: mutations increases, further reducing population size. So it is this
Speaker 2: feedback loop between the harmful mutations making the population smaller
Speaker 2: and thus increasing the effects of genetic drift compared to
Speaker 2: the effects of selection forces.
Speaker 1: Yeah, so at first you just have one wrong turn movie,
Speaker 1: and then you have two wrong turn movies, and before
Speaker 1: you know it, there's like twenty of them and you
Speaker 1: haven't seen a single one, but you know that they
Speaker 1: all have something to do with mutated hillbillies.
Speaker 2: Yes, it's a vicious cycle of some kind. And as
Speaker 2: a side note, by the way, this is not relevant
Speaker 2: to most of the species we'd be talking about, but
Speaker 2: just because I thought it was interesting. The authors in
Speaker 2: the context of this conservation biology book also mention how
Speaker 2: this applies in captive populations in a conservation context. So
Speaker 2: because captive populations of animals where you know there's concern
Speaker 2: for the species level survival, because those might those animals
Speaker 2: are not really competing in the wild to survive, It
Speaker 2: is very easy, in fact, for them to accumulate deleterious
Speaker 2: mutations in their genome because you have this genetic drift factor.
Speaker 2: But then also the normal selection pressures are not really
Speaker 2: applying at all, so once the population is reintroduced into
Speaker 2: the wild, the build up of all these deleterious mutations
Speaker 2: acquired through genetic drift can be quite harsh, and they
Speaker 2: say that this could explain some examples of basically poor
Speaker 2: performance of captive bread individuals of endangered species after being
Speaker 2: released into the wild.
Speaker 1: Yeah, there's so many factors to take into account with
Speaker 1: captive populations, because, yeah, on top of everything you just
Speaker 1: talked about, there's also the idea that some species will
Speaker 1: just then spontaneously asexually produce offspring, which of course is
Speaker 1: not going to that particular offspring is not going to
Speaker 1: be genetically diversified either, So yeah, you have this huge
Speaker 1: bottleneck potential.
Speaker 2: There one last thing from that book. The most common
Speaker 2: citations I see for the theoretical work on mutational meltdown
Speaker 2: are attributed to papers by Lynch published in the nineties
Speaker 2: in the nineteen nineties, but they do note also in
Speaker 2: this book chapter that there have been some studies that
Speaker 2: looked for so that's the theoretical work by Lynch, but
Speaker 2: there were some studies that look to try to find
Speaker 2: evidence of what they call greater genetic loads accumulations of
Speaker 2: mutations in small fruit fly populations. This was cited to
Speaker 2: Gilligan at all in two thousand and five, and they
Speaker 2: didn't find it. They didn't find evidence of these of
Speaker 2: these loads they expected. So I guess some questions about
Speaker 2: how the theory of mutational meltdown actually applies to populations
Speaker 2: in the wild.
Speaker 1: Yeah, yeah, it's my understanding that, Yeah, we are dealing
Speaker 1: with theories here, and there is a continued challenge for
Speaker 1: evolutionary biologies to find examples and potential examples of all
Speaker 1: of this and to define these breakthrough examples. It will
Speaker 1: help us better understand not only this whole question of
Speaker 1: potential mutational meltdown, but also just sort of a larger
Speaker 1: question again of like why is sexual reproduction more beneficial
Speaker 1: or seemingly more beneficial? Like why sexual reproduction at all?
Speaker 1: But anyway, as I understand it, based on what we're
Speaker 1: looking at here, yeah, we have Mueller's which is the
Speaker 1: theoretical process that then could bring us to this end
Speaker 1: game of mutational meltdown. Mutational meltdown in this regard would
Speaker 1: be considered a subclass of an extinction vortex. Extinction vortex
Speaker 1: is a larger classification entailing different environmental, genetic, and demographic factors.
Speaker 1: It's also worth noting and perhaps inflating the obvious here,
Speaker 1: and that is that extinction is in the long term inevitable.
Speaker 1: All species eventually face extinction, and I've read that something
Speaker 1: like more than ninety nine percent of all species to
Speaker 1: ever exist have gone extinct. Again, this is stuff that
Speaker 1: makes perfect sense when you spell it out, but also
Speaker 1: it can sort of mess with your short term, short,
Speaker 1: short lived human brain when you start again thinking about
Speaker 1: the really long term history of life on Earth. So,
Speaker 1: of course, one of the big obvious challenges to exploring
Speaker 1: all of this is that humans have only been around
Speaker 1: on Earth and in a position to look for examples
Speaker 1: of things like mutational meltdown for a very short period
Speaker 1: of time. And if most asexual species or populations don't
Speaker 1: last very long, do you know, theoretically to Muller's ratchet
Speaker 1: or to the stability of sexual reproduction outlined and things
Speaker 1: like the red queen hypothesis, then the various examples of
Speaker 1: ancient asexual species that we have that are more easy,
Speaker 1: you know, to look to, those are going to be
Speaker 1: exceptions to the rule. And then this creates additional additional
Speaker 1: questions arise, well, how has this asexual species been able
Speaker 1: to survive these challenges, these rigors that we're identifying in
Speaker 1: the data here. And you know, one of the sources
Speaker 1: I was looking at two thousand and eights quantifying the
Speaker 1: threat of extinction from Mueller's ratchet in the diploid Amazon
Speaker 1: molly This is from Low and Lamach. They point out that, yeah,
Speaker 1: these species are of considerable interest to researchers for these
Speaker 1: very reasons.
Speaker 2: That would be the Amazon molli.
Speaker 1: Well, just in general, these sorts of species species that.
Speaker 2: Oh I see, yeah, ancient asexual species.
Speaker 1: Sorry, right, in this particular paper, this particular paper that
Speaker 1: the main focus the Amazon mollie, though, is also really interesting.
Speaker 1: This is a small asexual fish species that seems just
Speaker 1: prime for mutational meltdown. However, in modeling out the rate
Speaker 1: of harmful mutations in the species, they ran into what
Speaker 1: they referred to in the paper as a genomic decay paradox.
Speaker 1: So in most of the models they ran, the expected
Speaker 1: time to extinction for this species was less than previous
Speaker 1: estimates on the age of the species, So it would
Speaker 1: seem that the species has outlived its genomic expiration date.
Speaker 1: If Mueller's ratchet and mutational meltdown is indeed a factor
Speaker 1: the author's right quote, several biological processes can individually or
Speaker 1: in combination solve this genomic decay paradox, including paternal leakage
Speaker 1: of undamaged DNA from sexual sister species, compensatory mutations, and
Speaker 1: many others, and they, of course conclude that more research
Speaker 1: is ultimately required. Another paper that looks into all this
Speaker 1: that I found quite interesting was Deleterious mutation Accumulation in
Speaker 1: Asexual Tymema stick Insects by Henry at All, published in
Speaker 1: twenty twelve in Molecular Biology and Evolution. In this paper,
Speaker 1: the researchers look at six independently derived asexual lineages and
Speaker 1: related sexual species of the temma stick insects. So we're
Speaker 1: talking about closely related species, some that reproduce sexually and
Speaker 1: others that reproduce asexually. The idea here, of course, is
Speaker 1: the closeness. They're the related closely related to each other,
Speaker 1: so this would make the accumulation of deletarious mutations stand
Speaker 1: out more in the asexual species versus the sexual species,
Speaker 1: and that seems to be what they found. Quote. We
Speaker 1: found signatures of increased coding mutation accumulation in all six
Speaker 1: asexual tymema and for each of the three analyzed genes,
Speaker 1: with three point six to thirteen point four fold higher
Speaker 1: rates in the asexuals as compared with the sexuals. They
Speaker 1: also point out that the coding mutations and the asexuals
Speaker 1: are likely associated with more strongly deletarious effects than the
Speaker 1: sexuals due to some specific molecular reasons that they outline
Speaker 1: in the article. They conclude that quote deletarious mutation accumulation
Speaker 1: can differentially affect sexual and asexual lineages and support the
Speaker 1: idea that deletarious mutation accumulation plays an important role in
Speaker 1: limiting the long term persistence of all female lineages.
Speaker 2: So, according to this, as we were alluding to earlier,
Speaker 2: a species that's mainly reproducing or totally reproducing asexually and
Speaker 2: just making clonal copies will will tend to one of
Speaker 2: the pressures acting against it will be the tendency to
Speaker 2: build up loads of mutations that are not helpful to survival.
Speaker 1: Yeah, so over time, worse mutations accumulate, and the asexual
Speaker 1: species who do not diversify via sexual recombination, they don't
Speaker 1: purify through purging harmful mutations via sexual reproduction either and
Speaker 1: in fact, the authors here specifically mentioned that sexual reproduction
Speaker 1: enhances the efficiency of purifying selection. This is fascinating, It's
Speaker 1: not I mean, certainly the authors are not arguing that
Speaker 1: this is the case, but it's obviously this is not
Speaker 1: like a smoking gun for the whole idea here, but
Speaker 1: it does seem to give us some interesting evidence to
Speaker 1: back up some of these ideas, though of course also
Speaker 1: raising additional questions about you know, what exactly going on. Now.
Speaker 1: There's a I've mentioned ted Ad before. There's a great
Speaker 1: ted ad video titled no Sex, No Problem, and I
Speaker 1: highly recommend checking that out. It has a nice overview
Speaker 1: of sort of the different the different strategies of asexual
Speaker 1: versus sexual reproduction, and and and briefly mentioned some of
Speaker 1: the concepts we're talking about here. Uh. One thing that
Speaker 1: I thought was interesting in this videos it points out
Speaker 1: that pa fits are a great example of an organism
Speaker 1: that utilizes both sexual reproduction and asexual reproduction. Uh Uh.
Speaker 1: But but depending on what the circumstances are, So with
Speaker 1: these particular a fits, when it's springtime, they are asexual reproducers.
Speaker 1: So it's like it's this is the these are the
Speaker 1: fat times, Like it's it's time to feed, it's time
Speaker 1: to red it's not time to worry too much about,
Speaker 1: you know, differentiating your product. It's about just getting product
Speaker 1: on the shelves, and so that's what they do. But
Speaker 1: then when autumn rolls around, then it's time for sexual reproduction.
Speaker 1: So it's like, Okay, this is our time to think
Speaker 1: about the product. This is our time to get experimental
Speaker 1: and see what we can do to change up our
Speaker 1: offering for the next season. So I thought that was
Speaker 1: just a really really interesting, like single species example that
Speaker 1: kind of sums up some of the benefits and some
Speaker 1: of the costs involved with asexual versus sexual reproduction, Like
Speaker 1: this is not the It's kind of like when you
Speaker 1: think about films in a series, for example, when it's
Speaker 1: time to make Wrong Turn two, you're not necessarily thinking about, well,
Speaker 1: how am I going to recreate? No, you don't recreate.
Speaker 1: You just do what worked the first time, accept more
Speaker 1: of it. This is the springtime of the wrong term franchise.
Speaker 1: Much later, when it's run out of gas, that's when
Speaker 1: you can you can sit down and think, yeah, that's
Speaker 1: when you can be like, how do we reanalyze this,
Speaker 1: how do we reconceptualize Wrong Turn for a new audience?
Speaker 1: And maybe we can hire Matthew Modine to be in
Speaker 1: it too, and.
Speaker 2: It makes sense they both be part of your content strategy,
Speaker 2: you know. Sometimes you do reruns, sometimes you do a
Speaker 2: crossover event.
Speaker 1: Yeah. No, I haven't actually seen a Wrong Turn movie,
Speaker 1: So please don't go out and see these movies just
Speaker 1: based on me casually mention them.
Speaker 2: Here Rob wrongly recommends the Wrong Turn franchise. I can't
Speaker 2: remember if I have or not. Is it is that
Speaker 2: the one there's like a guy in a muscle car
Speaker 2: who drives into the woods and then they meet some
Speaker 2: I don't know some people and they get chased by
Speaker 2: dudes with hatchets.
Speaker 1: That sounds likely. I think that it's basically it's the
Speaker 1: hills have Eyes except in the woods. And there's like
Speaker 1: a million of these films. It's one of there's something
Speaker 1: always kind of alarming to me when I realized there's
Speaker 1: like a whole franchise that has been around for years
Speaker 1: and years and I just not only have I not
Speaker 1: seen them, but I just have just a very surface
Speaker 1: level understanding of what they're about, you know, like I've
Speaker 1: maybe never even seen a trailer for one of them.
Speaker 2: Yeah, there are a lot of series like that, and
Speaker 2: I understand what you mean, Like it can be alarming, Like, oh,
Speaker 2: I didn't even see the first Purge. We're on Purged
Speaker 2: nine now, this is Yeah, I don't know what's going on.
Speaker 2: I kind of can't start at this point. I'm not
Speaker 2: going to see these movies.
Speaker 1: Yeah, the Purge franchise, which I haven't seen any of
Speaker 1: those either, but I've read a bit more about them,
Speaker 1: so I'm kind of intrigued by the way it has
Speaker 1: survived thus far. It seems like it is a franchise
Speaker 1: that definitely has its springtime and autumn cycles of how
Speaker 1: it puts out new content, Like some of these seem
Speaker 1: like definite, like Okay, it's time for another Purge, and
Speaker 1: then other times it's like what can we do different
Speaker 1: with the Purge this time? And then it's like cut
Speaker 1: that we're doing a TV series, so just like ten
Speaker 1: the Purges, and then we'll work about innovating after that.
Speaker 2: I like that you have read about the Purge. I
Speaker 2: haven't seen it, but you've on some research. Well, you know, it.
Speaker 1: Feels like it has more of a you know, you
Speaker 1: got to stay on top of culture, so you got
Speaker 1: to read about the Purge, whereas somehow Wrong Turn movies
Speaker 1: maybe were less important culturally, or so it seems to me.
Speaker 2: Wrong Turn movies, i'd say, are less high concept because
Speaker 2: Purge has an elevator pitch right there, unless I misunderstand
Speaker 2: the idea is all crime is legal on one night.
Speaker 1: Yeah, yeah, okay, so, and I think it lends itself
Speaker 1: well to referencing. You can be like, oh, wow, I
Speaker 1: tried to drive across town the other day and it
Speaker 1: was like the Purge out there. You know, that makes sense.
Speaker 1: It's like you're saying something about how bad traffic was.
Speaker 1: But I don't know, Wrong Turn franchises maybe just a
Speaker 1: little harder to you know, bring into your daily life.
Speaker 2: I guess some organisms also have more of an elevator
Speaker 2: pitch quality to them, though, you know, like the platypus.
Speaker 2: It is it is a furry, poisonous duck.
Speaker 1: But it's also kind of high concept.
Speaker 2: Yeah that's what I'm saying. Yeah, it's high concept.
Speaker 1: Yeah, good creature. Have we ever done an episode on
Speaker 1: the Platypus? I can't recall. Of course, it's diversified enough
Speaker 1: that it's inevitably come up at least in passing in
Speaker 1: any number of episodes.
Speaker 2: I don't know if we have. I just really I
Speaker 2: said poisonous, but I think the correct word would be venomous.
Speaker 2: I don't know. We'll have to sort that out later.
Speaker 1: All right, Well, on that note, I think we have
Speaker 1: we have reached mutational meltdown for this episode. But we'd
Speaker 1: love to hear from everyone out there. I mean, especially
Speaker 1: if there's anyone out there who is in the field
Speaker 1: of evolutionary biology. Perhaps you have some additional feedback, additional
Speaker 1: examples you'd like to bring to mind.
Speaker 2: Let us know.
Speaker 1: You know, this is a topic that it caught my attention,
Speaker 1: but I'd love to see some more data on it.
Speaker 1: I'd love to see some more studies of note. In
Speaker 1: the meantime, will remind you that Stuff to Blow Your
Speaker 1: Mind is primarily a science podcast, with core episodes on
Speaker 1: Tuesdays and Thursday Days. On Mondays we do listener mail,
Speaker 1: on Wednesdays we do a short form artifact or monster fact,
Speaker 1: and on Fridays we set aside most serious concerns to
Speaker 1: just talk about a weird film on Weird House Cinema.
Speaker 1: That's usually where our discussions of films about mutants would
Speaker 1: wind up, but sometimes those mutations accumulate in the core
Speaker 1: episodes as well.
Speaker 2: Huge thanks to our audio producer JJ Posway. If you
Speaker 2: would like to get in touch with us with feedback
Speaker 2: on this episode or any other, to suggest a topic
Speaker 2: for the future, or just to say hello, you can
Speaker 2: email us at contact at stuff to Blow your Mind
Speaker 2: dot com.
Speaker 3: Stuff to Blow Your Mind is production of iHeartRadio. For
Speaker 3: more podcasts from my Heart Radio, visit the iHeartRadio app,
Speaker 3: Apple Podcasts, or wherever you listen to your favorite shows.
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