Speaker 1: Welcome to Stuff to Blow Your Mind, a production of iHeartRadio.
Speaker 1: Hey you welcome to Stuff to Blow Your Mind. My
Speaker 1: name is Robert Lamb.
Speaker 2: And I'm Joe McCormick, and today we're going to be
Speaker 2: talking about a concept in the realm of genetics and reproduction,
Speaker 2: 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 concepts that I imagine we've covered on the show before,
Speaker 1: and many of you out there 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 reproduction
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 start with the basics, though, we're going
Speaker 1: to just approach it as if you're not really familiar
Speaker 1: with any of the topics that we're discussing here. So
Speaker 1: asexual reproduction versus sexual reproduction on a very basic level,
Speaker 1: here's how it all goes down. So, with sexual reproduction,
Speaker 1: you have the offspring of two genetic parents inheriting a
Speaker 1: mix of genes from those parents, genetically distinguishing itself from
Speaker 1: either parent. The resulting genetic variation is highly adaptive because
Speaker 1: it provides individuals with varying traits that may prove necessary
Speaker 1: for survival in an ever changing environment. The resulting genetic
Speaker 1: diversity makes the population more resistant to 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.
Speaker 1: Yeah. Yeah, So you end up with this, I mean,
Speaker 1: roughly speaking, you know, you have kind of like a
Speaker 1: randomization of these different traits, and the individuals that end
Speaker 1: up the offspring with that end up with 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
Speaker 1: ca is it's how they acquire their food. But in
Speaker 1: other cases it's how do they get a mate? How
Speaker 1: do they attract a mate or pursue a mate, and
Speaker 1: it ends up taking up a whole lot of time,
Speaker 1: a whole lot of energy. And what if you didn't
Speaker 1: have to do that? What if instead you could just
Speaker 1: essentially clone yourself.
Speaker 2: It would be very convenient and safer in a lot
Speaker 2: of cases, because I mean it varies by organism, but
Speaker 2: in many cases, yeah, if you have to go seeking
Speaker 2: out a mate, it is not only, you know, an
Speaker 2: energy expense to go looking around, but you're also often
Speaker 2: removing yourself from safe locations and going into dangerous ones.
Speaker 1: Yeah. I mean it's kind of like when you get
Speaker 1: some 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 wrecks
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 parthenogenesis, 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 vertebrates,
Speaker 2: like maybe some lizards or fish that can give birth
Speaker 2: without ever having without ever having their game meets fertilized
Speaker 2: 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 obligate 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 is I guess you could think of
Speaker 1: it as kind of a backup plan or some sort
Speaker 1: of a you know, an emergency button that can be pushed.
Speaker 1: And this has been the case with some of the
Speaker 1: famous examples of say sharks or lizards such as the
Speaker 1: Komodo dragon, reproducing in captivity, These so called virgin births
Speaker 1: 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 the.
Speaker 1: Right species correct, Yeah, and if I'm remembering correctly. This
Speaker 1: 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. This I'm just recalling from memory what mister
Speaker 2: DNA tells us, that that some frogs are able to
Speaker 2: spontaneously change sex in a single sex environment. And thus
Speaker 2: even though all of the dinosaurs in the park were
Speaker 2: supposed to be female, some changed into males and thus
Speaker 2: were 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,
Speaker 1: Like you know, an entire division of the species just
Speaker 1: for reproduction. Mating can prove fatal in and of itself,
Speaker 1: not necessarily in a way that actually has any impact
Speaker 1: on the species. But still it's like the again, you
Speaker 1: get into these situations where the male's whole role is
Speaker 1: reproduction and then afterwards has no purpose except maybe death.
Speaker 1: And it can of course also 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: It could be something very specific like well, while you're mating,
Speaker 1: it's possible that something could could prey on you. But
Speaker 1: also again, just think of all the links that creatures
Speaker 1: end up going to inmate selection and so forth. Various
Speaker 1: examples of this, even if it's just say sexual dimorphism,
Speaker 1: could mean that one member of the species is more
Speaker 1: 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, yes, particular places you know
Speaker 1: they have to travel to in order to engage in
Speaker 1: the mating, et cetera. But another drawback to sexual reproduction
Speaker 1: is that if it's your only option, it means that
Speaker 1: isolated members 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. Is 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 these different variations on
Speaker 1: the genetic code that could prove adaptive to change.
Speaker 2: Yeah, it gives you options, diversity.
Speaker 1: Yeah, yeah, diversifies your portfolio. Now, we mentioned disease and
Speaker 1: parasites already, so that's very much the case. If you
Speaker 1: just have a whole bunch of clones, then they all
Speaker 1: have the same susceptibility to illness or parasites. Overall, the
Speaker 1: big drawback is just a lack of genetic diversity, which
Speaker 1: can also result in the accumulation of harmful mutations. And
Speaker 1: another thing about the difference between the two though, that
Speaker 1: I guess I hadn't really thought about too much, is
Speaker 1: that it being a difference between short term and long
Speaker 1: term benefits. So, asexual reproduction is great for rapidly growing
Speaker 1: a population during a time of plenty, but the resulting
Speaker 1: population can run into problems long term. Meanwhile, sexual reproduction
Speaker 1: requires more energy and time, but generates diversity that may
Speaker 1: come in handy in the long term again when there
Speaker 1: are changes and obstacles that arise. Anyway, coming back to
Speaker 1: this idea that via asexual reproduction you can have this
Speaker 1: accumulation of harmful genetic changes. This brings us to the
Speaker 1: topic of Muller's ratchet, which is not something I was
Speaker 1: familiar with previously. The basic theory here is that long
Speaker 1: term reproduction, particularly a sexual reproduction, some of the studies
Speaker 1: we're looking at they're also looking at it with sexual reproduction. Basically,
Speaker 1: you see this accumulation of harmful genetic mutations, and after
Speaker 1: thousands of generations pass by, you can eventually reach a
Speaker 1: tipping point, which we refer to as mutational meltdown. And
Speaker 1: we'll get back to mutational meltdown in just a second.
Speaker 1: But interestingly, the namesake for Muller's ratchet is Hermann Joseph Muller,
Speaker 1: who lived eighteen ninety through nineteen sixty seven an American geneticist,
Speaker 1: mostly known for his work on mooda genesis, and for
Speaker 1: being like an outspoken critic and just sort of communicator
Speaker 1: on the dangers of radioactive fallout. He won the nineteen
Speaker 1: forty six Nobel Prize in Physiology or Medicine, and he
Speaker 1: was also the father of mathematician and computer scientist David E. Muller,
Speaker 1: who also has various things named after him. So you'll
Speaker 1: find a number of things in both genetic concepts and
Speaker 1: what have you, in genetics and mathematics that have the
Speaker 1: Mueller name attached 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 the 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 selections 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 rep produce will tend to disappear.
Speaker 2: But under certain circumstances, bad genes can accumulate. And one
Speaker 2: of the key concepts to understand here is what's known
Speaker 2: as genetic drift. So genetic drift is a change in
Speaker 2: the frequency of a particular gene variant also known as
Speaker 2: an allele in a population due to random chance rather
Speaker 2: than to natural selection. So random genetic drift is always happening.
Speaker 2: It's always going on in the background in the evolution
Speaker 2: of species. While you might think of natural selection as
Speaker 2: sort of acting in the foreground amplifying or diminishing alleles
Speaker 2: because they are helpful or harmful. So you might think of,
Speaker 2: say 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 birds
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 standard fluctuations in the sampling rate of the different
Speaker 2: alleles that get recombined in sexual reproduction. The smaller a
Speaker 2: population is, the more likely it is to be irreversibly
Speaker 2: changed by random trends in genetic drift. Now you might wonder,
Speaker 2: how would that work if the trends in genetic drift
Speaker 2: or just random, it's just chance, how would that cause
Speaker 2: irreversible changes? I think one way you might be able
Speaker 2: 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 into gamble with with the
Speaker 2: size of the population where the random genetic drift is happening.
Speaker 2: Genetic drift in a small population can easily drive certain
Speaker 2: alleeles extinct, even though those alleles had no negative effect
Speaker 2: on survival. The other side of the other side of
Speaker 2: that is that in small populations, random genetic drift can
Speaker 2: also so do the inverse. It can take an allele
Speaker 2: and make it the only version of that gene left
Speaker 2: 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
Speaker 2: fixed with it. Now the important thing to realize is
Speaker 2: that alleles don't have to be the best version of
Speaker 2: that gene. They don't have to be helpful to survival
Speaker 2: 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 at 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: rite quote. In small populations, the dominant genetic process is
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 reduces birth rates and reduces
Speaker 2: survival to the extent that the population declines, feedback between
Speaker 2: random genetic drift and mutation is set in motion. As
Speaker 2: the population size decreases, random genetic drift becomes a more
Speaker 2: significant force, and the rate of fixation of deleterious mutations increases,
Speaker 2: further reducing population size. So it is this feedback loop
Speaker 2: between the harmful mutations making the population smaller and thus
Speaker 2: increasing the effects of genetic drift compared to the effects
Speaker 2: 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, that this is not
Speaker 2: relevant to most of the species we'd be talking about,
Speaker 2: but just because I thought it was interesting. The authors
Speaker 2: in the context of this conservation biology book also mention
Speaker 2: how this applies in captive populations in a conservation context.
Speaker 2: So because captive populations of animals where you know, there's
Speaker 2: concern for the species level survival because those might those
Speaker 2: animals are not really competing in the wild to survive.
Speaker 2: It is very easy, in fact, for them to accumulate
Speaker 2: deleterious mutations in their genome because you have this genetic
Speaker 2: drift factor. But then also the normal selection pressures are
Speaker 2: not really applying at all. So once the population is
Speaker 2: reintroduced into the wild, the build up of all these
Speaker 2: deleterious mutations acquired through genetic drift can be quite harsh,
Speaker 2: and they say that this could explain some examples of
Speaker 2: basically poor performance of captive bread individuals of endangered species
Speaker 2: after being released into the wild.
Speaker 1: Yeah, there are so many factors to take into account
Speaker 1: with captive populations, because 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 there.
Speaker 2: One last thing from that book. The most common citations
Speaker 2: I see for the theoretical work on mutational meltdown are
Speaker 2: attributed to papers by Lynch published in the nineties, in
Speaker 2: the nineteen nineties, but they do note also in this
Speaker 2: book chapter that there have been some studies that looked
Speaker 2: for so that's the theoretical work by Lynch, but there
Speaker 2: were some studies that looked to try to find evidence
Speaker 2: of what they call greater genetic loads, these accumulations of
Speaker 2: mutations in small fruitfly populations. This was cited to Giligan
Speaker 2: at all in two thousand and five, and they didn't
Speaker 2: find it. They didn't find evidence of these of these
Speaker 2: loads they expected. So I guess some questions about how
Speaker 2: the theory of mutational meltdown actually applies to populations in
Speaker 2: 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 the 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, we have Mueller's ratchet, 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 it can sort of mess with your short
Speaker 1: term of short, short lived human brain when you start
Speaker 1: again thinking about the really long term history of life
Speaker 1: on Earth. So, of course, one of the big obvious
Speaker 1: challenges to exploring all of this is that humans have
Speaker 1: only been around on Earth and in a position to
Speaker 1: look for examples of things like mutational meltdown for a
Speaker 1: very short period of time. And if most asexual species
Speaker 1: or populations don't last very long, do you know, theoretically
Speaker 1: to Muller's ratchet or to you know, the stability of
Speaker 1: sexual reproduction outline and things like the red queen hypothesis.
Speaker 1: Then the various examples of ancient asexual species that we
Speaker 1: have that are more easy to you know, to look to,
Speaker 1: those are going to be exceptions to the rule. And
Speaker 1: then this creates additional additional questions arise, well, how has
Speaker 1: this asexual species been able to revive these challenges, these
Speaker 1: rigors that we're identifying in the data here. And you know,
Speaker 1: one of the sources I was looking at two thousand
Speaker 1: and eights quantifying the threat of extinction from Mueller's ratchet
Speaker 1: in the diploid Amazon molli. This is from Low and Lamach.
Speaker 1: They point out that, yeah, these species are of considerable
Speaker 1: interest to researchers for these 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 them 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 in 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. I'm
Speaker 1: in certain the authors are not arguing that this is
Speaker 1: the case, but it's obviously this is not like a
Speaker 1: smoking gun for the whole idea here, but it does
Speaker 1: seem to to give us some some some interesting evidence
Speaker 1: to back up some of these ideas, though of course
Speaker 1: also raising additional questions about you know, what exactly is
Speaker 1: going on. Now, there's a I've mentioned ted Ad before.
Speaker 1: There's a great ted AD video titled no Sex, No Problem,
Speaker 1: and I highly recommend checking that out. It does a
Speaker 1: nice overview of sort of the different the different strategies
Speaker 1: of asexual versus sexual reproduction, and and and briefly mentioned
Speaker 1: some of the concepts we're talking about here. Uh. One
Speaker 1: thing that I thought I thought was interesting in this
Speaker 1: videos it points out that pa fits are a great
Speaker 1: example of an organism that utilizes both sexual reproduction and
Speaker 1: asexual reproduction. Uh Uh, but depending on what the circumstances are.
Speaker 1: So with these particular eight, it's when it's springtime. They
Speaker 1: are a sexual reproducers. So it's like it's this is
Speaker 1: the these are the fat times, Like it's it's time to feed,
Speaker 1: it's time to reproduce. It's not time to worry too
Speaker 1: much about, you know, differentiating your product. It's about just
Speaker 1: getting product on the shelves. And so that's what they do.
Speaker 1: But then when autumn rolls around, then it's time for
Speaker 1: sexual reproduction. So it's like, Okay, this is our time
Speaker 1: to to think about the product. This is our time
Speaker 1: to to get experimental and see what we can do
Speaker 1: to change up our offering for the next season. So
Speaker 1: I thought that was just a really really interesting, like
Speaker 1: single species example that kind of sums up some of
Speaker 1: the benefits and some of the costs involved with asexual
Speaker 1: versus sexual reproduction, Like this is not the It's kind
Speaker 1: of like when you think about films in a series,
Speaker 1: for example, when it's time to make Wrong Turn two,
Speaker 1: you're not necessarily thinking about, well, how am I going
Speaker 1: to recreate? No, you don't recreate. You just do what
Speaker 1: worked the first time, accept more of it. This is
Speaker 1: the springtime of the Wrong Term franchise. Much later, when
Speaker 1: it's run out of gas, that's when you can you
Speaker 1: can sit down and think, Yeah, that's when you can
Speaker 1: be like, how do we reanalyze this, how do we
Speaker 1: reconceptualize Wrong Turn for a new audience? And maybe we
Speaker 1: can hire Matthew Modine to be in.
Speaker 2: It too, And it makes sense they both be part
Speaker 2: of your content strategy, you know. Sometimes you do reruns,
Speaker 2: sometimes you do a 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 mentioning 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 meets 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 Unpurged nine
Speaker 2: 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 their 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 tend
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 done some research.
Speaker 1: Well, you know, it feels like it has more of
Speaker 1: a you know, you got to stay on top of culture,
Speaker 1: so you got to read about the Purge, whereas somehow
Speaker 1: Wrong Turn movies maybe were less important culturally, or so
Speaker 1: 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
Speaker 1: sense 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.
Speaker 1: Duck, 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 said,
Speaker 2: I said poisonous, but I think the correct word would
Speaker 2: be venomous.
Speaker 1: I don't know.
Speaker 2: We'll have to sort that out later.
Speaker 1: All right. Well, on on that note, I think we
Speaker 1: have we have reached mutational meltdown for this episode. But
Speaker 1: we'd love to hear from everyone out there. I mean,
Speaker 1: especially if there's anyone out there who who is in
Speaker 1: the field of evolutionary biology. Perhaps you have some additional feedback,
Speaker 1: additional examples, you'd like to bring to mind. Let us know.
Speaker 1: You know, this is a topic that that 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 Blow your Mind
Speaker 1: is primarily a science podcast, with core episodes on Tuesdays
Speaker 1: and Thursdays. On Mondays we do listener mail, On Wednesdays
Speaker 1: we do a short form artifact or monster fact, and
Speaker 1: on Fridays we set aside most serious concerns to just
Speaker 1: talk about a weird film on Weird House Cinema. That's
Speaker 1: usually where our discussions of films about mutants would wind up,
Speaker 1: but sometimes those mutations accumulate in the core 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 topic for
Speaker 2: the future, or just to say hello, you can email
Speaker 2: us at contact stuff to Blow your Mind dot com.
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