May 2, 2024 • 39 mins
Asexual reproduction has its advantages, but organisms that depend too much on it also face potential extinction. In this classic episode of Stuff to Blow Your Mind, Robert and Joe discuss mutational meltdown and Muller's ratchet. (originally published 04/20/2023)
See omnystudio.com/listener for privacy information.
Mark as Played
Transcript
Episode Transcript
Available transcripts are automatically generated. Complete accuracy is not guaranteed.
Speaker 1 (00:06):
Hey, you welcome to Stuff to Blow Your Mind. My
name is Robert.
Speaker 2 (00:09):
Lamb and I am Joe McCormick, and today we are
bringing you an episode from the vault. This one originally
published April twentieth, twenty twenty three, and it is called
Mutational Meltdown.
Speaker 1 (00:22):
Yeah, this is one that I was drawn to initially
solely because of the title Mutational Meltdown, How can you resist?
But there's a lot of fascinating content in there as well,
beyond just sort of the knee jerk feel of imagining
some sort of mutant melting in some sort of a
sci fi context. Let's jump right in.
Speaker 3 (00:45):
Welcome to Stuff to Blow Your Mind, production of iHeartRadio.
Speaker 1 (00:55):
Hey you welcome to Stuff to Blow your Mind. My
name is Robert.
Speaker 2 (00:58):
Lamb and I'm Joe McCormick, and today we're going to
be talking about a concept in the realm of genetics
and reproduction, a concept known as mutational meltdown. Very enticing name, Rob.
I understand you became interested in mutational meltdown earlier this week.
(01:19):
What got you going on this?
Speaker 1 (01:21):
Well, it actually didn't have anything to do directly with
any melt movies. We might have been talking about on
Weird House Cinema. I actually, I think I was on
a walk with my family and I said, Hey, I
think we're going to need an episode for Thursday. What
should we do it on? And there my wife and
my son are like, Oh, you should do it on
asexual reproduction. So okay, let's just started looking around a
(01:44):
little bit. And yeah, this particular term kind of jumped
out at me. I wasn't familiar with it, and it
basically gets down into and I think for our purposes
here on the show, you know, it's a reason to
sort of provide an overview of sort of asexual reproduction
versus sexual reproduction as sort of competing ways of going
about sort of the same thing for an organism, but
(02:08):
one with more short term benefits versus long term benefits.
And I don't know, I just found it to be
kind of a neat way to re examine and think
about these these concepts that I imagine we've covered on
the show before, and many of you out there have
have encountered in varying formats.
Speaker 2 (02:29):
Sure well, I know over the years we have alluded
to the big question in biology of like where sex
comes from, the where when and why of sexual reproductions
as a part of the history of organisms on planet Earth.
Not going to solve that problem today, but yeah, I
think maybe this little subtopic could help shed a little
(02:50):
bit of light there.
Speaker 1 (02:52):
Yeah, so let's let's start with the basics. Though, we're
going to just approach it as if you know, you're
not really familiar with any of the topics that we're
discussing here, So asexual reproduction versus sexual reproduction on a
very basic level, here's how it all goes down. So,
with sexual reproduction, you have the offspring of two genetic
(03:13):
parents inheriting a mix of genes from those parents, genetically
distinguishing itself from either parent. The resulting genetic variation is
highly adaptive because it provides individuals with varying traits that
may prove necessary for survival in an ever changing environment.
The resulting genetic diversity makes the population more resistant to
(03:34):
disease as well.
Speaker 2 (03:35):
I think one of the theories we've talked about before
is that an advantage of sexual reproduction is that it
helps protect the host organism against various types of parasites
by introducing genetic variability that makes it harder for the
parasite to target each successive generation of the host.
Speaker 1 (03:56):
Yeah, this is a clumsy analogy at best, but I
can't help but think too about like to say that,
because essentially, when you're talking about asexual reproduction, you're talking
essentially about making a clone of oneself. And so the
clone army in the Star Wars prequels highly susceptible to say,
a single order coming out and telling them to turn
(04:18):
on the Jedi, that sort of thing. But that's just
a very very rough idea of how to think about it.
But more specifically for our purposes here, another key benefit
that comes up in the literature is looking at is
that you can think of sex and genetic recombination is
ultimately a means of purging deletarious mutations.
Speaker 2 (04:41):
Right, so the impact of mutations that might be harmful
to the organism can be blunted by sexual recombination. Yeah.
Speaker 1 (04:49):
Yeah, So you end up with this, I mean, roughly speaking,
you know, you have kind of like a randomization of
these different traits, and the individuals that end up the
offspring with that end up with the the negative traits,
the harmful traits. They don't survive the ones that have
been purged of those mutations do survive, and therefore it
can purge the mutation from a particular lineage. Okay, all right,
(05:13):
So moving on to asexual reproduction. This is a case
in which you have the offspring of a single genetic
parent inheriting the genes of the parent, making it a
clone identical to the parent. The advantage here is that
you can reproduce rapidly without all of the energy expenditure
of mating. And I mean that's a pretty big statement
(05:33):
to think about, because so many organisms we end up
discussing on the podcast. You know, what is the key
thing that makes them interesting? Well, in some cases, many cases,
it's how they acquire their food, But in other cases
it's how do they get a mate? How do they
attract a maid or pursue a mate, And it ends
up taking up a whole lot of time, a whole
lot of energy. And what if you didn't have to
(05:55):
do that? What if instead you could just essentially clone yourself.
Speaker 2 (06:00):
Convenient and safer in a lot of cases, because I
mean it varies by organism, but in many cases, yeah,
if you have to go seeking out a mate, it
is not only you know, an energy expense to go
looking around, but you're also often removing yourself from safe
locations and going into dangerous ones. Yeah.
Speaker 1 (06:19):
I mean it's kind of like when you get some
sort of new kit to a symbols of Ikia furniture, right,
and the first thing you notice is that on the
instructions it says, oh, you have to have two people
to do this, and you're like, oh, that totally recks
my day. Now, I've got to get my significant other
or a friend to help with this. We've got to
align our schedules, and we have to both work together
(06:39):
to build this thing, as opposed to one where I
can just build it myself and put it where it
needs to go in the house. Now, there are multiple
types of asexual reproduction, and we're not going to go
into all of them, but you have all sorts of
things like asexual budding and so forth. The sources I
was looking at dealt a lot with parthenogenis, which occurs
(07:00):
widely and invertebrates. This word stems from the Greek for
virgin creation parthenos plus genesis.
Speaker 2 (07:08):
Okay, so this would describe, for example, a lot of
vertebrates like maybe some lizards or fish that can give
birth without ever having without ever having their game meets
fertilized by a member of the opposite sex.
Speaker 1 (07:21):
Yeah, yeah, we're talking about lizards, geckos, various insects, particularly
some sharks. And it's of course very important to note
that there are obligates sexual reproducers and then they're obligate
asexual reproducers. But then there are also organisms that can
do either depending on environmental pressure. So a classic example
(07:44):
of a sexually reproducing organism engaging in asexual reproduction is,
of course, when an individual cannot find a mate. It's
kind of there as I guess you could think of
it as kind of a backup plan that or some
sort of a you know, an emergency button that can
be pushed. And this has been the case with some
of the famous examples of say sharks or lizards such
(08:06):
as the Komodo dragon reproducing in captivity, these so called
virgin births that will suddenly occur in shock zoo keepers.
Speaker 2 (08:15):
So the ideal is to mix and match your genetic
material with somebody else's, but in a pinch, you could
just make a copy of yourself if you're.
Speaker 1 (08:26):
The right species correct, Yeah, and if I'm remembering correctly,
This also pops up in the plot of Jurassic Park, right,
something to do with the way that they're recreating dinosaur
DNA using amphibian DNA.
Speaker 2 (08:36):
Well, I don't know if this is parthenogenesis or if
it would be different. I think what they say, at
least in the movie, I don't remember what happens in
the book. In the movie they say that because they
use some frog DNA to cover up patches in the
DNA sequence. I'm just recalling from memory what mister DNA
tells us that some frogs are able to spontaneously change
(08:59):
sex in a single sex environment, and thus, even though
all of the dinosaurs in the park were supposed to
be female, some changed into males, and thus we're sexually reproducing.
Speaker 1 (09:09):
Ah okay, I think that's the main thing. I'm either
misremembering that, or maybe there's something from one of the
later like Jurassic World films that I'm only like half
processing here. All Right, So you have these two basic
ways of reproducing, then this of course means that there
are drawbacks to either one. So in sexual reproduction again,
you got to put a whole lot of energy and
(09:30):
time into mating behaviors. It necessitates the existence of males,
which in some cases like do little or nothing else, Like,
you know, an entire division of the species just for reproduction.
Mating can prove fatal in and of itself, not necessarily
in a way that actually has any impact on the species.
(09:53):
But still it's like the again, you get into these
situations where the male's whole role is reproduction and then
afterwards it has no purpose except maybe death. And it
can of course also can be nutrition, could be nutrition. Yeah,
so it's not a complete ways. But also just mating
in general creates opportunities for predators in a number of ways.
(10:14):
You could it could be something very specific, like well,
while you're mating, it's possible that something could could prey
on you. But also again, just think of all the
the links that creatures end up going to inmate selection
and so forth. Various examples of this, even if it's
just say sexual dimorphism could mean that one member of
(10:35):
the species is more likely to be consumed than the other.
Speaker 2 (10:38):
Yeah, it makes me think about all of the I
don't know, like birds that essentially where male birds are
trying to attract mates, specifically by being conspicuous. Yeah, you
got to think that that also that comes with some
amount of predation risk, at least in many cases.
Speaker 1 (10:56):
Yeah, another thing could be a particular places you have
to travel to in order to engage in the mating,
et cetera. But another drawback to sexual reproduction is that
if it's your only option, it means that isolated members
just of a particular species or population just cannot reproduce.
And it also means that sufficiently reduced populations are just
(11:19):
already at a dead end. Now in asexual reproduction, there's
also a potential dead end there as well, because if
you don't have genetic variation occurring, if you're basically just
putting out the same model after the same model, after
the same model, it may well improve, it may well
prove impossible for the species to adapt or to change.
(11:42):
So it's you know, if you're just putting out the
same model after the same model, and like the market
is the same for that product, then I guess you
don't have anything to worry about so long as the
market doesn't change. It's suddenly, if the demand for a
particular you know, toy or item we were to alter
in some way and you couldn't alter the product, then
(12:03):
you'd be in trouble. And the same goes for any
kind of biological form. What happens when say, things begin
to dry up, or there's warming or cooling, or whatever
the case may be. Sexual reproduction is what gives you
the ability to bust out these different variations on the
genetic code that could prove adaptive to change. Yeah, it
(12:27):
gives you options, diversity, Yeah, yeah, diversifies your portfolio. Now,
we mentioned disease and parasites already, so that's very much
the case. If you just have a whole bunch of clones,
then they all have the same susceptibility to illness or parasites. Overall,
the big drawback is just a lack of genetic diversity,
(12:50):
which can also result in the accumulation of harmful mutations.
And another thing about the difference between the two though,
that I guess I hadn't really thought about too much,
is that it being a difference between short term and
long term benefits. So, asexual reproduction is great for rapidly
growing a population during a time of plenty, but the
resulting population can run into problems long term. Meanwhile, sexual
(13:13):
reproduction requires more energy and time, but generates diversity that
may come in handy in the long term again when
there are changes and obstacles that arise. Anyway, coming back
to this idea that via asexual reproduction you can have
this accumulation of harmful genetic changes. This brings us to
(13:35):
the topic of Mueller's ratchet, which is not something I
was familiar with previously. The basic theory here is that
long term reproduction, particularly a sexual reproduction, but some of
the studies we're looking at they're also looking at it
with sexual reproduction. Basically, you see this accumulation of harmful
(13:56):
genetic mutations, and after thousands of generations pass by, you
can eventually reach a tipping point, which we refer to
as mutational meltdown. And we'll get back to mutational meltdown
in just a second. But interestingly, the namesake for Muller's
ratchet is Hermann. Joseph Muller, who lived eighteen ninety through
(14:17):
nineteen sixty seven, an American geneticis mostly known for his
work on mooda genesis and for being like an outspoken
critic and just sort of communicator on the dangers of
radioactive fallout. He won the nineteen forty six Nobel Prize
in physiology or medicine. And he was also the father
(14:38):
of mathematician and computer scientist David E. Muller, who also
has various things named after him. So you'll find a
number of things in both genetic concepts and what have you,
in genetics and mathematics that have the Muller name attached
to them.
Speaker 2 (14:57):
Now, Rob, before you suggested this, I had never heard
of mutational meltdown or Mueller's ratchet, at least as far
as I know. But one of the things that I
got really interested in here is how it violates sort
of the simple assumptions that you make when you think
about evolution on a surface level, because, of course it
makes this reference to the idea of harmful genetic mutations
(15:20):
accumulating over time in a species, and at a surface level,
you might think, well, wait a minute, why would harmful
genetic mutations accumulate? Isn't natural selection supposed to get rid
of those? And so over time, with enough enough opportunities, yes,
mutations that bring more harm than benefit to an organism's
(15:41):
ability to survive and reproduce will tend to disappear. But
under certain circumstances, bad genes can accumulate. And one of
the key concepts to understand here is what's known as
genetic drift. So genetic drift is a change in the
frequency of a particular gene variant also known as an allele,
(16:04):
in a population due to random chance rather than to
natural selection. So random genetic drift is always happening. It's
always going on in the background in the evolution of species.
While you might think of natural selection as sort of
acting in the foreground, amplifying or diminishing alleles because they
(16:25):
are helpful or harmful. So you might think of, say
a gene for blue feathers in some kind of bird,
that gene might increase in the population, not for any
reason having to do with blue feathers making the bird
survive or reproduce more. Maybe it's just you know, sheer
(16:47):
luck one season. Or maybe there might be some kind
of random thing that happens in the popular lakes, maybe
a big population of blue feathered individuals come across a
big cache of food or something, or there is just
the the standard fluctuations in the sampling rate of the
different alleles that get recombined in sexual reproduction. The smaller
(17:08):
a population is, the more likely it is to be
irreversibly changed by random trends in genetic drift. Now you
might wonder, how would that work. If the trends in
genetic drift are just random, it's just chance, how would
that cause irreversible changes. I think one way you might
be able to compare this is if you think about gambling. Okay,
(17:32):
imagine you're making bets on somebody flipping a coin. If
you have an infinite pot of money to bet with,
you could just keep doing this forever, right Like, you
might get a run of good luck. You might get
a run of bad luck. You might call the coin wrong.
You know, I don't know how many times it would
be plausible eight times in a row and lose a
lot of money. But eventually, on average, you'd have a
(17:54):
winning streak again, and you'd win your money back as
long as you can keep gambling, as long as you've
got like an infinite pot to play from. But if
you are gambling with a fixed amount of money, you
eventually will hit a random run of bad luck and
lose it all. You will play to extinction.
Speaker 1 (18:13):
Very fitting, very fitting.
Speaker 2 (18:15):
So for my analogy here, you could compare the size
of your purse you're going in to gamble with with
the size of the population where the random genetic drift
is happening. Genetic drift in a small population can easily
drive certain alleles extinct, even though those alleles had no
negative effect on survival. The other side of the other
(18:38):
side of that is that in small populations, random genetic
drift can also do the inverse. It can take an
allele and make it the only version of that gene
left in the population, present in one hundred percent of individuals.
And there's a term for this, The population genetics term
for when an allele becomes present in the entire population
(19:01):
is fixation. When that allele is the only version of
that gene left, it is said to be fixed in
the population. Everybody's got it. And of course, once a
gene variant is fixed in a population, of course, that
means the individuals in that population are stuck with it,
you know, unless there is new information introduced. Now that
(19:22):
could be maybe a random mutation causes a new version
of that gene to appear and then it can maybe compete,
or there is inflow of new alleles of that gene,
maybe by interbreeding with another population or something like that.
But for a closed population, once a gene variant is fixed,
they're stuck with it. Now, the important thing to realize
(19:54):
is that alleles don't have to be the best version
of that gene. They don't have to be helpful to
survival or reproduction in order to become fixed in a population.
In big populations, harmful versions of genes will not tend
to dominate over time. They will tend to get removed
(20:14):
or remain in the background. But in small populations, because
you're essentially gambling with a small purse, those deleterious alleles
can become fixed just through bad luck. So you imagine,
maybe every season within a population, you pick a randomly
assorted number of the individuals in that population, You say,
(20:35):
whichever allele they've got, make another copy of that one,
and then you just keep doing that over and over.
You can get random results where suddenly a gene that's
not very good for the population is suddenly the only
one left. So that's how genetic drift can cause deleterious,
(20:55):
harmful genes to become fixed in a population. But I
was wondering, Okay, so what's the deal with this idea
of mutational meltdown. What's happening there? Well, I was reading
about this in a in a biology textbook I found
called Practical Conservation Biology edited by David Lindenmeyer and Mark Bergmann.
(21:15):
And you know, one of the things that the authors
mention is that every population carries some load in the
background of deleterious recessive genes. But the core theory of
mutational meltdown again, it's something that really applies in particular
to small populations. That's where it's really dangerous. The author's
(21:38):
write quote. In small populations, the dominant genetic process is drift.
If the size of the breeding population is very small,
then random drift can overwhelm natural selection and a population
can accumulate and become fixed for quite deleterious mutations. If
the decline in fitness that results from the accumulation of
(21:59):
new mutations reduces fecundity, so it reduces birth rates and
reduces survival to the extent that the population declines, feedback
between random genetic drift and mutation is set in motion.
As the population size decreases, random genetic drift becomes a
(22:20):
more significant force, and the rate of fixation of deleterious
mutations increases, further reducing population size. So it is this
feedback loop between the harmful mutations making the population smaller
and thus increasing the effects of genetic drift compared to
the effects of selection forces.
Speaker 1 (22:43):
Yeah, so at first you just have one wrong turn movie,
and then you have two wrong turn movies, and before
you know it, there's like twenty of them and you
haven't seen a single one, but you know that they
all have something to do with mutated hillbillies.
Speaker 2 (22:57):
Yes, it's a vicious cycle of some kind. And as
a side note, by the way, this is not relevant
to most of the species we'd be talking about, but
just because I thought it was interesting. The authors in
the context of this conservation biology book also mention how
this applies in captive populations in a conservation context. So
(23:17):
because captive populations of animals where you know there's concern
for the species level survival, because those might those animals
are not really competing in the wild to survive, It
is very easy, in fact, for them to accumulate deleterious
mutations in their genome because you have this genetic drift factor.
(23:40):
But then also the normal selection pressures are not really
applying at all, so once the population is reintroduced into
the wild, the build up of all these deleterious mutations
acquired through genetic drift can be quite harsh, and they
say that this could explain some examples of basically poor
(24:01):
performance of captive bread individuals of endangered species after being
released into the wild.
Speaker 1 (24:08):
Yeah, there's so many factors to take into account with
captive populations, because, yeah, on top of everything you just
talked about, there's also the idea that some species will
just then spontaneously asexually produce offspring, which of course is
not going to that particular offspring is not going to
be genetically diversified either, So yeah, you have this huge
(24:31):
bottleneck potential.
Speaker 2 (24:32):
There one last thing from that book. The most common
citations I see for the theoretical work on mutational meltdown
are attributed to papers by Lynch published in the nineties
in the nineteen nineties, but they do note also in
this book chapter that there have been some studies that
looked for so that's the theoretical work by Lynch, but
(24:54):
there were some studies that look to try to find
evidence of what they call greater genetic loads accumulations of
mutations in small fruit fly populations. This was cited to
Gilligan at all in two thousand and five, and they
didn't find it. They didn't find evidence of these of
(25:14):
these loads they expected. So I guess some questions about
how the theory of mutational meltdown actually applies to populations
in the wild.
Speaker 1 (25:23):
Yeah, yeah, it's my understanding that, Yeah, we are dealing
with theories here, and there is a continued challenge for
evolutionary biologies to find examples and potential examples of all
of this and to define these breakthrough examples. It will
help us better understand not only this whole question of
potential mutational meltdown, but also just sort of a larger
(25:45):
question again of like why is sexual reproduction more beneficial
or seemingly more beneficial? Like why sexual reproduction at all?
But anyway, as I understand it, based on what we're
looking at here, yeah, we have Mueller's which is the
theoretical process that then could bring us to this end
(26:07):
game of mutational meltdown. Mutational meltdown in this regard would
be considered a subclass of an extinction vortex. Extinction vortex
is a larger classification entailing different environmental, genetic, and demographic factors.
It's also worth noting and perhaps inflating the obvious here,
(26:27):
and that is that extinction is in the long term inevitable.
All species eventually face extinction, and I've read that something
like more than ninety nine percent of all species to
ever exist have gone extinct. Again, this is stuff that
makes perfect sense when you spell it out, but also
it can sort of mess with your short term, short,
(26:48):
short lived human brain when you start again thinking about
the really long term history of life on Earth. So,
of course, one of the big obvious challenges to exploring
all of this is that humans have only been around
on Earth and in a position to look for examples
of things like mutational meltdown for a very short period
(27:08):
of time. And if most asexual species or populations don't
last very long, do you know, theoretically to Muller's ratchet
or to the stability of sexual reproduction outlined and things
like the red queen hypothesis, then the various examples of
ancient asexual species that we have that are more easy,
(27:32):
you know, to look to, those are going to be
exceptions to the rule. And then this creates additional additional
questions arise, well, how has this asexual species been able
to survive these challenges, these rigors that we're identifying in
the data here. And you know, one of the sources
I was looking at two thousand and eights quantifying the
threat of extinction from Mueller's ratchet in the diploid Amazon
(27:55):
molly This is from Low and Lamach. They point out that, yeah,
these species are of considerable interest to researchers for these
very reasons.
Speaker 2 (28:05):
That would be the Amazon molli.
Speaker 1 (28:08):
Well, just in general, these sorts of species species that.
Speaker 2 (28:11):
Oh I see, yeah, ancient asexual species.
Speaker 1 (28:14):
Sorry, right, in this particular paper, this particular paper that
the main focus the Amazon mollie, though, is also really interesting.
This is a small asexual fish species that seems just
prime for mutational meltdown. However, in modeling out the rate
of harmful mutations in the species, they ran into what
they referred to in the paper as a genomic decay paradox.
(28:37):
So in most of the models they ran, the expected
time to extinction for this species was less than previous
estimates on the age of the species, So it would
seem that the species has outlived its genomic expiration date.
If Mueller's ratchet and mutational meltdown is indeed a factor
(29:01):
the author's right quote, several biological processes can individually or
in combination solve this genomic decay paradox, including paternal leakage
of undamaged DNA from sexual sister species, compensatory mutations, and
many others, and they, of course conclude that more research
is ultimately required. Another paper that looks into all this
(29:23):
that I found quite interesting was Deleterious mutation Accumulation in
Asexual Tymema stick Insects by Henry at All, published in
twenty twelve in Molecular Biology and Evolution. In this paper,
the researchers look at six independently derived asexual lineages and
related sexual species of the temma stick insects. So we're
(29:47):
talking about closely related species, some that reproduce sexually and
others that reproduce asexually. The idea here, of course, is
the closeness. They're the related closely related to each other,
so this would make the accumulation of deletarious mutations stand
out more in the asexual species versus the sexual species,
(30:08):
and that seems to be what they found. Quote. We
found signatures of increased coding mutation accumulation in all six
asexual tymema and for each of the three analyzed genes,
with three point six to thirteen point four fold higher
rates in the asexuals as compared with the sexuals. They
also point out that the coding mutations and the asexuals
(30:31):
are likely associated with more strongly deletarious effects than the
sexuals due to some specific molecular reasons that they outline
in the article. They conclude that quote deletarious mutation accumulation
can differentially affect sexual and asexual lineages and support the
idea that deletarious mutation accumulation plays an important role in
(30:54):
limiting the long term persistence of all female lineages.
Speaker 2 (30:58):
So, according to this, as we were alluding to earlier,
a species that's mainly reproducing or totally reproducing asexually and
just making clonal copies will will tend to one of
the pressures acting against it will be the tendency to
build up loads of mutations that are not helpful to survival.
Speaker 1 (31:19):
Yeah, so over time, worse mutations accumulate, and the asexual
species who do not diversify via sexual recombination, they don't
purify through purging harmful mutations via sexual reproduction either and
in fact, the authors here specifically mentioned that sexual reproduction
enhances the efficiency of purifying selection. This is fascinating, It's
(31:41):
not I mean, certainly the authors are not arguing that
this is the case, but it's obviously this is not
like a smoking gun for the whole idea here, but
it does seem to give us some interesting evidence to
back up some of these ideas, though of course also
raising additional questions about you know, what exactly going on. Now.
(32:10):
There's a I've mentioned ted Ad before. There's a great
ted ad video titled no Sex, No Problem, and I
highly recommend checking that out. It has a nice overview
of sort of the different the different strategies of asexual
versus sexual reproduction, and and and briefly mentioned some of
the concepts we're talking about here. Uh. One thing that
(32:31):
I thought was interesting in this videos it points out
that pa fits are a great example of an organism
that utilizes both sexual reproduction and asexual reproduction. Uh Uh.
But but depending on what the circumstances are, So with
these particular a fits, when it's springtime, they are asexual reproducers.
(32:54):
So it's like it's this is the these are the
fat times, Like it's it's time to feed, it's time
to red it's not time to worry too much about,
you know, differentiating your product. It's about just getting product
on the shelves, and so that's what they do. But
then when autumn rolls around, then it's time for sexual reproduction.
So it's like, Okay, this is our time to think
(33:16):
about the product. This is our time to get experimental
and see what we can do to change up our
offering for the next season. So I thought that was
just a really really interesting, like single species example that
kind of sums up some of the benefits and some
of the costs involved with asexual versus sexual reproduction, Like
(33:39):
this is not the It's kind of like when you
think about films in a series, for example, when it's
time to make Wrong Turn two, you're not necessarily thinking about, well,
how am I going to recreate? No, you don't recreate.
You just do what worked the first time, accept more
of it. This is the springtime of the wrong term franchise.
Much later, when it's run out of gas, that's when
you can you can sit down and think, yeah, that's
(34:00):
when you can be like, how do we reanalyze this,
how do we reconceptualize Wrong Turn for a new audience?
And maybe we can hire Matthew Modine to be in
it too, and.
Speaker 2 (34:10):
It makes sense they both be part of your content strategy,
you know. Sometimes you do reruns, sometimes you do a
crossover event.
Speaker 1 (34:16):
Yeah. No, I haven't actually seen a Wrong Turn movie,
So please don't go out and see these movies just
based on me casually mention them.
Speaker 2 (34:24):
Here Rob wrongly recommends the Wrong Turn franchise. I can't
remember if I have or not. Is it is that
the one there's like a guy in a muscle car
who drives into the woods and then they meet some
I don't know some people and they get chased by
dudes with hatchets.
Speaker 1 (34:39):
That sounds likely. I think that it's basically it's the
hills have Eyes except in the woods. And there's like
a million of these films. It's one of there's something
always kind of alarming to me when I realized there's
like a whole franchise that has been around for years
and years and I just not only have I not
seen them, but I just have just a very surface
level understanding of what they're about, you know, like I've
(35:02):
maybe never even seen a trailer for one of them.
Speaker 2 (35:05):
Yeah, there are a lot of series like that, and
I understand what you mean, Like it can be alarming, Like, oh,
I didn't even see the first Purge. We're on Purged
nine now, this is Yeah, I don't know what's going on.
I kind of can't start at this point. I'm not
going to see these movies.
Speaker 1 (35:21):
Yeah, the Purge franchise, which I haven't seen any of
those either, but I've read a bit more about them,
so I'm kind of intrigued by the way it has
survived thus far. It seems like it is a franchise
that definitely has its springtime and autumn cycles of how
it puts out new content, Like some of these seem
like definite, like Okay, it's time for another Purge, and
(35:43):
then other times it's like what can we do different
with the Purge this time? And then it's like cut
that we're doing a TV series, so just like ten
the Purges, and then we'll work about innovating after that.
Speaker 2 (35:54):
I like that you have read about the Purge. I
haven't seen it, but you've on some research. Well, you know, it.
Speaker 1 (36:03):
Feels like it has more of a you know, you
got to stay on top of culture, so you got
to read about the Purge, whereas somehow Wrong Turn movies
maybe were less important culturally, or so it seems to me.
Speaker 2 (36:15):
Wrong Turn movies, i'd say, are less high concept because
Purge has an elevator pitch right there, unless I misunderstand
the idea is all crime is legal on one night.
Speaker 1 (36:24):
Yeah, yeah, okay, so, and I think it lends itself
well to referencing. You can be like, oh, wow, I
tried to drive across town the other day and it
was like the Purge out there. You know, that makes sense.
It's like you're saying something about how bad traffic was.
But I don't know, Wrong Turn franchises maybe just a
little harder to you know, bring into your daily life.
Speaker 2 (36:45):
I guess some organisms also have more of an elevator
pitch quality to them, though, you know, like the platypus.
It is it is a furry, poisonous duck.
Speaker 1 (36:56):
But it's also kind of high concept.
Speaker 2 (36:58):
Yeah that's what I'm saying. Yeah, it's high concept.
Speaker 1 (37:01):
Yeah, good creature. Have we ever done an episode on
the Platypus? I can't recall. Of course, it's diversified enough
that it's inevitably come up at least in passing in
any number of episodes.
Speaker 2 (37:13):
I don't know if we have. I just really I
said poisonous, but I think the correct word would be venomous.
I don't know. We'll have to sort that out later.
Speaker 1 (37:22):
All right, Well, on that note, I think we have
we have reached mutational meltdown for this episode. But we'd
love to hear from everyone out there. I mean, especially
if there's anyone out there who is in the field
of evolutionary biology. Perhaps you have some additional feedback, additional
examples you'd like to bring to mind.
Speaker 2 (37:43):
Let us know.
Speaker 1 (37:43):
You know, this is a topic that it caught my attention,
but I'd love to see some more data on it.
I'd love to see some more studies of note. In
the meantime, will remind you that Stuff to Blow Your
Mind is primarily a science podcast, with core episodes on
Tuesdays and Thursday Days. On Mondays we do listener mail,
on Wednesdays we do a short form artifact or monster fact,
(38:05):
and on Fridays we set aside most serious concerns to
just talk about a weird film on Weird House Cinema.
That's usually where our discussions of films about mutants would
wind up, but sometimes those mutations accumulate in the core
episodes as well.
Speaker 2 (38:21):
Huge thanks to our audio producer JJ Posway. If you
would like to get in touch with us with feedback
on this episode or any other, to suggest a topic
for the future, or just to say hello, you can
email us at contact at stuff to Blow your Mind
dot com.
Speaker 3 (38:42):
Stuff to Blow Your Mind is production of iHeartRadio. For
more podcasts from my Heart Radio, visit the iHeartRadio app,
Apple Podcasts, or wherever you listen to your favorite shows.
Speaker 1 (39:00):
Said to wait. We had to write, to write, Tote
tot
Hey, you welcome to Stuff to Blow Your Mind. My
name is Robert.
Speaker 2 (00:09):
Lamb and I am Joe McCormick, and today we are
bringing you an episode from the vault. This one originally
published April twentieth, twenty twenty three, and it is called
Mutational Meltdown.
Speaker 1 (00:22):
Yeah, this is one that I was drawn to initially
solely because of the title Mutational Meltdown, How can you resist?
But there's a lot of fascinating content in there as well,
beyond just sort of the knee jerk feel of imagining
some sort of mutant melting in some sort of a
sci fi context. Let's jump right in.
Speaker 3 (00:45):
Welcome to Stuff to Blow Your Mind, production of iHeartRadio.
Speaker 1 (00:55):
Hey you welcome to Stuff to Blow your Mind. My
name is Robert.
Speaker 2 (00:58):
Lamb and I'm Joe McCormick, and today we're going to
be talking about a concept in the realm of genetics
and reproduction, a concept known as mutational meltdown. Very enticing name, Rob.
I understand you became interested in mutational meltdown earlier this week.
(01:19):
What got you going on this?
Speaker 1 (01:21):
Well, it actually didn't have anything to do directly with
any melt movies. We might have been talking about on
Weird House Cinema. I actually, I think I was on
a walk with my family and I said, Hey, I
think we're going to need an episode for Thursday. What
should we do it on? And there my wife and
my son are like, Oh, you should do it on
asexual reproduction. So okay, let's just started looking around a
(01:44):
little bit. And yeah, this particular term kind of jumped
out at me. I wasn't familiar with it, and it
basically gets down into and I think for our purposes
here on the show, you know, it's a reason to
sort of provide an overview of sort of asexual reproduction
versus sexual reproduction as sort of competing ways of going
about sort of the same thing for an organism, but
(02:08):
one with more short term benefits versus long term benefits.
And I don't know, I just found it to be
kind of a neat way to re examine and think
about these these concepts that I imagine we've covered on
the show before, and many of you out there have
have encountered in varying formats.
Speaker 2 (02:29):
Sure well, I know over the years we have alluded
to the big question in biology of like where sex
comes from, the where when and why of sexual reproductions
as a part of the history of organisms on planet Earth.
Not going to solve that problem today, but yeah, I
think maybe this little subtopic could help shed a little
(02:50):
bit of light there.
Speaker 1 (02:52):
Yeah, so let's let's start with the basics. Though, we're
going to just approach it as if you know, you're
not really familiar with any of the topics that we're
discussing here, So asexual reproduction versus sexual reproduction on a
very basic level, here's how it all goes down. So,
with sexual reproduction, you have the offspring of two genetic
(03:13):
parents inheriting a mix of genes from those parents, genetically
distinguishing itself from either parent. The resulting genetic variation is
highly adaptive because it provides individuals with varying traits that
may prove necessary for survival in an ever changing environment.
The resulting genetic diversity makes the population more resistant to
(03:34):
disease as well.
Speaker 2 (03:35):
I think one of the theories we've talked about before
is that an advantage of sexual reproduction is that it
helps protect the host organism against various types of parasites
by introducing genetic variability that makes it harder for the
parasite to target each successive generation of the host.
Speaker 1 (03:56):
Yeah, this is a clumsy analogy at best, but I
can't help but think too about like to say that,
because essentially, when you're talking about asexual reproduction, you're talking
essentially about making a clone of oneself. And so the
clone army in the Star Wars prequels highly susceptible to say,
a single order coming out and telling them to turn
(04:18):
on the Jedi, that sort of thing. But that's just
a very very rough idea of how to think about it.
But more specifically for our purposes here, another key benefit
that comes up in the literature is looking at is
that you can think of sex and genetic recombination is
ultimately a means of purging deletarious mutations.
Speaker 2 (04:41):
Right, so the impact of mutations that might be harmful
to the organism can be blunted by sexual recombination. Yeah.
Speaker 1 (04:49):
Yeah, So you end up with this, I mean, roughly speaking,
you know, you have kind of like a randomization of
these different traits, and the individuals that end up the
offspring with that end up with the the negative traits,
the harmful traits. They don't survive the ones that have
been purged of those mutations do survive, and therefore it
can purge the mutation from a particular lineage. Okay, all right,
(05:13):
So moving on to asexual reproduction. This is a case
in which you have the offspring of a single genetic
parent inheriting the genes of the parent, making it a
clone identical to the parent. The advantage here is that
you can reproduce rapidly without all of the energy expenditure
of mating. And I mean that's a pretty big statement
(05:33):
to think about, because so many organisms we end up
discussing on the podcast. You know, what is the key
thing that makes them interesting? Well, in some cases, many cases,
it's how they acquire their food, But in other cases
it's how do they get a mate? How do they
attract a maid or pursue a mate, And it ends
up taking up a whole lot of time, a whole
lot of energy. And what if you didn't have to
(05:55):
do that? What if instead you could just essentially clone yourself.
Speaker 2 (06:00):
Convenient and safer in a lot of cases, because I
mean it varies by organism, but in many cases, yeah,
if you have to go seeking out a mate, it
is not only you know, an energy expense to go
looking around, but you're also often removing yourself from safe
locations and going into dangerous ones. Yeah.
Speaker 1 (06:19):
I mean it's kind of like when you get some
sort of new kit to a symbols of Ikia furniture, right,
and the first thing you notice is that on the
instructions it says, oh, you have to have two people
to do this, and you're like, oh, that totally recks
my day. Now, I've got to get my significant other
or a friend to help with this. We've got to
align our schedules, and we have to both work together
(06:39):
to build this thing, as opposed to one where I
can just build it myself and put it where it
needs to go in the house. Now, there are multiple
types of asexual reproduction, and we're not going to go
into all of them, but you have all sorts of
things like asexual budding and so forth. The sources I
was looking at dealt a lot with parthenogenis, which occurs
(07:00):
widely and invertebrates. This word stems from the Greek for
virgin creation parthenos plus genesis.
Speaker 2 (07:08):
Okay, so this would describe, for example, a lot of
vertebrates like maybe some lizards or fish that can give
birth without ever having without ever having their game meets
fertilized by a member of the opposite sex.
Speaker 1 (07:21):
Yeah, yeah, we're talking about lizards, geckos, various insects, particularly
some sharks. And it's of course very important to note
that there are obligates sexual reproducers and then they're obligate
asexual reproducers. But then there are also organisms that can
do either depending on environmental pressure. So a classic example
(07:44):
of a sexually reproducing organism engaging in asexual reproduction is,
of course, when an individual cannot find a mate. It's
kind of there as I guess you could think of
it as kind of a backup plan that or some
sort of a you know, an emergency button that can
be pushed. And this has been the case with some
of the famous examples of say sharks or lizards such
(08:06):
as the Komodo dragon reproducing in captivity, these so called
virgin births that will suddenly occur in shock zoo keepers.
Speaker 2 (08:15):
So the ideal is to mix and match your genetic
material with somebody else's, but in a pinch, you could
just make a copy of yourself if you're.
Speaker 1 (08:26):
The right species correct, Yeah, and if I'm remembering correctly,
This also pops up in the plot of Jurassic Park, right,
something to do with the way that they're recreating dinosaur
DNA using amphibian DNA.
Speaker 2 (08:36):
Well, I don't know if this is parthenogenesis or if
it would be different. I think what they say, at
least in the movie, I don't remember what happens in
the book. In the movie they say that because they
use some frog DNA to cover up patches in the
DNA sequence. I'm just recalling from memory what mister DNA
tells us that some frogs are able to spontaneously change
(08:59):
sex in a single sex environment, and thus, even though
all of the dinosaurs in the park were supposed to
be female, some changed into males, and thus we're sexually reproducing.
Speaker 1 (09:09):
Ah okay, I think that's the main thing. I'm either
misremembering that, or maybe there's something from one of the
later like Jurassic World films that I'm only like half
processing here. All Right, So you have these two basic
ways of reproducing, then this of course means that there
are drawbacks to either one. So in sexual reproduction again,
you got to put a whole lot of energy and
(09:30):
time into mating behaviors. It necessitates the existence of males,
which in some cases like do little or nothing else, Like,
you know, an entire division of the species just for reproduction.
Mating can prove fatal in and of itself, not necessarily
in a way that actually has any impact on the species.
(09:53):
But still it's like the again, you get into these
situations where the male's whole role is reproduction and then
afterwards it has no purpose except maybe death. And it
can of course also can be nutrition, could be nutrition. Yeah,
so it's not a complete ways. But also just mating
in general creates opportunities for predators in a number of ways.
(10:14):
You could it could be something very specific, like well,
while you're mating, it's possible that something could could prey
on you. But also again, just think of all the
the links that creatures end up going to inmate selection
and so forth. Various examples of this, even if it's
just say sexual dimorphism could mean that one member of
(10:35):
the species is more likely to be consumed than the other.
Speaker 2 (10:38):
Yeah, it makes me think about all of the I
don't know, like birds that essentially where male birds are
trying to attract mates, specifically by being conspicuous. Yeah, you
got to think that that also that comes with some
amount of predation risk, at least in many cases.
Speaker 1 (10:56):
Yeah, another thing could be a particular places you have
to travel to in order to engage in the mating,
et cetera. But another drawback to sexual reproduction is that
if it's your only option, it means that isolated members
just of a particular species or population just cannot reproduce.
And it also means that sufficiently reduced populations are just
(11:19):
already at a dead end. Now in asexual reproduction, there's
also a potential dead end there as well, because if
you don't have genetic variation occurring, if you're basically just
putting out the same model after the same model, after
the same model, it may well improve, it may well
prove impossible for the species to adapt or to change.
(11:42):
So it's you know, if you're just putting out the
same model after the same model, and like the market
is the same for that product, then I guess you
don't have anything to worry about so long as the
market doesn't change. It's suddenly, if the demand for a
particular you know, toy or item we were to alter
in some way and you couldn't alter the product, then
(12:03):
you'd be in trouble. And the same goes for any
kind of biological form. What happens when say, things begin
to dry up, or there's warming or cooling, or whatever
the case may be. Sexual reproduction is what gives you
the ability to bust out these different variations on the
genetic code that could prove adaptive to change. Yeah, it
(12:27):
gives you options, diversity, Yeah, yeah, diversifies your portfolio. Now,
we mentioned disease and parasites already, so that's very much
the case. If you just have a whole bunch of clones,
then they all have the same susceptibility to illness or parasites. Overall,
the big drawback is just a lack of genetic diversity,
(12:50):
which can also result in the accumulation of harmful mutations.
And another thing about the difference between the two though,
that I guess I hadn't really thought about too much,
is that it being a difference between short term and
long term benefits. So, asexual reproduction is great for rapidly
growing a population during a time of plenty, but the
resulting population can run into problems long term. Meanwhile, sexual
(13:13):
reproduction requires more energy and time, but generates diversity that
may come in handy in the long term again when
there are changes and obstacles that arise. Anyway, coming back
to this idea that via asexual reproduction you can have
this accumulation of harmful genetic changes. This brings us to
(13:35):
the topic of Mueller's ratchet, which is not something I
was familiar with previously. The basic theory here is that
long term reproduction, particularly a sexual reproduction, but some of
the studies we're looking at they're also looking at it
with sexual reproduction. Basically, you see this accumulation of harmful
(13:56):
genetic mutations, and after thousands of generations pass by, you
can eventually reach a tipping point, which we refer to
as mutational meltdown. And we'll get back to mutational meltdown
in just a second. But interestingly, the namesake for Muller's
ratchet is Hermann. Joseph Muller, who lived eighteen ninety through
(14:17):
nineteen sixty seven, an American geneticis mostly known for his
work on mooda genesis and for being like an outspoken
critic and just sort of communicator on the dangers of
radioactive fallout. He won the nineteen forty six Nobel Prize
in physiology or medicine. And he was also the father
(14:38):
of mathematician and computer scientist David E. Muller, who also
has various things named after him. So you'll find a
number of things in both genetic concepts and what have you,
in genetics and mathematics that have the Muller name attached
to them.
Speaker 2 (14:57):
Now, Rob, before you suggested this, I had never heard
of mutational meltdown or Mueller's ratchet, at least as far
as I know. But one of the things that I
got really interested in here is how it violates sort
of the simple assumptions that you make when you think
about evolution on a surface level, because, of course it
makes this reference to the idea of harmful genetic mutations
(15:20):
accumulating over time in a species, and at a surface level,
you might think, well, wait a minute, why would harmful
genetic mutations accumulate? Isn't natural selection supposed to get rid
of those? And so over time, with enough enough opportunities, yes,
mutations that bring more harm than benefit to an organism's
(15:41):
ability to survive and reproduce will tend to disappear. But
under certain circumstances, bad genes can accumulate. And one of
the key concepts to understand here is what's known as
genetic drift. So genetic drift is a change in the
frequency of a particular gene variant also known as an allele,
(16:04):
in a population due to random chance rather than to
natural selection. So random genetic drift is always happening. It's
always going on in the background in the evolution of species.
While you might think of natural selection as sort of
acting in the foreground, amplifying or diminishing alleles because they
(16:25):
are helpful or harmful. So you might think of, say
a gene for blue feathers in some kind of bird,
that gene might increase in the population, not for any
reason having to do with blue feathers making the bird
survive or reproduce more. Maybe it's just you know, sheer
(16:47):
luck one season. Or maybe there might be some kind
of random thing that happens in the popular lakes, maybe
a big population of blue feathered individuals come across a
big cache of food or something, or there is just
the the standard fluctuations in the sampling rate of the
different alleles that get recombined in sexual reproduction. The smaller
(17:08):
a population is, the more likely it is to be
irreversibly changed by random trends in genetic drift. Now you
might wonder, how would that work. If the trends in
genetic drift are just random, it's just chance, how would
that cause irreversible changes. I think one way you might
be able to compare this is if you think about gambling. Okay,
(17:32):
imagine you're making bets on somebody flipping a coin. If
you have an infinite pot of money to bet with,
you could just keep doing this forever, right Like, you
might get a run of good luck. You might get
a run of bad luck. You might call the coin wrong.
You know, I don't know how many times it would
be plausible eight times in a row and lose a
lot of money. But eventually, on average, you'd have a
(17:54):
winning streak again, and you'd win your money back as
long as you can keep gambling, as long as you've
got like an infinite pot to play from. But if
you are gambling with a fixed amount of money, you
eventually will hit a random run of bad luck and
lose it all. You will play to extinction.
Speaker 1 (18:13):
Very fitting, very fitting.
Speaker 2 (18:15):
So for my analogy here, you could compare the size
of your purse you're going in to gamble with with
the size of the population where the random genetic drift
is happening. Genetic drift in a small population can easily
drive certain alleles extinct, even though those alleles had no
negative effect on survival. The other side of the other
(18:38):
side of that is that in small populations, random genetic
drift can also do the inverse. It can take an
allele and make it the only version of that gene
left in the population, present in one hundred percent of individuals.
And there's a term for this, The population genetics term
for when an allele becomes present in the entire population
(19:01):
is fixation. When that allele is the only version of
that gene left, it is said to be fixed in
the population. Everybody's got it. And of course, once a
gene variant is fixed in a population, of course, that
means the individuals in that population are stuck with it,
you know, unless there is new information introduced. Now that
(19:22):
could be maybe a random mutation causes a new version
of that gene to appear and then it can maybe compete,
or there is inflow of new alleles of that gene,
maybe by interbreeding with another population or something like that.
But for a closed population, once a gene variant is fixed,
they're stuck with it. Now, the important thing to realize
(19:54):
is that alleles don't have to be the best version
of that gene. They don't have to be helpful to
survival or reproduction in order to become fixed in a population.
In big populations, harmful versions of genes will not tend
to dominate over time. They will tend to get removed
(20:14):
or remain in the background. But in small populations, because
you're essentially gambling with a small purse, those deleterious alleles
can become fixed just through bad luck. So you imagine,
maybe every season within a population, you pick a randomly
assorted number of the individuals in that population, You say,
(20:35):
whichever allele they've got, make another copy of that one,
and then you just keep doing that over and over.
You can get random results where suddenly a gene that's
not very good for the population is suddenly the only
one left. So that's how genetic drift can cause deleterious,
(20:55):
harmful genes to become fixed in a population. But I
was wondering, Okay, so what's the deal with this idea
of mutational meltdown. What's happening there? Well, I was reading
about this in a in a biology textbook I found
called Practical Conservation Biology edited by David Lindenmeyer and Mark Bergmann.
(21:15):
And you know, one of the things that the authors
mention is that every population carries some load in the
background of deleterious recessive genes. But the core theory of
mutational meltdown again, it's something that really applies in particular
to small populations. That's where it's really dangerous. The author's
(21:38):
write quote. In small populations, the dominant genetic process is drift.
If the size of the breeding population is very small,
then random drift can overwhelm natural selection and a population
can accumulate and become fixed for quite deleterious mutations. If
the decline in fitness that results from the accumulation of
(21:59):
new mutations reduces fecundity, so it reduces birth rates and
reduces survival to the extent that the population declines, feedback
between random genetic drift and mutation is set in motion.
As the population size decreases, random genetic drift becomes a
(22:20):
more significant force, and the rate of fixation of deleterious
mutations increases, further reducing population size. So it is this
feedback loop between the harmful mutations making the population smaller
and thus increasing the effects of genetic drift compared to
the effects of selection forces.
Speaker 1 (22:43):
Yeah, so at first you just have one wrong turn movie,
and then you have two wrong turn movies, and before
you know it, there's like twenty of them and you
haven't seen a single one, but you know that they
all have something to do with mutated hillbillies.
Speaker 2 (22:57):
Yes, it's a vicious cycle of some kind. And as
a side note, by the way, this is not relevant
to most of the species we'd be talking about, but
just because I thought it was interesting. The authors in
the context of this conservation biology book also mention how
this applies in captive populations in a conservation context. So
(23:17):
because captive populations of animals where you know there's concern
for the species level survival, because those might those animals
are not really competing in the wild to survive, It
is very easy, in fact, for them to accumulate deleterious
mutations in their genome because you have this genetic drift factor.
(23:40):
But then also the normal selection pressures are not really
applying at all, so once the population is reintroduced into
the wild, the build up of all these deleterious mutations
acquired through genetic drift can be quite harsh, and they
say that this could explain some examples of basically poor
(24:01):
performance of captive bread individuals of endangered species after being
released into the wild.
Speaker 1 (24:08):
Yeah, there's so many factors to take into account with
captive populations, because, yeah, on top of everything you just
talked about, there's also the idea that some species will
just then spontaneously asexually produce offspring, which of course is
not going to that particular offspring is not going to
be genetically diversified either, So yeah, you have this huge
(24:31):
bottleneck potential.
Speaker 2 (24:32):
There one last thing from that book. The most common
citations I see for the theoretical work on mutational meltdown
are attributed to papers by Lynch published in the nineties
in the nineteen nineties, but they do note also in
this book chapter that there have been some studies that
looked for so that's the theoretical work by Lynch, but
(24:54):
there were some studies that look to try to find
evidence of what they call greater genetic loads accumulations of
mutations in small fruit fly populations. This was cited to
Gilligan at all in two thousand and five, and they
didn't find it. They didn't find evidence of these of
(25:14):
these loads they expected. So I guess some questions about
how the theory of mutational meltdown actually applies to populations
in the wild.
Speaker 1 (25:23):
Yeah, yeah, it's my understanding that, Yeah, we are dealing
with theories here, and there is a continued challenge for
evolutionary biologies to find examples and potential examples of all
of this and to define these breakthrough examples. It will
help us better understand not only this whole question of
potential mutational meltdown, but also just sort of a larger
(25:45):
question again of like why is sexual reproduction more beneficial
or seemingly more beneficial? Like why sexual reproduction at all?
But anyway, as I understand it, based on what we're
looking at here, yeah, we have Mueller's which is the
theoretical process that then could bring us to this end
(26:07):
game of mutational meltdown. Mutational meltdown in this regard would
be considered a subclass of an extinction vortex. Extinction vortex
is a larger classification entailing different environmental, genetic, and demographic factors.
It's also worth noting and perhaps inflating the obvious here,
(26:27):
and that is that extinction is in the long term inevitable.
All species eventually face extinction, and I've read that something
like more than ninety nine percent of all species to
ever exist have gone extinct. Again, this is stuff that
makes perfect sense when you spell it out, but also
it can sort of mess with your short term, short,
(26:48):
short lived human brain when you start again thinking about
the really long term history of life on Earth. So,
of course, one of the big obvious challenges to exploring
all of this is that humans have only been around
on Earth and in a position to look for examples
of things like mutational meltdown for a very short period
(27:08):
of time. And if most asexual species or populations don't
last very long, do you know, theoretically to Muller's ratchet
or to the stability of sexual reproduction outlined and things
like the red queen hypothesis, then the various examples of
ancient asexual species that we have that are more easy,
(27:32):
you know, to look to, those are going to be
exceptions to the rule. And then this creates additional additional
questions arise, well, how has this asexual species been able
to survive these challenges, these rigors that we're identifying in
the data here. And you know, one of the sources
I was looking at two thousand and eights quantifying the
threat of extinction from Mueller's ratchet in the diploid Amazon
(27:55):
molly This is from Low and Lamach. They point out that, yeah,
these species are of considerable interest to researchers for these
very reasons.
Speaker 2 (28:05):
That would be the Amazon molli.
Speaker 1 (28:08):
Well, just in general, these sorts of species species that.
Speaker 2 (28:11):
Oh I see, yeah, ancient asexual species.
Speaker 1 (28:14):
Sorry, right, in this particular paper, this particular paper that
the main focus the Amazon mollie, though, is also really interesting.
This is a small asexual fish species that seems just
prime for mutational meltdown. However, in modeling out the rate
of harmful mutations in the species, they ran into what
they referred to in the paper as a genomic decay paradox.
(28:37):
So in most of the models they ran, the expected
time to extinction for this species was less than previous
estimates on the age of the species, So it would
seem that the species has outlived its genomic expiration date.
If Mueller's ratchet and mutational meltdown is indeed a factor
(29:01):
the author's right quote, several biological processes can individually or
in combination solve this genomic decay paradox, including paternal leakage
of undamaged DNA from sexual sister species, compensatory mutations, and
many others, and they, of course conclude that more research
is ultimately required. Another paper that looks into all this
(29:23):
that I found quite interesting was Deleterious mutation Accumulation in
Asexual Tymema stick Insects by Henry at All, published in
twenty twelve in Molecular Biology and Evolution. In this paper,
the researchers look at six independently derived asexual lineages and
related sexual species of the temma stick insects. So we're
(29:47):
talking about closely related species, some that reproduce sexually and
others that reproduce asexually. The idea here, of course, is
the closeness. They're the related closely related to each other,
so this would make the accumulation of deletarious mutations stand
out more in the asexual species versus the sexual species,
(30:08):
and that seems to be what they found. Quote. We
found signatures of increased coding mutation accumulation in all six
asexual tymema and for each of the three analyzed genes,
with three point six to thirteen point four fold higher
rates in the asexuals as compared with the sexuals. They
also point out that the coding mutations and the asexuals
(30:31):
are likely associated with more strongly deletarious effects than the
sexuals due to some specific molecular reasons that they outline
in the article. They conclude that quote deletarious mutation accumulation
can differentially affect sexual and asexual lineages and support the
idea that deletarious mutation accumulation plays an important role in
(30:54):
limiting the long term persistence of all female lineages.
Speaker 2 (30:58):
So, according to this, as we were alluding to earlier,
a species that's mainly reproducing or totally reproducing asexually and
just making clonal copies will will tend to one of
the pressures acting against it will be the tendency to
build up loads of mutations that are not helpful to survival.
Speaker 1 (31:19):
Yeah, so over time, worse mutations accumulate, and the asexual
species who do not diversify via sexual recombination, they don't
purify through purging harmful mutations via sexual reproduction either and
in fact, the authors here specifically mentioned that sexual reproduction
enhances the efficiency of purifying selection. This is fascinating, It's
(31:41):
not I mean, certainly the authors are not arguing that
this is the case, but it's obviously this is not
like a smoking gun for the whole idea here, but
it does seem to give us some interesting evidence to
back up some of these ideas, though of course also
raising additional questions about you know, what exactly going on. Now.
(32:10):
There's a I've mentioned ted Ad before. There's a great
ted ad video titled no Sex, No Problem, and I
highly recommend checking that out. It has a nice overview
of sort of the different the different strategies of asexual
versus sexual reproduction, and and and briefly mentioned some of
the concepts we're talking about here. Uh. One thing that
(32:31):
I thought was interesting in this videos it points out
that pa fits are a great example of an organism
that utilizes both sexual reproduction and asexual reproduction. Uh Uh.
But but depending on what the circumstances are, So with
these particular a fits, when it's springtime, they are asexual reproducers.
(32:54):
So it's like it's this is the these are the
fat times, Like it's it's time to feed, it's time
to red it's not time to worry too much about,
you know, differentiating your product. It's about just getting product
on the shelves, and so that's what they do. But
then when autumn rolls around, then it's time for sexual reproduction.
So it's like, Okay, this is our time to think
(33:16):
about the product. This is our time to get experimental
and see what we can do to change up our
offering for the next season. So I thought that was
just a really really interesting, like single species example that
kind of sums up some of the benefits and some
of the costs involved with asexual versus sexual reproduction, Like
(33:39):
this is not the It's kind of like when you
think about films in a series, for example, when it's
time to make Wrong Turn two, you're not necessarily thinking about, well,
how am I going to recreate? No, you don't recreate.
You just do what worked the first time, accept more
of it. This is the springtime of the wrong term franchise.
Much later, when it's run out of gas, that's when
you can you can sit down and think, yeah, that's
(34:00):
when you can be like, how do we reanalyze this,
how do we reconceptualize Wrong Turn for a new audience?
And maybe we can hire Matthew Modine to be in
it too, and.
Speaker 2 (34:10):
It makes sense they both be part of your content strategy,
you know. Sometimes you do reruns, sometimes you do a
crossover event.
Speaker 1 (34:16):
Yeah. No, I haven't actually seen a Wrong Turn movie,
So please don't go out and see these movies just
based on me casually mention them.
Speaker 2 (34:24):
Here Rob wrongly recommends the Wrong Turn franchise. I can't
remember if I have or not. Is it is that
the one there's like a guy in a muscle car
who drives into the woods and then they meet some
I don't know some people and they get chased by
dudes with hatchets.
Speaker 1 (34:39):
That sounds likely. I think that it's basically it's the
hills have Eyes except in the woods. And there's like
a million of these films. It's one of there's something
always kind of alarming to me when I realized there's
like a whole franchise that has been around for years
and years and I just not only have I not
seen them, but I just have just a very surface
level understanding of what they're about, you know, like I've
(35:02):
maybe never even seen a trailer for one of them.
Speaker 2 (35:05):
Yeah, there are a lot of series like that, and
I understand what you mean, Like it can be alarming, Like, oh,
I didn't even see the first Purge. We're on Purged
nine now, this is Yeah, I don't know what's going on.
I kind of can't start at this point. I'm not
going to see these movies.
Speaker 1 (35:21):
Yeah, the Purge franchise, which I haven't seen any of
those either, but I've read a bit more about them,
so I'm kind of intrigued by the way it has
survived thus far. It seems like it is a franchise
that definitely has its springtime and autumn cycles of how
it puts out new content, Like some of these seem
like definite, like Okay, it's time for another Purge, and
(35:43):
then other times it's like what can we do different
with the Purge this time? And then it's like cut
that we're doing a TV series, so just like ten
the Purges, and then we'll work about innovating after that.
Speaker 2 (35:54):
I like that you have read about the Purge. I
haven't seen it, but you've on some research. Well, you know, it.
Speaker 1 (36:03):
Feels like it has more of a you know, you
got to stay on top of culture, so you got
to read about the Purge, whereas somehow Wrong Turn movies
maybe were less important culturally, or so it seems to me.
Speaker 2 (36:15):
Wrong Turn movies, i'd say, are less high concept because
Purge has an elevator pitch right there, unless I misunderstand
the idea is all crime is legal on one night.
Speaker 1 (36:24):
Yeah, yeah, okay, so, and I think it lends itself
well to referencing. You can be like, oh, wow, I
tried to drive across town the other day and it
was like the Purge out there. You know, that makes sense.
It's like you're saying something about how bad traffic was.
But I don't know, Wrong Turn franchises maybe just a
little harder to you know, bring into your daily life.
Speaker 2 (36:45):
I guess some organisms also have more of an elevator
pitch quality to them, though, you know, like the platypus.
It is it is a furry, poisonous duck.
Speaker 1 (36:56):
But it's also kind of high concept.
Speaker 2 (36:58):
Yeah that's what I'm saying. Yeah, it's high concept.
Speaker 1 (37:01):
Yeah, good creature. Have we ever done an episode on
the Platypus? I can't recall. Of course, it's diversified enough
that it's inevitably come up at least in passing in
any number of episodes.
Speaker 2 (37:13):
I don't know if we have. I just really I
said poisonous, but I think the correct word would be venomous.
I don't know. We'll have to sort that out later.
Speaker 1 (37:22):
All right, Well, on that note, I think we have
we have reached mutational meltdown for this episode. But we'd
love to hear from everyone out there. I mean, especially
if there's anyone out there who is in the field
of evolutionary biology. Perhaps you have some additional feedback, additional
examples you'd like to bring to mind.
Speaker 2 (37:43):
Let us know.
Speaker 1 (37:43):
You know, this is a topic that it caught my attention,
but I'd love to see some more data on it.
I'd love to see some more studies of note. In
the meantime, will remind you that Stuff to Blow Your
Mind is primarily a science podcast, with core episodes on
Tuesdays and Thursday Days. On Mondays we do listener mail,
on Wednesdays we do a short form artifact or monster fact,
(38:05):
and on Fridays we set aside most serious concerns to
just talk about a weird film on Weird House Cinema.
That's usually where our discussions of films about mutants would
wind up, but sometimes those mutations accumulate in the core
episodes as well.
Speaker 2 (38:21):
Huge thanks to our audio producer JJ Posway. If you
would like to get in touch with us with feedback
on this episode or any other, to suggest a topic
for the future, or just to say hello, you can
email us at contact at stuff to Blow your Mind
dot com.
Speaker 3 (38:42):
Stuff to Blow Your Mind is production of iHeartRadio. For
more podcasts from my Heart Radio, visit the iHeartRadio app,
Apple Podcasts, or wherever you listen to your favorite shows.
Speaker 1 (39:00):
Said to wait. We had to write, to write, Tote
tot
Stuff To Blow Your Mind News
Hosts And Creators
Robert Lamb
Joe McCormick
Popular Podcasts
Dateline NBC
Current and classic episodes, featuring compelling true-crime mysteries, powerful documentaries and in-depth investigations.
The Nikki Glaser Podcast
Every week comedian and infamous roaster Nikki Glaser provides a fun, fast-paced, and brutally honest look into current pop-culture and her own personal life.
Stuff You Should Know
If you've ever wanted to know about champagne, satanism, the Stonewall Uprising, chaos theory, LSD, El Nino, true crime and Rosa Parks, then look no further. Josh and Chuck have you covered.