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Evolution's Binary Choice - Why 2 Sexes Are Better Than 3 or 5 or 1000 | Future IQ

4,609 views Wait, is this logic right? • Oct 03, 2025
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Why are there only two sexes in humans and animals? In this episode, we explore the fascinating evolutionary and mathematical reasons behind the existence of just two sexes, and why sexual reproduction is so much more complex than simple bacterial splitting. You’ll discover how variation and diversity drive survival, why eggs and sperm evolved to be so different, how mitochondria shaped this process, and why evolution picked exactly two sexes instead of three or more. We also look at examples from yeast and plants, the problem of self-pollination, and how men and women’s bodies diverged through evolutionary pressures including the surprising Fisherian runaway effect. If you’ve ever wondered “Why does sex even exist?” this episode has the answers.

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Chapters
00:00 Intro
01:08 Why are there 2 sexes?
06:14 Why can't bacteria crossover?
08:22 Why do multicellular organisms have 2 different sexes?
13:22 What's the scientific proof?!
16:10 But why do they HAVE to be different?
19:10 Why do the male and female look so different?

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Transcript

Why are there two sexes and why are they so different? Right? I mean, wouldn't life be so much simpler if there weren't two different sexes, right? Okay. Evolutionarily, it causes a lot of complications and we just accept this fact, right? So, we need to question that's what this channel is about, right? Just confirming we are talking biology and not any of the other political discussions. We are going to talk about biology and evolution and mathematics. There are fascinating mathematical reasons why there are two sexes and not three or four or seven or just one. Okay, fair enough.
So, and during this journey, we are going to talk about bacteria. We will talk about plants and flowers and birds and bees. Okay, birds and bees. Really, we going to have the birds and bees. No, no, no, no, no, no. So, this episode is safe for any 13year-old or above child who has been introduced to reproduction in school. Okay. Ah, okay. I was worried we were going to have birds and bees talks before the parents had a chance to have the talk with their kids. But if there are any 13 year or younger kids, uh please defer this at a later date.
Yeah. Okay. So why are there two sexes? The simple answer which many of you might kind of vaguely remember from school is that this is for genetic diversity, right? For having variations in our genes. So the basic theory is that two organisms get together and they have a lot of offspring who have many different combinations of genes. Correct? Some of those will be good genes which increase chances of survival and some are bad genes which decrease chances of survival. Correct? So the offspring which have bad genes will die out. The ones with good genes will live longer and slowly over time that just means that the whole population will have better and better genes. Make sense?
Yeah. That is what we are usually taught and told in school. Yeah. This answer is too simple. Okay. Doesn't quite work because this answer doesn't require two success. Even bacteria can do this. Okay. Yeah. I mean, think about it. Okay. The way bacteria reproduce is by splitting. Correct. Right. One bacteria makes two copies of of itself. So it both sets are getting the same set of genes. Correct? Except that there is also mutation happening. Right? So the two copies get slightly different genes because of mutations.
Not only that, but bacteria have machinery in there which can increase the rate of mutation or decrease it. Right? Okay. So when they want to adapt to the environment, right? when things are bad and they're having difficulty surviving what they do is they increase the rate of mutation so that there is much more variation and as a result more of them die because of bad mutations but there will be some who survive because of good mutations. when you say more of them die the lineages die along with the bacteria also because they just can't reproduce right and also what the bacteria do is that when the environment is good the bacteria have found a set of genes that
work well in this situation then they reduce the rate of mutation and then they're just making copies with fewer changes right so all of this works yeah I mean that's what I was just thinking all of this sounds good enough it works but you're saying it's not good enough correct Because now imagine the following problem okay which the bacteria can face that there is one particular organism which has some good genes and some bad genes. Fair. Okay. The good genes are really good. So that is helping them survive.
But there are bad genes which are kind of a drag. Okay. Now what happens is that because of those good genes this particular lineage of bacteria they keep surviving and they keep growing but the bad genes also keep surviving and growing. So this is pretty much you know the bad genes are hitchhiking a ride on the good genes and the mutation system is not precise enough to just get rid of those bad genes. Right? Once in a while if it was just one bad gene then a random mutation could take it out. But usually genes are a combination of many different things. So it becomes quite difficult.
Okay. Now compare this with the sexual reproduction which is not splitting but it is called crossover. So there are two different organisms. They have two sets of DNA. And then you take half the DNA from here a randomly picked half and half the DNA from the other side. So now you have two halves. Okay. these two half cells uh which are called gameamtes but I'll just refer to them as half cells right these two half cells which have half the genes of the parent when they mix together that's when you get an offspring correct which means that you get in a single offspring you get a chance to get rid of a whole lot of genes from this side and
pick a whole lot of genes from that side right right so now just through randomness exactly all of this is still random So there is a possibility that some bad will also come. Oh yeah yeah yeah no no think about this the offspring they will have okay let's say that there are just this parent has good genes and bad genes okay and this parent is just neutral just for simplicity okay okay there will be some offspring who have both the good genes and the bad genes there'll be some offspring who have only the bad some who have only the good and some who have neither of those okay right you will notice that the good plus
bad the only bad and neither of those all three of these are inferior. Correct? Whereas the only good is clearly better than all these. So that lineage will continue. Correct. So now just through sheer randomness you produced a lot of offspring out of which only the good ones will continue and we managed to get rid of the bad. Oh, so crossover is clearly uh superior to splitting which was the original example you gave. So why can't bacteria do crossover? That's an excellent question, right? Crossovers have been discovered. They've been around for 3 billion years. So why didn't bacteria use that? Right? A couple of simple answers, right? One is that doing crossover requires much more
complex molecular machinery which the bacteria don't have. Second is that bacteria are single-sellled animals, right? So the rate of mutation for bacteria is fairly low and so they don't really need this level of sophistication. Okay, it's only when you get more complex organisms, multisellular organisms, the rate of mutation grows. Okay, and as a result, that is where it becomes necessary and important to have more sophisticated methods of getting rid of bad genes. Makes sense. Makes sense. Right. And this is where mitochondria also come in, right? The point is that multisellular animals, right? Because they're more complex, their DNA needs to have many more genes with much more instructions on how to build that complex organism, right?
So mutation rate is higher. But also because they are bigger, they need much more energy to survive, right? And that energy only comes from mitochondria, the powerhouse of the cell. Right? There is a big negative to mitochondria because it is the powerhouse of the cell. It is converting a lot of energy and as a result the side effects of that energy process, right? Is a whole bunch of oxidants which actually increase the mutation rate. They cause a whole bunch of damage. And so if you have a mitochondria, you definitely need crossover because you know there is so much more damage in the DNA that needs to be fixed. So mitochondria are providing the energy to
make this crossover happen but in the process they are also polluting the environment and therefore mutations are happening. AB: Absolutely. Yes. Correct. Okay. Uh but then the question arises why do multisellular organisms have two different sexes? Why can't they do it with just one sex unisex? Another excellent question, right? In fact, this exists really plants. There are no male plants and female plants separately, right? The same plant does have male parts and female parts. The pollen is the male part and the stigma. Stigma is the uh female part. Right?
I remember biology. Correct. But the point is that all the plants of say a rose are more or less the same type. Okay. And here any plant can mate with any other pollinate with any other plant to produce offspring. Right? You don't this particular rose doesn't have to go out searching only for female roses if you know what I mean. Okay. And so the life of a rose is much simpler like I said. What is the problem with this situation? Okay. Now for a plant like this there are two types of reproduction.
There is cross-pollination in which pollen from this plant pollinate the stigma of this plant. Correct? Or there is self-pollination where Poland from the same plant can pollinate the stigma of the plant and result in offspring which have more or less the same DNA. So it is a little bit like bacteria but it is not exactly splitting. Okay. So, so far this is clear. Somewhat. Somewhat. So, when a plant pollinates with itself, you get the same kind of plant, I'm guessing. Yeah. The DNA will be same. So, everything will be pretty much identical.
So, a red rose self-pollinating with itself will create red roses. Yeah. A red rose pollinating with a yellow rose creates an orange rose. I'm just making things up at this point. No, that that's a very rare situation, right? The more common thing is that there is this red rose which is resistant to a certain kind of pest. Okay. and this red rose which can thrive in less water. Okay? Right? So when these two combine you will get a red rose which is resistant to the pest and needs less water.
Whereas if this one just self-pollinates you will get the same rose which is resistant to the pest but doesn't have the advantages of that and vice versa. Right? So this is what is going on roughly. Got it? Okay. The problem with all of this is that cross-pollination is difficult. You literally have to involve the bees and the birds to do the cross-pollination, right? It's difficult. It takes more effort and it might not work and all of that. Whereas self-pollination is so easy that if given a choice, everybody would be doing self-pollination.
I mean, we all love ourselves. So, yeah. Right. So you can just imagine if you told a party and just spend time with yourself and your phone in a corner go and you have to talk to a stranger which will most guys prefer right so if I have if I'm told I have to go to talk to a stranger then I will of course do that but I would honestly prefer sometimes just being on the phone right but see here is the main point that crosspollination has all the advantages that we talked about earlier yes right whereas self-pollination has disadvantage advantages but evolution is lazy. So self-pollination is what will happen and then that species will die
out right whereas one of those species just through random mutation develops something called self incompatibility right where which means that pollen from this plant cannot uh fertilize the stigma from the same plant because of some way that the molecules are set up. Right. Right. This is the case of where you are forced. You are not given a phone. So you are forced to go talk to a stranger. Right? In that case that particular uh linage is going to cross-pollinate. They don't have a choice and then they will have all the advantages of cross-pollination and they will keep thriving more and more and slowly they will drive out the others. Right. any examples of this self incompatibility
that you just explained in so first of all in plants there are lots right so I mean if you just go through the list I mean there is for example tobacco has self- incompatibility there's a long list but what also happened is that those plants later on figured out how to do the self incompatibility by splitting into males and females where now the entire animal kingdom is male males and females which is this forced self incompatibility. A male cannot mate with itself himself itself. Yeah. Whatever it is. Correct. Got it.
So that's that's the reason why you have two different sexes which are incompatible with the self. Right? So that forces the cross-pollination to happen which is a good thing. What you've said so far makes complete sense to me. But I also noticed that a lot of this is mathematics and theory. So I assume that there has to be some experimental validation of this of crossover being superior or whatever. I'm glad you asked that. Right. This channel is about data and science and experiments. So what they did was experiments with yeast. Okay. Yeast has the ability to do self-pollination as well as cross-pollination.
Okay. So what the scientists did was they set up experimental conditions so that this yeast on one side they were forced to self-pollinate. With yeast is it called self-pollination? No, it is called asexual reproduction versus sexual reproduction. But it's these terms are easier to understand. Okay fine. Self-pollination, asexual reproduction, crosspollination, sexual reproduction. Go on. So on one side the yeast was forced to self-pollinate. on the other side it was forced to cross-pollinate. Okay. And then they looked at the results right of the different genes uh in there and the survival rates. Okay. Now in these graphs these graphs look a little difficult but I'm going to explain right. So every line on this graph is one of the genes.
Okay. Okay. A orange line is a good gene. A blue line is a bad gene. Okay. And the y-axis height shows how many of that genes survived. Right. Okay. So basically a line going up. Wait, the x-axis is over time over multiple reproduction generations. Okay. Right. So a line going up means that that gene has succeeded and a line going down means that that gene died out. Right. Correct. Now the upper graph is the self-pollination asexual reproduction and you can see that a whole bunch of orange genes have succeeded very early on.
Along with them a whole bunch of blue jeans have also succeeded and in fact you can see that every blue gene that succeeded is near some orange. So that is clearly a blue gene free riding upon an orange gene. The hitchhiking thing you talked about hitchhiking right? But in sexual reproduction it starts out similar but soon you see that the blue and the orange get separated. The blue get kicked out and they die out whereas the orange remain. Right? So clear clear proof that cross-pollination is superior to selfpollination.
Okay. So this graph explains the importance of crosspollination of sexual reproduction of crossover whatever you want to call it. And it means that it has to be two different sexes. It has to be a Poland and a stigma. It has to be a male and a female. But why do they have to be different? Yeah, great question, right? And to understand this, you have to understand that survival isn't just about good genes and gene variation. Okay? Survival has one more important part. Imagine the little offspring that has been created, the embryo, right? It has to grow. While growing, it needs a bunch of things. First of all, it needs energy and nutrition to grow.
It needs some sort of protection, a protective shell or something else. It needs sensors to check whether the environment is conducive or not. Uh whether it should go to the next stage or not. All of these things, right? Now, imagine how this could be implemented. Right? You're doing crossover. So, you have these two half cells, these gamts, right? Now which of these gamts is carrying around all these extra resources in addition to just the good genes, right? It is like a person traveling with three huge trunks.
Okay. This would just make it difficult for them to travel around and look around for uh partners. Right. Right. Mathematically, it is so much more easier if a small number of them carried all these extra resources and stayed in one place whereas all the others traveled light and they're running around trying to look for these people with resources. Right? This is exactly what is happening. The small number carrying around all these resources are the egg which contains the yolk which is the nutrition and a shell and a whole bunch of other things and they stay in one place. Whereas the travel light with so many different variations these are the sperm 100 million of them all going around and
then when an egg and a sperm meet they have a great combination. You got a lot of variation and then you got this embryo which has a lot of resources by which you can survive. Makes sense? Make sense? Right? So mathematically it is just so much more optimal for these two half cells to split into two different functions. One which is stationary and has resources and the other which is mobile and travels light. Make sense? Make sense. This also explains why there are not three or four or five successes, right? Just mathematically think about this. These are our two ends, right?
Mobility versus resources. So, it makes sense for one type to go all the way to this end, another type to go all the way to this end. If there was a third sex, where would it go? If it is like in the middle, it is like, you know, it'll get out competed by the female for resources and out competed by the male for mobility, right? what what what's the purpose of this middle one to exist right so mathematically this is more optimal just having two which are at two ends of our spectrum I just want to point out this is not a gender discourse this is a sexual reproduction discourse biology and not gender we are talking in terms of that
please do not mistake that but coming back to my original question what I also meant to ask was why do the male and female of a species look so different. I mean it could basically be one same shell with just different parts inside. Right? So using the same arguments, you can say that it makes sense for evolution to say that the female is the one in charge of nurturing the child, right? Whereas the male is in charge of going out and finding resources and food and uh so on.
Right? Now because the females are in charge of resources even when the child is gestating and after the child is born that's why biologically they have to be different right wide hips they have to have breasts and so on which the males don't need make sense so far yes and similarly for the males it makes sense for them to have more endurance for them to have bigger muscles and later on in you know when tribes and societies are formed for them to be the ones with more status, right? And who's respected by the tribe kind of because while the female is gestating the embryo, uh she will have to remain in seclusion for 9 months, she
will have to be protected for 9 months. So the male has to take on the role of that protector and make sure that the female of the species I mean in early evolution it has to be the male who is stronger and in later evolution it has to be the male who has better status in the tribe kind of it evolves from there socially which is another discussion. We will not have that right now but makes sense. Okay. The problem is that this goes too far, right? So the male is looking for the hips and the breasts and so on, right?
And evolution works by making the male attracted to these things. As a result, the female start developing more of those and this just keeps continuing and goes past the point of being useful. Right? And this is called something called fisherian runaway. Okay? Okay, so this is just unnecessarily evolving in that direction so that now males are attracted to things which don't really produce any advantages. But you know what are you going to do? Did you just explain the male fetishes of large size? It is called fisherian runaway problem.
Okay, but let's get back to the basic point. Okay, the summary of this episode, yes, is that cross-pollination has significant advantages over, you know, just splitting and bacterial and asexual reproduction. Yes, it is to do with interspecies. It is not about interspecies competition, right? Within the species, it is competition between good genes versus bad genes and good genes versus better genes and how to get the best genes in one combination, right? And the more important point I want to make is that everything even the things you take most for granted has a good mathematical reason if you just ask why enough times right.
Yeah. First principles. It all keeps coming back to the first principles. And in this case uh even though it is a subject of biology the first principles as we just established were mathematics and statistics which is fascinating when you think about it. And I never thought that the answer to the question why are there exactly two sexes would be given by mathematics in conjunction with biology. Biomathematics some kind of but there it is and uh biology and evolution in that sense is a very fascinating thing to think about in terms of this mathematical statistical lens. And if you like thinking in that lens, there is another episode for you that we'll line up now which discusses how the eye must
have evolved. Uh go check that out. Shriant Naven, Future IQ.