Why is there sex and the separation of men and women?
It would seem that it would be so simple and logical if there were no division into the male and female sexes in nature. Why do we need sexual reproduction, if you can perfectly share one organism until the end of time? After all, the search for a sexual partner takes time and effort. And, if nobody likes you, or, in general, finds nobody, then you will not breed at all.
The biological meaning of sex is the exchange of genetic material between two organisms.
Why change genes at all?
To begin with, I want to conduct some thought experiments.
Suppose there is a bacterium that multiplies only by division. Of course, she does not live alone, but she and her next of kin form a population. Suddenly, our bacteria developed a mutation. This mutation is very useful and gives our bacteria an advantage over all other members of the population. But at the same time, she got another mutation, harmful. And the harm from this mutation is greater than the benefit from the beneficial. It turns out that even if this bacterium multiplies, it is worse than the rest, and this useful mutation disappears from the population.
For the next experiment, we need a little theoretical background. The fact is that beneficial mutations appear much less often than harmful ones. Intuitively, this can be understood in such a way that by accidentally changing the details in something that works, it is easier to break something than to improve. So, in the same population there is a common, unremarkable bacterium. And, suddenly, she had a harmful mutation. If the mutation is not very harmful, then our bacterium will be able to multiply, but the likelihood that in the same place the reverse mutation will occur later and correct the old, insignificant, which means that now all its descendants will carry this mutation. At some point, a mutation occurred in the descendant line again, and with a very high probability it is also harmful. And this will further reduce the fitness of the line. Thus, from generation to generation, bacteria will wither away,
If two different organisms have two beneficial mutations, then we will never get an organism combining both of them, because they cannot exchange them.
In all cases, the problem could be solved if bacteria were given the opportunity to exchange genes.
In the first case, a bacterium with a useful and harmful mutation could replace the damaged gene with a normal version from another bacterium, and distribute the useful gene to its relatives. In the second case, when a gene is damaged, a working version of it could be borrowed from relatives, and, in a timely manner, replace poorly working genes with normal ones. In the third case, two organisms could exchange their improved genes and distribute them to their comrades, which would benefit the entire population.
And, indeed, all bacteria can exchange parts of the genome. This phenomenon is called horizontal gene transfer (GPG). The bacterium simply takes and transfers another piece of its genome, while the other accepts and uses it as its own. It is GPG that is responsible for the resistance of bacteria to antibiotics. As soon as one bacterium accidentally becomes resistant to the drug, it very quickly distributes this resistance to the rest.
By the way, GPG between distant organisms sometimes confuses all the maps during reconstruction of the evolution of organisms .
Realization in more advanced organisms
As you can see, with the help of GPG, evolution can begin to work not with individual genomes, but with the gene pools of entire populations, as with a single whole, combining different variants of the genomes and selecting the best ones. But, GPG has some fundamental limitations that deserve a separate article, and with its help it will not be possible to fully mix the genomes, and the efficiency of this process will always be low. And with weak mixing of the genomes, the removal of harmful mutations and the combination of useful ones will always be ineffective, which means that there will be restrictions on the complexity of organisms. It is because of this that bacteria could not become large and complex. The largest bacterial genomes are 250 times shorter than the human. And that means that those organisms that could become complex and large (like us) invented a way to better mix genomes.
To begin with, I will understand that sex in biology is far from always associated with reproduction, it is the process of complete mixing of the genomes of two organisms, and there are several options for how it can occur. For example, during sex at ciliates, two of them fit together, exchange genes, and diverge, and no reproduction occurs. And they do it in conditions of hunger or poor living conditions, when it makes sense to try new combinations of genes.
By the way, nature has made a lot of efforts to ensure that genes are randomly and independently selected when sex cells are formed. But for genes located next to each other, this is often not possible, and they are transmitted together more often than those located far away, and the pioneers of genetics determined the distance between the two genes by the probability of their linked inheritance.
And why in humans and all other multicellular animals, the sexual process occurs during reproduction? Why is there not and cannot be such an option that, as in ciliates, two organisms could meet, exchange genes, and move on? Firstly, the problem arises of conveying changes in the genome to each of the trillions of cells in our body, but, most importantly, they have many genes that act at the developmental stage, and if we want to modify some large structure, for example, the eye, then we often cannot redo the already adult eye, but it is necessary that it re-develops according to a slightly different program. Therefore, it is logical to carry out the sexual process before the beginning of the development of the body, so that it is already completely created according to the new genetic program.
Why do men and women exist?
Oh well. Why sexual reproduction is needed, it seems, it became clear. Well, why do we need men and women to exist? After all, it turns out that only half of the individuals directly produce offspring. Wouldn't it be more logical if everyone fertilized each other and became pregnant all? Then they would multiply twice as fast.
There is one interesting hypothesis. Imagine a species that has sexual reproduction (that is, the sexual process that occurs during reproduction) and, all representatives of which are hermaphrodites, fertilizing each other. Such species do exist, but there are few of them. Obviously, after sex, a representative of this species will be forced to spend a lot of resources on the development of descendants for whom he is a mother. The contribution of the male to the offspring is much lower than the contribution of the female. The male should only produce sperm, and the female should spend resources on the development of offspring until they are able to feed themselves. Therefore, in such species, one can sometimes observe a struggle over who will play the role of a male, because everyone wants to transfer genes without straining.
in the photo two flatworms are fighting on the penis for the right to play the role of a male
And, if all representatives of the species have both reproductive systems, then, after each mating, they will have to spend time and effort on growing offspring. If a mutant appears that will have only the male reproductive system, then he will be able to fertilize many other organisms with a little resources, which means he will have many descendants and his “male gene” will quickly spread throughout the population. At some point, hemaphrodites will become fewer, and, males will already begin to compete for them, and, since they will perform the role of a male more efficiently than hermaphrodites, over time they will become females.
PS In the comments they write about the theory of the origin of sex, which says that nature "experiments" on men, and women are more genetically stable, but I did not find authoritative confirmation of this theory, and, in general, it has many weaknesses. For example, she does not explain at all how males and females appeared, but if something appeared during the evolution, then “why it works” is equivalent to “how it appeared”, and not “what is the use of it,” but, most importantly generally speaking, sex in many organisms is not determined genetically, but under the influence of temperature, pheromones and the devil still knows what. And someone can even change throughout life. So the option that “males have more mutations” does not work here at all.
PPS Before you write "author goof, everything is arranged differently" make sure that you have a biological education.
The biological meaning of sex is the exchange of genetic material between two organisms.
Why change genes at all?
To begin with, I want to conduct some thought experiments.
Suppose there is a bacterium that multiplies only by division. Of course, she does not live alone, but she and her next of kin form a population. Suddenly, our bacteria developed a mutation. This mutation is very useful and gives our bacteria an advantage over all other members of the population. But at the same time, she got another mutation, harmful. And the harm from this mutation is greater than the benefit from the beneficial. It turns out that even if this bacterium multiplies, it is worse than the rest, and this useful mutation disappears from the population.
For the next experiment, we need a little theoretical background. The fact is that beneficial mutations appear much less often than harmful ones. Intuitively, this can be understood in such a way that by accidentally changing the details in something that works, it is easier to break something than to improve. So, in the same population there is a common, unremarkable bacterium. And, suddenly, she had a harmful mutation. If the mutation is not very harmful, then our bacterium will be able to multiply, but the likelihood that in the same place the reverse mutation will occur later and correct the old, insignificant, which means that now all its descendants will carry this mutation. At some point, a mutation occurred in the descendant line again, and with a very high probability it is also harmful. And this will further reduce the fitness of the line. Thus, from generation to generation, bacteria will wither away,
If two different organisms have two beneficial mutations, then we will never get an organism combining both of them, because they cannot exchange them.
In all cases, the problem could be solved if bacteria were given the opportunity to exchange genes.
In the first case, a bacterium with a useful and harmful mutation could replace the damaged gene with a normal version from another bacterium, and distribute the useful gene to its relatives. In the second case, when a gene is damaged, a working version of it could be borrowed from relatives, and, in a timely manner, replace poorly working genes with normal ones. In the third case, two organisms could exchange their improved genes and distribute them to their comrades, which would benefit the entire population.
And, indeed, all bacteria can exchange parts of the genome. This phenomenon is called horizontal gene transfer (GPG). The bacterium simply takes and transfers another piece of its genome, while the other accepts and uses it as its own. It is GPG that is responsible for the resistance of bacteria to antibiotics. As soon as one bacterium accidentally becomes resistant to the drug, it very quickly distributes this resistance to the rest.
By the way, GPG between distant organisms sometimes confuses all the maps during reconstruction of the evolution of organisms .
Realization in more advanced organisms
As you can see, with the help of GPG, evolution can begin to work not with individual genomes, but with the gene pools of entire populations, as with a single whole, combining different variants of the genomes and selecting the best ones. But, GPG has some fundamental limitations that deserve a separate article, and with its help it will not be possible to fully mix the genomes, and the efficiency of this process will always be low. And with weak mixing of the genomes, the removal of harmful mutations and the combination of useful ones will always be ineffective, which means that there will be restrictions on the complexity of organisms. It is because of this that bacteria could not become large and complex. The largest bacterial genomes are 250 times shorter than the human. And that means that those organisms that could become complex and large (like us) invented a way to better mix genomes.
To begin with, I will understand that sex in biology is far from always associated with reproduction, it is the process of complete mixing of the genomes of two organisms, and there are several options for how it can occur. For example, during sex at ciliates, two of them fit together, exchange genes, and diverge, and no reproduction occurs. And they do it in conditions of hunger or poor living conditions, when it makes sense to try new combinations of genes.
By the way, nature has made a lot of efforts to ensure that genes are randomly and independently selected when sex cells are formed. But for genes located next to each other, this is often not possible, and they are transmitted together more often than those located far away, and the pioneers of genetics determined the distance between the two genes by the probability of their linked inheritance.
And why in humans and all other multicellular animals, the sexual process occurs during reproduction? Why is there not and cannot be such an option that, as in ciliates, two organisms could meet, exchange genes, and move on? Firstly, the problem arises of conveying changes in the genome to each of the trillions of cells in our body, but, most importantly, they have many genes that act at the developmental stage, and if we want to modify some large structure, for example, the eye, then we often cannot redo the already adult eye, but it is necessary that it re-develops according to a slightly different program. Therefore, it is logical to carry out the sexual process before the beginning of the development of the body, so that it is already completely created according to the new genetic program.
Why do men and women exist?
Oh well. Why sexual reproduction is needed, it seems, it became clear. Well, why do we need men and women to exist? After all, it turns out that only half of the individuals directly produce offspring. Wouldn't it be more logical if everyone fertilized each other and became pregnant all? Then they would multiply twice as fast.
There is one interesting hypothesis. Imagine a species that has sexual reproduction (that is, the sexual process that occurs during reproduction) and, all representatives of which are hermaphrodites, fertilizing each other. Such species do exist, but there are few of them. Obviously, after sex, a representative of this species will be forced to spend a lot of resources on the development of descendants for whom he is a mother. The contribution of the male to the offspring is much lower than the contribution of the female. The male should only produce sperm, and the female should spend resources on the development of offspring until they are able to feed themselves. Therefore, in such species, one can sometimes observe a struggle over who will play the role of a male, because everyone wants to transfer genes without straining.
in the photo two flatworms are fighting on the penis for the right to play the role of a male
And, if all representatives of the species have both reproductive systems, then, after each mating, they will have to spend time and effort on growing offspring. If a mutant appears that will have only the male reproductive system, then he will be able to fertilize many other organisms with a little resources, which means he will have many descendants and his “male gene” will quickly spread throughout the population. At some point, hemaphrodites will become fewer, and, males will already begin to compete for them, and, since they will perform the role of a male more efficiently than hermaphrodites, over time they will become females.
PS In the comments they write about the theory of the origin of sex, which says that nature "experiments" on men, and women are more genetically stable, but I did not find authoritative confirmation of this theory, and, in general, it has many weaknesses. For example, she does not explain at all how males and females appeared, but if something appeared during the evolution, then “why it works” is equivalent to “how it appeared”, and not “what is the use of it,” but, most importantly generally speaking, sex in many organisms is not determined genetically, but under the influence of temperature, pheromones and the devil still knows what. And someone can even change throughout life. So the option that “males have more mutations” does not work here at all.
PPS Before you write "author goof, everything is arranged differently" make sure that you have a biological education.