Why Are There Men and Women? Column in ComputerraOnline #4
At a superficial comparison, the type of reproduction characteristic of humans turns out to be the least advantageous. Why, then, in the course of evolution did organisms with sexual reproduction, and specifically dioecious ones, gain the advantage?
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"Ecology" and liars. Why are there men and women? A super-task for education.
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I promised to discuss the biological roots of our behavior, and then I started writing about other things – about textbooks, about "greens"… I'm correcting myself. We are so used to the fact that there are men and women among people that the very posing of the question about the reasons for this phenomenon causes surprise. Often, an answer is given that concerns not the reasons for sexual dimorphism, but simply the mechanism of sex determination. "People belong to different sexes because they have two types of sex chromosomes. Half of the sperm carry the Y chromosome, and half carry the X chromosome. These sperm fuse with egg cells carrying the X chromosome; those organisms that receive the XX set will develop into girls and women, and those that get the XY set will develop into boys and men." All this is correct, but this is an answer to the immediate "why," not the distant one. The distant "why" concerns the phenomenon of sexual dimorphism itself. Different organisms exhibit different mechanisms of sex determination. In humans, drosophila, and sorrel, males have different sex chromosomes, while in chickens, frogs, and strawberries, females do. And that's not all. Sexual reproduction is also characteristic of hermaphrodites! Let's identify the three simplest types of reproduction (without touching on cases of alternation of generations with different modes of reproduction, or less common variants such as self-fertilizing hermaphroditism in tapeworms or semi-clonal inheritance in hybrid green frogs). Three simplest variants. Type I (case of Amoeba proteus). Clonality. Each individual reproduces by producing offspring genetically identical to itself. Type II (case of the garden snail). Hermaphroditism. Two individuals, each of which has both female and male reproductive organs, fertilize each other. Type III (human case). Sexual dimorphism. Some individuals have male reproductive organs, some have female. Males fertilize females, after which females produce offspring. With your permission, I will use the story of Robinson Crusoe to compare these types of reproduction. Throughout the history of humanity and its ancestors, it has happened that an individual, like Alexander Selkirk, found himself in a new habitat alone. What awaited him? Suppose humans reproduced clonally, according to type I. Over time, Robinson would have produced a considerable number of his genetic copies, and together they would have begun to build a happy life on their island. If humans had type I reproduction, Robinson would have populated the island with his identical offspring. If humans were cross-fertilizing hermaphrodites (had type II reproduction), Robinson would have needed a partner. Since any two sexually mature hermaphrodites can fertilize each other, meeting Friday would have been enough. These two would have populated the island with their recombinant offspring, who would have differed from each other as a result of recombination (formation of new gene combinations). With type II reproduction, Robinson and Friday would have populated the island with their recombinant offspring. Two randomly selected sexually dimorphic individuals, with a probability of 1/2, belong to the same sex and cannot leave offspring (let's assume an equal number of sexes for simplicity). Moreover, if Robinson were lucky and a native woman, Subbota, arrived on the island, they together with Robinson could produce only half the number of offspring than a pair of hermaphrodites! With type III reproduction, Robinson would be left without offspring. Thus, at first glance, the type of reproduction characteristic of humans turned out to be the least advantageous. Why did organisms with sexual reproduction, and specifically sexual dimorphism, gain an advantage in the course of evolution? Many of their offspring lost their sex, but sexual dimorphism remained the most common option. This problem can be divided into two parts. Why is sexual reproduction more effective than asexual reproduction, and why is sexual dimorphism more effective than hermaphroditism. I will briefly describe the approaches to solving the first problem here, and my explanation for the reasons for sexual dimorphism – sometime later. Many works are devoted to the first problem, such as the article by K.Yu. Popadin. Its author emphasizes that the origin of sexual reproduction is the "queen of evolutionary problems" and the "greatest mystery of evolutionary biology." It is especially difficult for those who, like most proponents of sociobiology, believe that biological evolution is simply a consequence of gene selection, to accept this. The English biologist John Haldane expressed the essence of this approach as follows: "I am willing to sacrifice myself for two brothers or eight cousins." The fact is that the relatedness of brothers is 1/2, and of cousins is 1/8; in two brothers, as well as in eight cousins, there are as many of John Haldane's genes as in John Haldane himself. Of course, if he wants to spread his genes, it is better to sacrifice himself for only three brothers or nine cousins – then the genes will get their "profit." Why can a female sacrifice herself for her offspring? Let's imagine that such behavior is determined by a certain gene. Copies of this gene are present in half of her offspring. If such a female sacrifices herself for four of her young, the number of copies of these genes will double in the next generation. The behavior determined by them will become more widespread. This concept is called kin selection. Logical? Quite. Now think about it. With sexual reproduction, a female, by producing an egg cell, halves her set of genes. The other half of the set will be delivered by the sperm. With asexual reproduction, all her offspring carry her entire set of genes! Thus, from the perspective of gene selection, sexual reproduction would be almost impossible. But it occurs everywhere! From my point of view, this circumstance alone is enough to prove the limitations (specifically, limitations, not falsehood) of the gene selection concept… From the standpoint of group selection, sexual reproduction is incomparably easier to explain. Consider a simple scenario. Parasites perfect the mechanisms of their transmission from one host to another. For example, the F-factor, a short fragment of DNA located in the cytoplasm of a bacterium, can be considered a parasite of E. coli. The F-factor ensures its transmission (along with a piece of the bacterium's own genetic information) through a special outgrowth – a pilus. This is not reproduction yet, but simply recombination – a "shuffling" of genetic information. The parasite (entirely in accordance with the "selfish gene" concept) spreads in the population, and the hosts become genetically more diverse. Here is an electron micrograph of the sexual process in E. coli. In general, genetic variability has two sources: mutations (introducing changes into the genetic text) and recombination (the appearance of new combinations of individual fragments). Mutations are predominantly undirected "noise" (at least, at first approximation); they more often destroy than create. And with recombination, an organism receives genetic texts that have already passed selection, i.e., with a significantly higher probability, useful ones. If new adaptations need to be developed, those lines of organisms that have a recombination mechanism will be ahead of clonal ones. This is shown in theory, in models, and in experiments. The perfection of parasites leads to the perfection of sexual recombination (more details in the aforementioned article by Popadin). More interesting is something else. Why are we sexually dimorphic? You will find far fewer explanations for this. Although about this – another time.
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"Ecology" and liars. Why are there men and women? A super-task for education.
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