THE PRESENCE OF TWO SEXES AND SEXUAL REPRODUCTION LEAD TO A SHARP INCREASE IN THE GENETIC DIVERSITY OF INDIVIDUALS IN A SPECIES POPULATION.

If reproduction is asexual (by budding, division, or self-fertilization), then all descendants have the same genotype as the original maternal organism. As a result, the entire isolated population becomes filled with individuals having an identical genotype. However, under sexual reproduction, the genes of two different individuals with differing genotypes are mixed in the offspring. As a result, the genetic composition of their offspring will differ substantially from both the mother and the father.

2. WHY DID NATURE NEED SEXUAL REPRODUCTION AND THE RESULTING DIVERSITY OF GENOMES WITHIN THE SAME SPECIES POPULATION?

There are several reasons for this that are critical to the survival of a species. We shall list them in ascending order of importance.

THE FIRST REASON - if a beneficial mutation occurs among individuals reproducing asexually, there is no opportunity for this mutation to spread widely through the population, nor for the individual - to acquire genes from other lineages of its species in which their own beneficial mutations may also have arisen. But this is far from the main reason – after all, selection can ultimately simply increase the number of precisely those descendants that carry the beneficial mutation (even if this is not as efficient).

THE SECOND REASON is already more important. Sexual reproduction mixes alleles. The point is that some instances of hereditary variation may be advantageous only when one mutation combines with another mutation, while sexual reproduction increases the probability that such a combination will occur at all.

Note that both of the factors listed above contribute to positive selection only when the environment in which the species exists is constantly changing, and the organism is forced to adapt to it again and again through beneficial mutations. It might seem that if the natural environment is unchanging, many species can afford not to change their genome for millions of years. However, in most cases, even under strictly conservative environmental conditions, this is not at all what happens. And the reason here – is the constant struggle of every species against its parasites. Even unicellular organisms have parasites; thus, bacteria are forced to fight constantly against bacteriophages that devour them. And any multicellular organisms have a vast multitude of parasites dangerous to them. Viruses, bacteria, fungi, worms; there are even vertebrate parasites (fish that penetrate inside the body). A special role (and one of the most dangerous to the life of a multicellular organism) is occupied by parasites that have split off from among its own cells. These are breakaway cells that have unsealed in their genome the ancient selfish programs of their unicellular ancestors and have ceased to perform specialized functions useful to the organism as a whole, instead beginning to reproduce uncontrollably and fraudulently at the organism’s expense. These are cancerous tumors. These cells are set on the fraudulent, parasitic path either by viruses integrating into the genome or by random mutations. Moreover, some forms of cancer are highly transmissible. Thank God, humans currently have no such forms, but they remain in nature to this day in certain multicellular species and even in mammals.

A constant arms race is taking place between a multicellular organism and its parasites. The organism improves its external defenses (preventing parasites from entering), while with respect to those that have already entered it constantly modifies the mechanisms of immune defense, including both means of recognizing “foreigners” and methods of destroying them. Parasites, in turn, also constantly mutate and, through mutations beneficial to themselves, learn to “hide,” mimic, evade the scanners of the immune system, and also - defend themselves against the means used to destroy them. For example, at first the antibiotic penicillin killed practically all known bacteria, but bacteria throughout the world needed only a few years to begin mass-producing the enzyme penicillinase against it, which completely blocked the antibiotic’s lethal action. The same thing later happened with tetracycline, which was very effective at first and is completely useless today. Similar adaptation through constant restructuring of the genome occurs in bacteria and fungi not only in relation to drugs, but also in relation to the internal agents of the organism’s immune system. Viruses also constantly change their genotype. As soon as the immune system or the pharmaceutical industry has only just found a parasite’s vulnerable point, it has – just like that – already changed its genotype, and the means of destruction cease to recognize and kill it. In particular, influenza and the human immunodeficiency virus (HIV) constantly mimic in this way, thereby escaping both the organism’s immune agents and drugs.

The principle of an “evolutionary arms race” between species existing alongside one another (in particular, when one of them is a multicellular victim and the other – its smaller parasite) was first formulated in 1973 by the biologist Leigh Van Valen. And in 1993, Matt Ridley’s very well-known popular-science book “Sex and the Evolution of Human Nature” was published (in Russian it was published in 2011), in which he summarized the work of his predecessors and linked the “evolutionary arms race” to the necessary emergence in the course of evolution of such a means by which a large victim combats small parasites as sexual reproduction. In turn, Ridley in his book dubbed the constant “evolutionary arms race” the “Red Queen Principle” - the point here is that the following dialogue took place in L. Carroll’s fairy tale “Alice in Wonderland” between Alice and another heroine of the tale, the Red Queen:

”— Where we come from, — Alice said, breathing hard, — when you run as fast as you can for a long time, you are certain to get somewhere else.

— What a slow sort of country! — cried the Queen. — Now, here, you see, you have to run as fast as you can just to stay in the same place.”

Since then, the term “Red Queen Principle” has begun its rapid march through the pages of the scientific biological literature.

Thus, the “Red Queen Principle,” that is, the evolutionary arms race between a multicellular species and its numerous parasites that does not stop for even a minute, is the THIRD, AND AT THE SAME TIME THE MOST IMPORTANT REASON FOR THE EMERGENCE IN ALL MULTICELLULAR ORGANISMS OF THE DIVISION INTO SEXES AND OF SEXUAL REPRODUCTION.

Let us explain this reason. Parasites are small; they reproduce more often and faster; as a result of mutations, they update their genotype faster. That is, they have an initial advantage over the multicellular organism in the arms race – the speed of their evolution is higher. What can a species of multicellular organism oppose to this that is comparable in effect? Its genetic diversity in a population containing many similar multicellular organisms!

The cholera bacillus kills up to 70% of infected people. But 30% survive – and the reason here lies not only in the characteristics of their immunity, but also simply in the differing pathogenicity of the cholera vibrio for different people. The point is that cholera toxins in the intestine cause dehydration of the organism, which is what leads to death. But this dehydration itself occurs differently in different people – with different intensity, depending on their genotype. Therefore, some die while others survive. The same applies to plague bacilli – even independently of differences in immunity, different people are also differently sensitive to the toxins of this pathogenic bacterium. And what matters to the species as a whole? That at least someone survives and gives rise to new offspring that will no longer, in every case, necessarily respond to this pathogenic parasite by dying.

You know, of course, that our intestines contain several kilograms of bacteria? Most likely, many of their species (for example, Escherichia coli) were once dangerous to humans, but some people survived, adaptation to these bacteria occurred, and over time even a mutually beneficial symbiosis of our organisms arose.

Another example is well known from a school biology textbook. Some people have a gene that causes a hereditary disease – sickle-cell anemia. An unpleasant thing, but not fatal. On the other hand, it makes it possible not to become infected with malaria, that is, with something much more dangerous and fatal (the malaria plasmodium simply does not survive in erythrocytes carrying the sickle-cell anemia gene). As should be expected, in regions of the globe affected by malaria, the sickle-cell anemia gene is far more widespread in the human population than in the North.

Thus, the greater the genetic diversity of individuals in a population of a multicellular species, the less damage its parasites can inflict on the species as a whole. At the very least, the entire population will not die out all at once from some epidemic. Take, for example, the measles virus. Europeans adapted to it long ago, and in them the disease proceeds in a mild form. But after the New World was discovered by Columbus, the Native Americans had no evolutionarily developed adaptations against this virus introduced to them by Europeans – America had not previously known measles, and therefore this virus was fatal to the Native Americans. If all Native Americans had been completely uniform genetically, then every one of them without exception would have died from the measles epidemic. As it was – only 90-95% of the population died (including among the Aztecs), although everyone contracted the disease. From 5 to 10 % survived, despite everything! And for their descendants this virus was already not so frightening! The population declined sharply, but persisted and subsequently restored its numbers.

And what provides this very genetic diversity of individuals in the population that is so desirable for the survival of a species? Sexual reproduction, thanks to which individuals with very substantially differing genotypes are constantly circulating in the population, and these individuals with different genotypes mix them in their offspring, which as a result will likewise not be exactly like either their father or their mother. Evolution additionally took care that, when searching for a sexual partner for reproduction, each individual SPECIFICALLY sought and selected a partner with a genotype maximally different from its own.

In socionics, it is known that among humans the search for a sexual partner relies to a considerable extent on behavioral psychological characteristics (some people are more sexually attractive, others less so, and, for some reason, duals turn out to be more attractive).

The duality of marriages – is not a socionics legend. In the author’s study, which continues to this day, it has ALREADY been shown that in close relationships people reliably pair with their duals more often (several times more often than on average in other types of relationships). Moreover, the proportion of conflict pairs (in which the genes also differ strongly) exceeds the average level as well (although to a lesser extent), whereas marriages between psychological identicals are almost never encountered.

A similar result (but now without any socionics, and indeed without psychology at all) has been obtained in at least one recent study. Its participants did not complete any questionnaires, but provided saliva for genetic analysis, by which their histocompatibility genes (that is, tissue-compatibility genes – this set of genes is always tested in organ transplantation so that rejection does not occur) were determined. It turned out that women prefer the smell of those men whose set of histocompatibility genes differs most strongly from their own set. Proteins produced by foreign histocompatibility genes always cause an immune rejection reaction. Consequently, spouses pair according to such a principle so that, if necessary, the parents’ tissues would be recognized by the immune system of their offspring as maximally foreign. (Or vice versa).

And here we come to the FOURTH REASON for the emergence of sexual reproduction in multicellular organisms – and, possibly, the most important and decisive one in that distant period about a billion years ago when sexual reproduction was first formed by evolution. This reason – is transmissible cancer. We already wrote briefly about it above. Today, transmissible cancer is found in only three known multicellular species (in particular, in the “Tasmanian devil”), but a billion years ago, when multicellular organisms had surged ahead in their evolution and parasites had not yet had time to adapt to them, it was possibly the principal threat to all newly emerging multicellular species. A cancerous tumor consists of the organism’s own cells, only very slightly genetically altered – as a result of mutation or viral damage, the ancient programs of unicellular selfish behavior (selfish - at the expense of the resources of the entire organism) and uncontrolled division and reproduction are “unpacked” in these cells. In a population of multicellular organisms reproducing asexually (for example, by budding), transmissible forms of cancer had enormous scope in which to run rampant, while the programs of cooperative cell behavior in the first multicellular organisms especially often “failed,” releasing the ancient selfish programs. The mother transmitted her cancer to her daughter bud; it was also transmitted through simple physical contact between two different individuals – because of their genetic identity, they could easily infect one another with their cancer cells.

Even the immune system of modern organisms, improved over the past billion years, has difficulty combating cancerous tumors, because it is almost unable to distinguish them from its own healthy and “disciplined” cells. What, then, can be said of the very first multicellular organisms? Neither means of recognition nor means of combating them. For the first multicellular organisms, the only way to preserve the population and the species as a whole from destruction by transmissible cancer was genetic diversity alone, which would allow the immune system of one organism to recognize and then destroy cancer cells that had entered it from a foreign organism.

Metastasizing cancer cells are capable of penetrating even the placental barrier. And only because of the difference between the histocompatibility genes of the mother and her fetus (and ultimately – because of sexual reproduction and the targeted selection of a sexual partner with maximally differing histocompatibility genes), today even a mother with cancer will give birth to a healthy child – its immune system will cope with foreign maternal cells that have entered its organism (because, by the set of their histocompatibility genes, they are easily distinguishable from its own).

SUMMARY THESES:

1. Sexual reproduction, and subsequently division into two stable sexes with different functions, arose in the evolution of multicellular organisms as a means of sharply increasing the genetic diversity of the population.

2. The psychological diversity of a population reflects its internal genetic diversity.

3. Genetic diversity is vitally necessary for any population of multicellular organisms – without it, species perish in the struggle against parasites and also, when reproducing exclusively asexually, reduce their evolutionary fitness to the environment because beneficial combinations of genes arise less frequently.

4. The constant struggle of multicellular organisms against their parasites is part of the never-ending “evolutionary arms race” (its other name – the “Red Queen effect”)

5. In the course of the evolutionary arms race, small parasites have an evolutionary advantage over the victim and over the rate at which the victim updates its “immune weaponry,” because parasites accumulate mutations faster and undergo cycles of selection more often.

6. To prevent parasites from destroying the population as a whole, the “victims” find it necessary to maintain broad genetic diversity. Because of the genetic diversity of the population (which is achieved through its sexual reproduction), no parasites can destroy the population as a whole; at least a small part of it that is resistant to these parasites always remains.

7. It may be supposed that an even more important factor in the emergence and consolidation in evolution of sexual reproduction, and subsequently the division of the sexes, a billion years ago, when the first multicellular species were forming, was the need for the first multicellular species to resist transmissible forms of cancer. To prevent one individual in a population from being infected by cancer cells from another individual, evolution had only one option – to separate the histocompatibility genes of different individuals in the population as widely as possible. That is, to make the cells of one individual of a species (including its cancer cells) recognizable by other individuals of the same species as unquestionably foreign. Asexual reproduction could not provide this in any way, and only sexual reproduction accomplished this task.

8. In modern humans, the search for a sexual partner also occurs in such a way as to ensure the maximum difference between the histocompatibility genes of the second partner and the first partner’s own set of genes. The corresponding search and selection rely partly on differences in the odors of people with different sets of histocompatibility genes, and partly – on differences in their observable behavioral (psychological) stereotypes, that is, on the search for people who differ maximally from the subject on a number of psychological temperamental factors.

9. The formation of marital unions predominantly between people who differ maximally psychologically, as well as the almost complete absence of marriages between psychological identicals, have already been confirmed by the socionics statistics collected by V. Talanov to date on pairs in sexual or friendly relationships. Whereas in sexually oriented relationships (both opposite-sex and same-sex!) dual pairs predominate, in purely friendly relationships (also both opposite-sex and same-sex) – socionically identical pairs predominate.

10. Until today, socionics had one obvious explanation for the evolutionary basis of predominantly dual marriages among humans, namely in the form of the principle of conservative selection – the population resists excessive divergence of the phenotypes of its individuals because extreme cases are less viable. In order to prevent cases in which offspring will have paired copies of the same gene (which increases the risk of nonviability), evolution pushes parents to seek a partner with maximally differing psychological (and therefore genetic) properties.

11. This factor of conservative selection may indeed be important, but the factor of resistance to the transmission of lethal parasites and lethal transmissible forms of cancer between parents and children, as well as between different individuals in the population, especially members of the same pack (family), appears no less important. It is possible that evolution also for this purpose encourages individuals to seek sexual partners with a maximally differing set of histocompatibility genes – in order to increase resistance to the transmission of diseases between members of one family.

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OUR NEXT POST WILL BE DEVOTED TO HOW NATURAL SELECTION AFFECTS PSYCHOTYPES AND THEIR RELATIVE FREQUENCIES (in particular, it will discuss certain mechanisms that I. Romanov did not consider in his article)

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APPENDIX – REVIEW OF CERTAIN FACTS AND EXPERIMENTS IN EVOLUTIONARY BIOLOGY DIRECTLY RELATED TO THE TOPIC:

1) CONFIRMATION OF THE THEORY OF THE EVOLUTIONARY ARMS RACE.”

Biologists from Belgium, Switzerland, and France conducted an illustrative experiment by isolating eggs of Daphnia (Daphnia magna) and spores of their parasites — the bacterium Pasteuria ramosa[en] from layers of bottom sediment formed over 30 years in a small pond in Belgium. 8 layers, each 2 cm thick, were taken, on the assumption that each such layer had formed from organic sediment over 2—4 years, a period during which 10—20 generations of Daphnia were replaced. Three groups from each layer were infected with bacteria from the same layer, as well as - the overlying and underlying layers. The results of the observations fully corresponded to the theory of the “evolutionary arms race”: only 35 % of Daphnia proved resistant to infection by “contemporary” parasites, 43 % — to infection by parasites from the overlying later layer, and 45 % were resistant to parasites from the underlying earlier layer.

http://elementy.ru/news/430645

elementy.ru/ Alexander Markov Modern Parasites Are More Dangerous than Past and Future Ones (Source: Ellen Decaestecker et al. Host-parasite ‘Red Queen’ dynamics archived in pond sediment // Nature. 2007. V. 450. P. 870—873)

2) THE IMPORTANCE OF SEXUAL REPRODUCTION FOR RESISTANCE TO PARASITES

Curt Lively and coauthors conducted experimental observations of Poeciliopsis fish (Poeciliopsis) and their trematode parasites, which cause white, clearly visible spots to erupt on the fishes’ bodies, making it convenient to analyze the degree of infection in the population. In 1976, as a result of a desiccating drought, only three Poeciliopsis individuals survived in one pond; their offspring, reproducing asexually and constituting a highly inbred clone, subsequently populated the entire pond. These fish proved to be heavily infected with trematodes. After several males were introduced into the pond, the Poeciliopsis switched to sexual reproduction and the disease almost completely receded.

The nematode Caenorhabditis elegans, divided into two sexes — males and hermaphrodites, was also examined for scientific purposes. Under asexual reproduction in this species, self-fertilization of hermaphrodites occurred, which did not allow gene recombination. In nature, the proportion of nematodes reproducing sexually, as a rule, is — 1—30 %. When a lethally dangerous bacterial parasite was periodically introduced into the population, during the first ten generations the proportion of individuals reproducing sexually reached 80 %, after which it again fell to the natural level. After bacteria were selected for virulence (parasites were taken from the corpses of nematodes that had died within the first day), in the population under study the proportion of individuals reproducing sexually reached 80 %, after which it did not decrease again, but after 20 generations reached 90 %. The first case was conventionally called “evolution,” and the second — “coevolution.” If males were removed from the population and it was subjected to “coevolution,” then after 20 generations it died out completely.

http://elementy.ru/news/431626 elementy.ru/ Elena Naimark The Benefit of Males Has Been Demonstrated Experimentally (Source: Levi T. Morran, Olivia G. Schmidt, Ian A. Gelarden, Raymond C. Parrish II, Curtis M. Lively. Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sex // Science. 2011. V. 333. P. 216—218)

Recombination creates new genotypes that are capable of resisting diseases caused by pathogenic microorganisms and /or parasites (that is, the Red Queen hypothesis), thereby maintaining host fitness despite endlessly evolving virulent pathogens / parasites. Several empirical studies support this hypothesis; for example, in the facultatively sexual crustacean Daphnia magna, sexually produced offspring were twice as resistant to parasites infecting the parents as asexual offspring:

Auld SKJR Tinkler SK, Tinsley MC. Sex as a strategy against rapidly evolving parasites. Proceedings of the Royal Society B: Biological Sciences. 2016;283(1845). http://scholar.google.com/scholar?q=Tinsley+MC.+Sex+as+a+strategy+against+rapidly+evolving+parasites+Auld+SKJR+2016

3) SEXUAL REPRODUCTION CAN QUICKLY CREATE A BENEFICIAL GENETIC ALLELE:

Lehtonen J, Jennions MD, Kokko H. The many costs of sex. Trends in Ecology & Evolution. 2012; 27 (3): 172-8. PMID: 22019414 https://doi.org/10.1016/j.tree.2011.09.016

4) INDIRECT FACTS SUPPORTING THE IMPORTANCE OF CANCER FOR THE EMERGENCE OF SEXUAL REPRODUCTION

Species that are not affected by cancer, such as prokaryotes and unicellular eukaryotes, should periodically return to asexual reproduction. Accordingly, bacteria and archaea reproduce mainly by asexual reproduction, usually by division into two parts, while some genetic exchange and recombination occasionally occur through horizontal gene transfer [ Arber W. Horizontal Gene Transfer among Bacteria and Its Role in Biological Evolution. Life (Basel, Switzerland). 2014;4(2):217–24. pmid:25370194. https://doi.org/10.3390/life4020217 ]. Most protists and fungi reproduce asexually by division, budding, or spore formation [ Otto Sarah P. The evolutionary enigma of sex. The American Naturalist. 2009;174(S1):S1–S14. pmid:19441962 https://doi.org/10.1086/599084 ].

5) MULTICELLULAR EUKARYOTES THAT ARE STRONGLY AFFECTED BY THE EMERGENCE AND PROLIFERATION OF MALIGNANT CELLS SHOULD PREDOMINANTLY HAVE OBLIGATE SEXUAL REPRODUCTION.

Obligate sex (that is, rigidly fixed, rather than virtual) is indeed the dominant mode of reproduction in many lineages of complex eukaryotes [ Bell G. The Masterpiece of Nature: The Evolution and Genetics of Sexuality. Berkeley, CA: University of California Press.; 1982.].

6) SEXUAL REPRODUCTION AROSE BECAUSE OF TRANSMISSIBLE CANCER

Frédéric Thomas of France’s University of Montpellier and his colleagues suggest that, in fact, the main driving force of evolution that compelled our ancestors to switch to sexual reproduction was not “external enemies” such as viruses or parasites, but a peculiar internal “fifth column.” According to the French evolutionary biologists, transmissible cancer cells played this role.

According to Thomas, this idea occurred to him after he drew attention to accounts of how DFTD, a peculiar transmissible cancer, almost completely destroyed Tasmanian devils. A similar “infection” is today wiping out populations of several mollusk species in the Atlantic at once, while among dogs a similar cancer has been spreading sexually for tens of thousands of years.

According to the evolutionary biologist, the emergence of such forms of tumors became possible because the level of genetic diversity among marsupial predators, mollusks, and the ancestors of dogs was record-low. This allows cancer cells to integrate into the organism of a new host, passing themselves off as “self,” and to use its resources for uncontrolled growth.

According to the evolutionary biologist, all this was also characteristic of the first multicellular creatures reproducing asexually. The level of genetic diversity among them was initially low and subsequently changed almost not at all, which allowed the first such cancer cells to spread unhindered throughout the entire population and infect any individual.

“The simplest way to protect against this is – to become unlike others. Sexual reproduction allows two individuals to produce unique offspring with an absolutely “new” genome. This will simultaneously protect them from invasion by already existing cancer cells, including those from their parents’ bodies, and make their own tumors incompatible with other representatives of their species,” — the evolutionary biologist continues.

In support of this, the scientist believes, are several considerations, including the fact that sexual reproduction is not characteristic of bacteria, archaea, unicellular protists, and many primitive fungi and plants, among which cancer in principle does not occur or arises extremely rarely.

In addition, cancer, unlike parasites, viruses, and other “external” threats, will always exist in a population of multicellular animals because mutations constantly accumulate in the DNA of every cell under the influence of cosmic rays, the Sun’s ultraviolet radiation, aggressive molecules, other random events, and simply with every division. This explains well why animals reproduce only sexually rather than combining sexual with asexual reproduction.

Frédéric Thomas et al. Transmissible cancer and the evolution of sex (June, 2019) https://doi.org/10.1371/journal.pbio.3000275 https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3000275

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REFERENCES:

• Matt Ridley “Sex and the Evolution of Human Nature,” — Moscow: Eksmo, 2011, pp.82-86. ISBN 978-5-699-48641-0

• (Eng.) Leigh Van Valen “A new evolutionary law”. Evolutionary Theory 1: 1—30. 1973