Evolutionary theory of sex

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The Evolutionary Theory of Sex was proposed in 1965 by V. Geodakian and now provides explanation of many sex-related phenomena such as sexual dimorphism, sex chromosomes,[1] asymmetry of brain[2] and hands, [3] reciprocal effects, and congenital heart defects.[4]

The theory was included in the textbooks,[5][6], college study programs,[7][8] was covered in numerous newspaper and magazine articles[9] (two in the US[10][11]) and TV programs. [12][13][14]

Contents

[edit] Analysis of a sex riddle

The sex notion consists of two fundamental phenomena: the sexual process (conjugation of genetic information of two persons) and the sexual differentiation (partitioning this information into two parts). Depending on the presence (+) or absence (-) of these phenomena, the whole variety of existing reproduction models can be divided into three basic forms: asexual (-, -), hermaphrodite (+, -), and bi-sexual reproduction (+, +).

The sexual process and sexual differentiation are distinct, and moreover, directly opposite phenomena. Indeed, the sexual process diversifies genotypes, which is its objective in evolution, whereas differentiation halves the resulting diversity. For instance, in an asexual population of size N, the maximum theoretically possible variability of offspring genotypes is N, given that the genotypes of all parents are different. Since the offspring of each asexual individual is a clone with the same genotype, the variability of the offspring σ is always lower than N.

In the sexual process, the variability of offspring is squared. In hermaphroditic organisms, each of N individuals can mate with all individuals except itself, i.e., N - 1; but, as the cross of individual A with individual B is the same as that of individual B with individual A (there is no reciprocal effect), at N >> 1 , σ = N(N - 1)/2 ≈ N2/2; with the reciprocal effect, σ = N2.

In dioecious forms, sex differentiation that excludes same sex combinations (male-male, female-female), decreases the amount of diversity possible in hermaphrodites by at least two times: σ = N/2 x N/2 = N2/4 (each female with each male, with the same number of males and females equal to N/2). The offspring diversity in a population of dioecious organisms also depends on the sex ratio in the parental generation: it is the highest at a 1:1 sex ratio and decreases with any deviation from it.

The maximum progeny diversity of the asexual, hermaphrodite, and bi-sexual populations of the same size N are related as N  : N2/2 : N2/4, i.e., the diversity is at least halved while passing from hermaphrodite to bisexual reproduction! Then, it becomes completely unclear what the differentiation is intended for, if it halves the main bonus provided by the sexual process. Why are all progressive species bi-sexual, since the asexual process is much more efficient and simple, and hermaphrodites produce a more diversified progeny? This is the essence of the sex puzzle.

The fact that this problem is still unsolved is primarily due to the lack of a clear understanding that the sexual process and sexual differentiation are opposite phenomena. Researchers make attempts at understanding the advantage of the sexual reproduction (hermaphrodite and bi-sexual forms) over the asexual one, although it is necessary to understand the advantage of bi-sexuals over hermaphrodites.

The purpose of the sexual process is clear, and consists of diversifying. It is needed to comprehend the objective of the sexual differentiation. Although it is recognized that, because bi-sexual methods have no visible advantages over asexual ones, bi-sexual reproduction should provide us with significant evolutionary bonuses, the sex problem is commonly considered as a reproduction problem but not an evolutionary one.

[edit] Conservative-operative specialization of sexes

Sex differentiation is specialization in preserving and changing the genetic information of population. One of the sexes should be informationally more closely connected with the environment, and be more sensitive to the environmental factors. Higher mortality and vulnerability of the males to all harmful factors of the environment make one believe that it is the operative, ecological subsystem of the population. While the females are the conservative subsystem that preserves the existing genotype distribution in the population. A series of mechanisms were developed during different stages of sex evolution to provide this specialization. Compared to females, males experience more mutations, inherit fewer properties of their parents, have narrower reaction norm, higher aggressiveness and inquisitiveness, riskier behavior and other properties that move them closer to the environment. All these properties, moving the male sex to the frontline of the evolution, provide for receiving of ecological information. The second group of properties includes great superfluity of male gametes, their small size and high mobility, the greater activity of mobility of males, their inclination towards polygamy and other ethologic and psychological qualities. Long periods of pregnancy, feeding and taking care of the descendants among the female population in reality increases the efficient concentration of the males, turning the male sex into superfluous, and thus cheap, while the female sex is turned into deficit and thus more expensive. As a result of conservative-operative specialization of sexes, asynchronous evolution takes place: new characters first appear in the operative subsystem (males), are tested there, and then are passed on to the conservative subsystem (females).

[edit] Receiving the Ecological Information from the Environment

First, change of factors of environment can eliminate most sensitive to the given factor part of individuals, as a result of natural selection. Second, environmental changes create discomfort conditions. As a result other part of a population can be partially or completely discharged from duplication, due to sexual selection. Third, the changed environment modifies survived part of a population, creating morpho-physiological, behavioral and other non inherited adaptations, due to a norm of reaction. For example in a cold conditions the animal’s tails are shortened, the fur becomes thicker, the hypodermic fat layer increases, humans use caves, clothes, fire.

First two processes (elimination and discrimination) remove some genotypes from a reproduction pool. The third process (modification), on the contrary, allows some genotypes to survive under the shell of the modified phenotype and to get in genes of posterity. That is someone should be broken, killed, discharged, and someone—bended, “educated”, and altered.

To receive the ecological information from the environment males should have a greater phenotypic variation which can be a consequence of their wider genetic variation. It can also reflect wider reaction norm of females which allows them to leave zones of elimination and elimination discomfort. Wider genetic variation of males can be due to their higher mutation rate. More additive inheritance of parental characters by female offspring can decrease their variation compare to males.

[edit] Mechanisms of regulation of the population parameters

There are two mechanisms that control population parameters in animals – stress and sex hormones. The plants receive ecological information through the amount of pollen.[15] It seems that particular nature of the environmental factor which causes the discomfort of the organism has no significance for starting up these mechanisms. The cause of the discomfort (frost, dry periods, famine or enemies) makes no difference. The “generalized” ecological information has one dimension only (“good” or “bad”) and its cause is unimportant.

[edit] Wider reaction norm of females

Wider reaction norm of females was theoretically predicted in 1973.[16][17] It means that in males the share of "hereditary component" must be larger and of the "environmental one" smaller than in females. Therefore in males the phenotypical distribution in population better reflects the genetic distribution. In females the environmental influence in ontogenesis is stronger, therefore any ontogenetic shift, any "education" or "training" is more efficient.

If the hypothesis is valid, the differences between monozygous female twins must be greater than between the male ones. At the same time in dizigous twins like in common siblings, everything must be vice versa. Two studies conducted on 44 monozygous pairs[18] and 53 monozygous and 38 dizigous pairs of twins[19] confirmed the predictions.

Much direct and indirect evidence can be presented in favor of the hypothesis. For example, greater conformism of females well known to psychologists has not been adequately interpreted up till now.[20][21][22]

[edit] Ontogenetic plasticity

The wide reaction norm makes females more flexible in ontogenesis (adaptive). It enables the females to leave elimination and discomfort zones and to be gathered in the comfort zone around the population norm. It narrows the phenotypical dispersion of females and decreases their mortality.

Contrary, the narrow reaction norm of males makes them less flexible in ontogenesis, does not permit the phenotypical dispersion to be narrowed, i.e. to leave the elimination and discomfort zones. Greater phenotypic variation of male sex makes it more sensitive to the environment and increases its damageability and mortality. Slightly exaggerating, it is possible to tell, that informational relationship of a population with the environment is based upon the elimination of males and the “education” (ontogenetic shift) of females.

[edit] High mortality of males

In a course of ontogenesis the sex ratio for many species of plants, animals and humans goes down. It is related to the raised death rate and damageability of male’s systems in comparison with female ones at almost all ontogenesis stages and at all levels of organization. Whether we study various species (the humans, animals or plants), different levels of the organization (an individual, organ, tissue or a cell) or stability to different harmful factors of environment (low and high temperature, starvation, poisons, parasites, diseases, etc.)—anywhere the same picture is observed: the raised death rate or damageability of male’s systems in comparison with corresponding female’s.

Hamilton (1948) reviewed differential gender death rate for 70 species, including such various forms of a life, as nematodes, mollusks, crustaceans, insects, arachnoidea, birds, reptiles, fishes and mammals. According to these data, for 62 species (89%) average life of males is shorter, than females; for the majority of the remaining 11% there is no difference, and only on occasion males live longer, than females.[23]

Higher mortality of males is one of the puzzles of sexuality, a general biological phenomenon, which no theory could explain satisfactorily. In new theory it is interpreted as a “payment” for new ecological information, as a useful form for population to get new information from the environment. For example males have higher susceptibility to all “new” diseases of our century (infarction, arteriosclerosis, cancer, schizophrenia and others).

[edit] The extreme environment and "reversibility" of males

Under extreme conditions, as a rule, more males are extinct and simultaneously more males are required for selection. Both males’ mortality and males’ birth-rate increase imply “reversibility” of males increase. In 1965 it was proposed that besides the direct relation, there exists a negative feedback between secondary and tertiary sex ratio

The feedback is represented in cross pollinating plants by the amount of pollen caught on the female flower and in animals by the intensity of sexual activity expressed via unequal aging of X- and Y-sperms and different affinity of the fresh and aged eggs to these latter. The small amount of pollen, intensive sexual activity of males, fresh sperm and aged eggs are factors leading to increase in the number of males.[24][25]

[edit] Channel "cross-section" of the information transfer to the progeny

Father and mother transfer each to descendant approximately identical amount of the genetic information, but the quantity of progeny that males can produce is much more compared to female. One male basically can transfer the information to the entire generation of a population. The females can not do that.

[edit] Transmission of genetic information to the progeny. Sexual dimorphism in one generation

In a strictly monogamous population the number mothers and fathers is equal. Contrary, in a panmictic or polygamous population the number of mothers is always greater than the number of fathers. It means that the hereditary ("old") information concerning the distribution of genotypes in panmictic population is better, more completely and representatively transmitted by the females.

Whereas a wide communication channel between males and their progeny makes possible better transmission of ecological ("new") information to the offspring. By leaving more offspring rare male organisms can multiply their ecologically valuable genotypes. So, in changing environment, different reaction norm and channel to the progeny create genotypic sexual dimorphism in the first generation

Is this "new" information gets leveled at fertilization or gets preserved? The existence of the reciprocal effects suggest that genetic mechanisms exist that prevent complete mixing of all genetic information. This is accomplished via Y chromosome which is transferred from father to son.[26]

[edit] Sexual dimorphism on different characters

In relation to sex the characters of the organisms can be divided into three groups.

[edit] Characters equal in both sexes

The first group includes the characters which show no difference between sexes. Distribution of such characters in males and females in the population is similar. Among these are the majority of specific characters (number of organs, extremities plan and general structure of the body and many others). There is no sexual dimorphism for these characters in the norm. (Sometimes distinct sexual dimorphism is observed at some when these specific characters are disturbed. Such sexual dimorphism may be related to the evolution of the given character. For example, among 2000 newborns with one kidney there were twice as much boys, while among 4000 newborns with three kidneys there were 2.5-fold more girls. It does not seem accidental.)

[edit] Characters present in one sex

The second group of characters is those inherent only to one sex, while in the other one they are absent. In this case they are present or absent in the norm in all the individuals of the given sex. Sexual dimorphism of this type can be named the "organismic" one. It has an absolute pattern; it distinguishes any male individual from the female one. These include all primary and secondary sexual characters (internal and external sex organs, mammary glands, beard in man, mane in lion, and many economically valuable characters—egg, milk, caviar production). Quantitative estimation of such character and of its distribution pattern in the population is sensible only for one sex, for another one it equals zero. However they are genetically distributed in both sexes (e.g. information concerning egg yield of the breed lies in genotypes of hen and rooster as well). Because phenotype of one sex lucks the character, one can judge genotypic sexual dimorphism by reciprocal effects

[edit] Characters present in both sexes

The third group of characters are those which are presented in both sexes but are differently pronounced or/and are met in the population with different frequency depending on sex. This group of characters according to the expressivity of sexual dimorphism (which is above zero and below one) is the intermediate one between the first (the sexual dimorphism equals zero) and second (sexual dimorphism equals one) ones. The pattern of sexual dimorphism for these characters is not an absolute, organismic, but the populational one (different distribution of the character in males and females in a population). Such "populational" sexual dimorphism may be determined as the difference between mean values of this character for males and females of the population. It is valid for the entire population but can be inverse for a pair of individuals. While organismic sexual dimorphism is valid for any individual of the population (any mammalian female has mammary glands, and the male has none). Populational sexual dimorphism can exist for such characters as stature, weight, size, proportions, many morpho-physiological and ethologo-psychological characters.

[edit] Sexual dimorphism and evolution of the characters

Consequently sexual dimorphism must be first of all closely related to the evolution of the character; it must be minimal for unchanging stable characters and maximal for the characters in the course of evolution (appearing, disappearing or altering). It could be expected that sexual dimorphism must be more pronounced for phylogenetically younger evolutionizing characters. Then, as we have seen already, sexual dimorphism is related to the population structure: in strictly monogamous populations it must be minimal, since they use sex specialization only at the organismic but not at the population level; their sexual dimorphism is mostly the organismic one.

[edit] See also

[edit] References

  1. ^ Geodakyan V. A. (2000). Evolutionary Chromosomes And Evolutionary Sex Dimorphism. “Biology Bulletin” 27 N 2, 99–113.
  2. ^ Geodakyan V. A. (1992). Evolutionary Logic of the Functional Asymmetry of the Brain. “Doklady Biological Sciences” 324 N 1-6, 283–287.
  3. ^ Geodakyan V. A., Geodakyan K. V. (1997). A New Concept on Lefthandedness. “Doklady Biological Sciences” 356 N 1-6, 450–454.
  4. ^ Geodakyan V. A., Sherman A. L. (1971). Svjaz' vrozdennych anomalij razvitija s polom (Relation of birth defects with sex). “Zh. Obsh. Biol.” 32 N 4, 417–424.
  5. ^ Vasiltshenko G. S. (1977, 2005) General sexopathology. Moscow, Medicine 488 p.
  6. ^ Nartova-Boschaver S. K. (2003) Differential psychology: Textbook. Moscow. Flinta Moscow psychological-social institute.
  7. ^ Moscow Institute of Physics and Technology. Department of Molecular and Biological Physics. Lectures for the 1st grade “Biology basics”. Lecture #24 Evolutionary Theory of Sex. Biotechnology. Immunology. Signal transmission in the body. http://www.fizhim.ru/student/files/biology/biolections/lection24/
  8. ^ Kharkov national university (Ukraine). Faculty of psychology. Department of general psychology. Cycle of lectures “Gender studies in psychology” Lecture #11. Studies of gender differences in brain organization and cognition. http://www.gender.univer.kharkov.ua/speckursy-025.shtml
  9. ^ Bibliography
  10. ^ Rahlis L. (1998) Why God created Adam and Eve? Russia House (Atlanta, GA), September N 9 (68), 4.
  11. ^ Rahlis L. (1999) Supplementing each other. Russia House (Atlanta, GA), February N 2 (73) 5.
  12. ^ Gordon A. (2002) Evolutionary theory of sex. “Program «00:30»” NTV, June 06.
  13. ^ Gordon A. (2002) Evolutionary theory of sex-2. “Program «00:30»” NTV, Apr 15.
  14. ^ Gordon A. (2003) Theory of brain asymmetry. “Program «00:30»” NTV, Dec 09.
  15. ^ Geodakyan V. A. (1977). The Amount of Pollen as a Regulator of Evolutionary Plasticity of Cross-Pollinating Plants. “Doklady Biological Sciences” 234 N 1-6, 193–196.
  16. ^ Geodakyan V. A. (1973). Differential Sex Mortality and Reaction Norm. “Biol. Zh. Arm.” 26 N 6, 3–12.
  17. ^ Geodakian V. A. (1974). Differential Mortality and Reaction Norm of Males and Females. Ontogenetic and Phylogenetic Plasticity (russ) “Zh. Obshch. Biol.” 35 N 3, 376-385.
  18. ^ Vandenberg S. G., McKusick V. A., McKusick A. B. (1962). Twin data in support of the Lyon hypothesis. “Nature”. 194 N 4827 505–506.
  19. ^ Chovanova E., Bergman P., Stukovsky K. (1980) Abstracts of communications of II Congress of European Anthropological Association, Brno, 136.
  20. ^ Harper E. B., e.a. (1965). Young children's yielding to false adult judgment. “Child. Development”. 36 175–183.
  21. ^ McCoby E.E. (1966). The Development of Sex Differences, Stanford, Stanford Univ. Press.
  22. ^ Kon I.S. (1967). Sociology of Personality, Moscow, "Politizdat".
  23. ^ Hamilton J. B. (1948). The role of testicular secretions as indicated by the effects of castration in man and by studies of pathological conditions and the short life span associated with maleness. “Recent Progress in Hormone Research” 3 N.Y., Acad. Press, 257–322.
  24. ^ Geodakyan V. A. (1977). The Amount of Pollen as a Regulator of Evolutionary Plasticity of Cross-Pollinating Plants. “Doklady Biological Sciences” 234 N 1-6, 193–196.
  25. ^ Geodakyan V. A. Geodakyan S.V. (1985). Is there a negative feedback in sex determination? “Zurnal obschej biol.” 46 N 2 201-216.
  26. ^ Geodakian V. A. (2000). Evolutionary Chromosomes And Evolutionary Sex Dimorphism. “Biology Bulletin” 27 N 2, 99–113.

[edit] External links

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