Theory of Asynchronous Evolution


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The Evolutionary Theory of Sex: Sexual Dimorphism—Forms

According to the evolutionary theory of sex sexual dimorphism consists from two components: reproductive sexual dimorphism (RSD) and earlier unknown evolutionary sexual dimorphism (ESD) predicted by the theory.

Reproductive sexual dimorphism is permanent. It involves primary and secondary sexual traits different in the male and female sex that have direct relation to reproduction (gametes, gonads, genitals, androgen-estrogen ratio, and all traits determined by them: reaction norm, mammary glands, beard in man, lion’s mane, heel of cock, etc.). These are fundamental species-specific traits. According to the theory, genes for these characters should be common for both sexes. Since there are no genetic differences, the reproductive sexual dimorphism is hormonal; i.e., phenotypic. Its function is to set up programs for the two sexes.

Modificational sexual dimorphism is temporary and is limited to Ontogeny. It appears as a result of female sex changes due to its broader reaction norm. It precedes evolution of any trait. The wider the reaction norm for the trait, the higher the associated sexual dimorphism. The purpose of modificational sexual dimorphism is to protect the female sex from selection, until new genes appear after been tested in the male genome. An example of such dimorphism is adaptations of females living in the Arctic: thick layer of subcutaneous fat, short legs, and high mineralization of skeleton.

Evolutionary sexual dimorphism appears as a consequence of asynchronous evolution of any trait as a result of any type of selection—natural, sexual, or artificial. It represents the “distance” between sexes. The vector of sexual dimorphism (from the female form of the trait to the male one) corresponds to the direction of trait evolution. The female form of the trait indicates the past state, while the male form indicates the future state.

Any population that exists in the stable environment for a long time has only reproductive sexual dimorphism. Evolutionary sexual dimorphism is absent, and modificational sexual dimorphism is only of a variational nature, since the phenotypic variance of the male sex in a stabilizing environment is greater than that of the female sex.

In changing environment the evolutionary sexual dimorphism is a sum of new information ( In ) that has already entered the male sex (but did not yet enter the female), and old information ( Io ) that is still retained in the female sex (but has already been lost by the male sex). It should be pointed out that when two populations interbreed (races or ethnic groups), the common information undergoes mixing after the first cross, while the new and the old information remain segregated throughout the period of sexual dichrony. This view easily explains differences of interspecies, interracial or interethnic reciprocal hybrids associated with the direction of crosses.

Sexual Dimorphism—“Last News of the Evolution”

In relation to sex the characters of the organisms can be divided into three groups. The first group includes the characters which show no difference between males and females. 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. It is observed only at some pathological conditions, and expressed in different frequency of some congenital anomalies in males and females. The idea as to classifying congenital defects of development into “atavistic” (regressions or interruption of development) and “futuristic” ones (search for new pathways) permits in some cases to follow in such sexual dimorphism general trends predicted by the theory. 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. Is it accidental or this fact implies a certain evolutionary tendency of oligomerization of multiple organs? Note that some worms have in each body segment a pair of specialized excretory gland—metanephridia. Consequently occurrence of three kidneys can be considered as an “atavistic” trend, and of one kidney—as a “futuristic” one. Other example: congenital hip luxation occurs 4–5 times more often in girls than in boys. It should be noted that infants with this defect better than normal ones run on all fours and climb the trees. The third example—anencephaly is also twice more often observed in girls.

A special study has been conducted on congenital defects of the heart and main blood vessels.

The second 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. These are quantitative characters such as height, weight, size and proportion, many morpho-physiological and ethologo-psychological characters. Sexual dimorphism on such characters can be determined as the difference between mean values of this character for males and females of the population. Such "populational" sexual dimorphism can show direction of the character evolution.

For example, the evolution of most vertebrates is accompanied by enlargement, whereas most insects and arachnids become smaller.  Therefore, vertebrate males should be larger than females, whereas male insects and arachnids should be smaller than females.  It is exactly so (Fig.).

The phylogenetic rule of sexual dimorphism was successfully checked up on a large group (173 species) of lower Crustacean (Geodakian, Smirnov, 1968). Males in all cases are smaller than females. According to the rule we can assume that this group has a common evolution trend of size reduction. In fact, it is well-known that morphologically more primitive forms of Crustacean are larger. Inside the group we can pick out arrays of forms with sequential size and number of extremities reduction, and specialization according to phylogenetic preemptivity and smaller male size.

The same reasoning is applicable to separate species and populations. For example, dog's ancestors (wolf, fox, and jackal) seem to have average size as compared to large (mastiff, St. Bernard weighing up to 70 kg) and small (Chihuahua— 2.5-3 kg, Toy Terrier up to 400g) dogs. The theory predicts larger-sized males in large breeds and females in small ones.

The same tendencies should be observed inside smaller taxa, say in mammals females of small forms are larger than males, while large forms has larger males. For example African savanna elephant males weight up to 6.5 ton, but females up to 3.5 ton only. Small forms—some bats, flying squirrels, spotted hyenas, dwarf mongooses, rabbits and others frequently has larger females.

This rule is also valid for plants. For example, in poplar female specimens have more elongated leaves, the male ones more rounded ones. Leaves of ginkgo female tree have even edges and are smaller, of male ones larger and cut. As known, poplar phylogenetic ancestors had narrow (like willows) leaves, while gingko ancestors—the uncut ones.

The third group is the characters inherent to one sex only. 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. Sexual dimorphism for these characters is genotypic, because they are absent in the phenotype of one sex. Still information about these characters is written in the genotype of both sexes. If these traits undergo evolution, the genotypical sexual dimorphism on them should exist and it can be revealed in the form of reciprocal effects. In humans all social, psychological characters related to the large hemispheres cortex of the brain, and to their asymmetry (primarily, speech, abstract thinking, spatial imagination, humor and other creative abilities) can be attributed to the “new” characters.

Thus the interpretation of sexual dimorphism as a phylogenetic “distance” between the sexes, as evolutionary “news” having already arrived to males, but not to females is applicable to all characters of humans, animals and plants for which sexual dimorphism is observed. Only in case of specific characters the regularity is manifested in pathological fields, of populational ones in the norm, while in case of sex characters as the “paternal effect”.

If there is sexual dimorphism according to a certain trait, then with age this trait will change, as a rule, from the female form to the male one. So, the female form of a trait is more characteristic of the initial, juvenile stage, while the male form is more characteristic of the definitive stage (mature, adult).


More about Sexual Dimorphism:

Sexual dimorphism. Geodakyan V. A. In: Evolution and morphogenesis. (Mlikovsky J., Novak V. J. A., eds.), Academia, Praha, 1985, p. 467–477.




Copyright 2005-2009 S. Geodakyan. All rights reserved.


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