Theory of Asynchronous Evolution

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The Evolutionary Theory of Sex:
High Male Mortality (II
) The duration of life, genotype and environment

The duration of life, just as any other character, is determined by the genotype and environment. Let us imagine the following idealized situations: (1) If the genotypic diversity in population is eliminated (i.e. the clone of genetically identical individuals) and unfavorable environmental factors are removed (i.e. optimal conditions are created for a complete realization of the single genotype) the life span of all the individuals in such a clone will be similar, and the death rate curve will be of a rectangular shape (variation σ =0  Figure 2A). (2) If instead of the clone a heterogeneous population is taken in an optimum environment which permits realization of all the genotypes, some variance in the life span will appear which is conditioned only by genotypic variability (variation σgen Figure 2). (3) Now if the clone again will be taken, but placed into a real environment, the variance in the life span will be conditioned by the environment (variation σenv Figure 2). (4) If a genetically heterogeneous population is placed into real environment, the overall variation will include both, the genotypic and environmental components (σ = σgen + σenv) Figure 2). (5) Now let us imagine a situation when the death rate is determined only stochastically, incidentally (by the environment). Then the genotype will make no contribution to the life span, and the latter will depend neither on age, nor on health. In this case the death rate curve will have an exponential pattern (Figure 2).

Text Box: Survival, %
Figure 2

Death rate curves determined to varying degree by genotype and environment.
A—clone in ideal environment (σgen = 0, σenv = 0); B—heterogeneous population in ideal environment (σgen ≠ 0, σenv = 0);
C— clone in real environment (σgen = 0, σenv ≠ 0); D—heterogeneous population in real environment (σgen ≠ 0, σenv ≠ 0);
E—death rate is determined only stochastically.
I—average duration of intrauterine life; II—average of appearance of reproductive capacity,
III—average age of loss of reproductive capacity,
● —average life span, ○ —average duration of Ontogeny stages.

It is evident that the curves (lB), (1C), (1D) (as all the real cases met in life) are intermediate between the extreme types (lA) (the death rate is totally controlled by the genotype) and (lE) (it is under total control of the environment), therefore they can be presented as a superposition of these extreme types. Consequently, as the population gets “emancipated” from the environment, the pattern of its death rate approximates the rectangular type and moves away from the exponential type, and vice versa. Hence, the more optimum is the environment, the nearer to the rectangular type is the death rate pattern of the population; the more extreme it is—the closer to the exponential type.

Now let us see how the transition of the population from the optimum environment to the extreme one affects the duration of Ontogeny and its stages. It is evident that mortality strongly affects the average life span and the post reproductive period and has practically no effect on the average duration of periods of intrauterine development, growth, and reproduction (Figure 2). This means that a decrease in the average life span in extreme environmental conditions is the result of the environmental component of mortality and not the genotypic. Thus, the application of the law of sexual dimorphism is a basis for the hypothesis that genotypic average life span of men must be greater than that of women. And if it was possible to eliminate the effect of environment completely (to place the population in an ideally optimal environment), then men might possibly live longer than women. Such are the further conclusions that might be drawn from the theory. Now let us see what facts say about this.

Comparing the groups differently “emancipated” from the environment, such as men and women of one population, blacks and whites from one country, the same country in the course of history, etc. (Comfort 1964), one can see that the pattern of the death curve changes regularly in the direction indicated by the theory. The more is the group “emancipated” from the environment, the nearer is its mortality curve to the rectangular type and vice versa (see Figure 3). Greater “emancipation” of females as compared to males results from a wider hereditary reaction norm of the women. Greater “emancipation” of white population in the U.S.A. compared to the African-American one, as well as of modern population as compared with those of the former years is conditioned by social and economical factors (nutrition, medical service, etc.) (Figure 3).

 

Text Box: Number of survived, *1000
Text Box: Number of survived, *1000
Figure 3

Death rate curves (USA). А: Number of survived per 100 000 (USA, 1939–1941): I—African-American men; 
II— African-American women; III—white men; IV— white women. B: life span in USA in different years (Comfort, 1964).

Thus, the hypothesis suggested removes the contradictions between the known correlations and the sexual dimorphism rule. It explains why the average life span of women is longer, while the “champions” of longevity are men. For example, in the central part of southern Transcaucasia, in the 1840s, 14 of 15 persons aged 110-140 were men. In Abkhazia in the 1920’s all upper rungs of the age ladder were also occupied by men (Arkhipov, 1978).

This hypothesis permits also to understand the seemingly paradoxical fact that the phenomenon of long-living is encountered in the population living under far from optimum conditions (Comfort 1964, Davidovskij 1966).

If long-living is really conditioned by an increased dispersion of the life span under extreme environmental conditions, it should be closely related with stress as a transmitter of ecological information in animals. It should be accompanied by an increased death rate (the infants one first of all) by an increased dispersion of other characters, by the increased secondary sex ratio (birth of boys) and by a rise of sexual dimorphism. The populations in which long-living individuals are found should have the death rate curve nearer to the exponential type, characteristic of extreme conditions. While the death rate curve of the population in an optimum environment should be nearer to the rectangular type, where the long-living is not observed.

 

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More about High Male Mortality:
Sexual Dimorphism and Evolution of Duration of Ontogeny and its Stages. Geodakyan V. A. Doklady Biological Sciences, 1982, v. 263, N 1-6, p. 174–177. Translated from Doklady Akademii Nauk. SSSR, Genetika, Vol. 263, No. 6, pp. 1475-1480, April, 1982.

 

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

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