Theory of Asynchronous Evolution

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The Evolutionary Theory of Sex: High Male Mortality and Gamete Type

In a course of Ontogeny 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 Ontogeny 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 reviewed 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.

According to United Nations data, women live longer than men in most countries. Higher death rate of men is observed almost from all illnesses, with just a few exceptions (pertussis, some gonococcal infections, etc.). Males have also higher rates of unintentional as well as intentional injuries as a consequence of more risky behavior, aggression and job preference (police, fireman, soldiers etc). Summarizing, it is possible to tell, that the raised death rate of males is a general biological phenomenon, it is observed at plants, animals and humans for all levels of organization from all extreme factors of environment.

The existing theories are not capable to explain the phenomenon of the raised males’ death rate. The Evolutionary Theory of Sex considers raised death rate and damageability of males as a favorable to a population form of information contact with environment, a payment for the new (ecological) information. This is achieved due to the greater variation of male attributes. In other words on all attributes, males should be more various than females. Wider phenotypic variation of males allows population “to pay” for the new information mostly by male individuals.

Gamete Type and Sex

With respect to sex, the conflict on conservative and operative evolution tendencies was raised at least twice. First time, when isogamy on the cell level existed, the conflict requirements to their dimensions were arisen. The operative task was to find another cell. In order to accomplish this task, small size and mobility were required. Conservative task was to preserve the formed zygote, to supply nutrition and energy resources, protective membranes etc., which is related to a big size. As a result the gametes were differentiated by size and mobility on ova and spermatozoids.

The differentiation by the gamete type had the same problem. It was necessary to try recessive genes before including them permanently in the genome. It can be accomplished in autosomes in homozygous state, and in gonosomes only in hemizigous state in heterogametic set of chromosomes (XY). This is operative tendency. Heterozygous gene combination in autosomes and in homogametic set (XX) realize conservative tendency, because defective recessive gene are not manifested.

In the process of evolution most species had both operative subsystems (small gametes and heterogametic constitution XY) in the male sex and both conservative subsystems (large gametes and homogametic constitution XX) in the female one. These are the species with Drosophila type of gamety (Table).

Table  Conservative and operative subsystems on different
levels of organisation in mammals and birds.

Organization
level
Heterogamety
Male Female
Organismic
Cellular Spermatozoids Ova Spermatozoids Ova
Chromosomal XY XX XX XY

But, in the evolution of some species the directions of these two differentiations did not match. The conservative ova combined with the operative heterogamety found themselves in the female sex, and combination of operative spermatozoids with conservative homogamety—in the male sex. These are species with Abraxas type of gamety. So, from the presented point of view, the Drosophila type of gamety is consistent, but the Abraxas type of gamety is contradictory. This can explain the fact that there are a lot more of Drosophila species, assuming that the selection of type was purely stochastic (independent of sex).

The role of gamety type and polygamy

The effect of differential death rate of sexes consists of two components (Figure 1). The first component, explained well by the theory of imbalance of genes, is the contribution of gamety type. The second component—is a contribution from specialization of sexes at the population level. The population effect arises in panmictic, freely crossed population only, and is depends, probably, upon a degree of polygamy. Therefore, to explain various death rates of sexes one needs to take into account a gamety type as well as mono- or polygamy of a given species.

Figure 1

The impact of sex specialization at the population level (the top part of figure) and the contribution of gamety
type (the bottom part of figure) in differential death rate of sexes. Population effect arises only in freely crossed
(polygamous) population, and is not valid for monogamous species. For polygamous species the higher death rate
of males is common. A male sex and heterogametic constitution (XY) are responsible for the operative task, while
female sex and homogametic constitution (XX) – for the conservative one. The directions of these two differentiations
do not coincide in some species (for example, for birds). Thus for polygamous species with female heterogamety,
the population effect is directed against the effect of gamety, while for polygamous species with male heterogamety
these effects work in the same direction.

For a monogamous species the population effects are minimal and the increased death rate is observed for heterogametic sex (for males of Drosophila type and females of Abraxas type). The facts are well described by the theory of genes imbalance. For the polygamous species of the Drosophila type, the population effect is imposed upon the effect from gamety thus strengthening it. This explains the maximal difference in death rate observed for such species. For polygamous species of Abraxas type, the population effect is directed against the effect of gamety: heterogametic constitution leads to the reduction of female longevity, while the population effect—the male one. Therefore it is possible to expect, that the difference in death rate for such species will be less expressed, than for the polygamous species of the Drosophila type with the same degree of polygamy. The consequence from abovementioned statements is that monogamous species of the Abraxas type should have higher female mortality that actually was observed.

Therefore, the new approach explains the observed pattern of differential mortality of sexes well. Strictly speaking, for the realization of the population mechanisms of sex specialization it is important that male mortality should “precede” the female one. At an adult age man’s death rate exceeds female. It partly can be connected by that all “new” illnesses, illnesses of the “century” or “civilization” (arteriosclerosis, hypertony, cancer, AIDS, coronary diseases and schizophrenia), as a rule, are illnesses of a male.

Existing theories—chromosomal imbalance and metabolic—consider differential mortality of sexes as a passive consequence of chromosomal constitution or level of metabolism. They explain the mechanism of a phenomenon and substitute the evolutionary problem by genetic (imbalance theory) or physiological one (metabolic theory). New theory considers increased mortality of male sex as an active feature, increasing evolutionary stability of the population. New approach clarifies evolutionary meaning of differential mortality relating it with different reaction norm of the sexes.

                The duration of life, genotype and environment   

 

 

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

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