Theory of Asynchronous Evolution


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The Evolutionary Theory of Sex: Sex Ratio

Sex ratio (SR) is one of the main characteristics of sexual population. Generally it is determined by the number of males per 100 females, or in percents. In relation to Ontogeny stage we distinguish primary (I SR), secondary (II SR) and tertiary (III SR) SR. Primary is zygote SR after fertilization; secondary - SR at birth; and tertiary - SR of mature organisms.

The main sex determination mechanism in many animals and plants species is chromosomal. Since the gametogenesis process produces an equal number of X- and Y-gametes, one may consider that this mechanism provides nearly equal quantities of sexes.

According to the new concept tertiary SR determines the proportion between maintenance and variation tendencies as well as the species evolutionary flexibility. At different stages of evolution, and also in different environmental conditions population needs different evolutionary flexibility (and consequently there exists a definite optimal tertiary SR value for each of these conditions). And this value is not necessarily equal to 1:1. Secondary SR is also a variable dependent on the environment, rather than a constant specific for a species, as it was believed. In stable environment secondary SR is at its optimum level.

The extreme environment and "turnover" 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 “turnover” of males increase. In 1965 it was proposed that besides the direct relationship, there exists a negative feedback between secondary SR and tertiary one (Fig.).

Figure. Negative feedback, regulating population sex ratio

Two negative feedback mechanisms are possible:

1. The initially genetically determined probability to have offspring of given sex is equal for all males and females in population, but environmental conditions may bring about change in this probability. This mechanism may be called organismic, or physiological. It may regulate SR only in polygamous or panmictic population.

2. Different organisms have genetically determined different probability to produce offspring of a given sex and this probability is related to hierarchical, social range of an animal. With this mechanism regulation may be on the population level, through greater or lesser participation of individuals, giving birth to more males or females. This mechanism may be called populational. For the negative feedback to work, the dominant (α-range) males should have low probability to produce more males in the progeny, while subordinate (Ω-range) males should have more males. Regulation is accomplished by changing the participation in reproduction of individuals of different range. Unlike the organismic mechanism, the populational mechanism may regulate the SR not only in polygamous or panmictic, but also in fully monogamous populations.

Mechanisms of organismic regulation

For the organismic type of regulation these factors are: a) the amount of pollen caught on the female flower: the amount is directly proportional to the number of male flowers surrounding the female flower, and consequently, tertiary SR; b) pollen aging and elimination: the more male plants there are around the female plant, the less time is needed to pollination and vise versa; c) for polygamous or panmictic animals - the intensity of sexual activity, which is directly related to the number of the same sex, and reversely proportional to the number of the opposite sex, thus also dependent on tertiary SR; d) male or female gametes aging and elimination (delayed fertilization); the aging probability in the organism of abundant sex is always greater than in the organism of deficient sex, so there exists a relation between gamete aging and tertiary SR (Fig.).

Negative feedback in insects. Bees and other hymenoptera, ticks have an unusual negative feedback mechanism. From fertilized eggs only females (or females and males) are born, while from unfertilized ones - only males are developed. Generally, the fewer males there are in the initial population, the fewer eggs are fertilized, and more males appear in offspring.

SR and sexual activity. Intensity of sexual activity (ISA) may be the chain link between the tertiary SR and the secondary SR in animals (Geodakian, 1965). On the one hand, it depends on the tertiary SR: for each sex the activity diminishes with the increase of the number of individuals of one's own sex, and increases, when the opposite sex number rises. On the other hand, ISA is related with organismic physiological parameters. Consequently, if negative feedback relation exists, and is realized through ISA, then high male birth-rate on high ISA of males will be observed. Low male ISA will result in high female birth-rate. For females the picture is reversed: high female ISA results in an increased female birth-rate probability, low - males. So, high ISA increases the probability of the birth of a child of the same sex, low ISA - the birth of a child of opposite sex.

The negative feedback concept was confirmed in direct experiments on at least 3 species: 1 plant and 2 animal ones. Many plant, and animal species and also humans have necessary mechanisms for its existence (4 plant species from 3 families, and 16 animal species from 11 families).

Humans are not strongly monogamous, therefore different secondary sex ratio deviations from 1:1 may arise. For Nigerians Thomas (1913)
has reported secondary sex ratio values in relation to wives number (Figure). It may be concluded that there exists a reverse relation
between secondary and tertiary sex ratio. Furthermore, if a negative feedback exists, an increase of the secondary sex ratio can be
observed in harems. Figure shows data on three harems: Chu Juanchshan (U Han, 1980) (1328-1398, China), Ramses II (Ebers, 1965)
 (1317-1251 B. C., Egypt), and Mauli Ismail (Asia and Africa today, 1970) (1646-1727, Morocco). These data are not easy to account for
by purely stochastic sex determination. The probability of random deviation from 1:1 under such boy excess is 10-15. The number of
mothers and progeny makes the effect statistically significant; however the number of fathers is small.

Populational mechanisms of sex ratio regulation

Two things are required for the realization of the populational mechanism. First it is necessary that in a population there was a genetically caused polymorphism on probability to leave posterity with the certain sex ratio. And second, that this probability should be in reverse correlation with a reproductive rank of a person—the higher the reproductive rank, the more offspring of an opposite sex the person should produce. Reproductive rank of a male (access to marriage partners) correlates, as a rule, with their social-hierarchical rank (access to resources in general). Females may have a reverse correlation, because their hierarchical rank, as well as for males, is defined by their strength and aggressiveness, but reproductive one can be determined more by their appeal and compliance.

Expected sex of offspring in various conditions

In natural conditions disturbances of tertiary sex ratio from optimum always make one sex “deficient” and the other—“abundant” with opposite changes of their states. Deficient sex, in average, has more ISA, frequently have sex saturation, in fertilization participate more freshly gametes etc. And abundant sex, reversely, has less ISA, frequently has sex starvation; in fertilization, older gametes participate etc., compared with the same characteristics of optimal sex ratio population. Deviations from tertiary sex ratio optimum influence animals with low reproductive range first. In monogamic populations they remain without mating partners, and in panmictic populations—their ISA decreases. Such disturbances always lead to the increase of deficit sex birth-rate independently of organismic or population mechanism occurs and where it acts—inside male or female organism (Table).

Table.  Theoretically expected sex of offspring in the relation to the intensity of sexual
                activity (ISA) for organismic or populational types of feedback, acting inside male
                and female organism, for natural and artificial disturbances of ISA optimum.

Disturbance of ISA optimum

ISA of

Negative feedback



In the organism of:


















Artificial situations are also possible, when the state of both sexes is exposed to similar changes. For example, males and females have low ISA and old gametes when kept in isolation, since both sexes are in abundant state. The result of sex ratio regulation depends on which of the parent organism has a negative feedback mechanism. Analysis of natural secondary sex ratio deviations may show the type of negative feedback mechanism: organismic or populational.

More about Sex Ratio:

Is there a negative feedback in sex determination? Geodakyan V. A., Geodakyan S. V., Translated from Zurnal obschej biol. 1985, 46, p. 201-216.

The Amount of Pollen as a Regulator of Evolutionary Plasticity of Cross-Pollinating Plants. Geodakyan V. A. Doklady Biological Sciences, 1977, v. 234, N 1-6, p. 193–196. Translated from Doklady Akademii Nauk, Vol. 234, No. 6, pp. 1460-1463, May, 1977.


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


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