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The Evolutionary Theory of Sex: “Paternal Effect”

“Paternal Effect” helps explain centuries-old biological observation that both mule and hinny look closer to their fathers.


Some characters belong to one sex only (primary and secondary sexual traits, many useful characters—egg, milk, caviar production). Because phenotype of one sex lucks the character, one can judge genotypical sexual dimorphism by reciprocal effects. It follows that while with “old” stable characters the genetic contribution of father to the offspring is somewhat smaller than that of mother due to maternal effect stipulated by cytoplasmic inheritance and uterine development, with “new”, evolving characters paternal contribution must exceed the maternal one. This may result in the compensation of maternal effect for such characters and even in the initiation of opposite “paternal effect”. In other words, when “new” characters are transmitted paternal characters must to some extent dominate over the maternal ones.

Reciprocal “paternal” effect allows distinguishing the character that undergoes evolution from the stable one. The direction of character evolution can be determined based on genotypical sexual dimorphism and heterosis.

Hence considering heterosis as a sum of new evolutionary achievements acquired through divergence it can be suggested that paternal contribution to heterosis must exceed the maternal one. In the light of new views, it becomes clear why in heterosis we, as a rule, observe increasing of the characters useful for the human, but not for the species itself. In addition, this phenomenon is independent of species undergoing heterosis. Heterosis gives nothing to the species and can be even unhealthy. But since selection can be considered as human-forced artificial evolution for those species, the direction of this evolution and the direction of heterosis are in accordance with human’s interest, but not with the interest of the particular species.

Considering evolution of the character in Phylogeny as some kind of an abstract “movement”, one can speak about a “distance” between male and female sex on this character. Suppose that an initial form diversified in Phylogeny into two different forms (breed, line or race) by this character. Then, according to “phylogenetic rule of sexual dimorphism,” we can expect that males from both forms (A and B) should be more advanced compare to females. So, one can speak about the “distances” between breeds according to the trait (how fare they are gone from each other) and between males and females within each breed (Figure). It’s possible to distinguish the “impact of breed” and “impact of sex” in hybridization. The effect of heterosis can tell about the “distance” between the breeds and sexual dimorphism—about the “distance” between the sexes. The direction (“maternal” or “paternal” effect) and value of reciprocal effect can tell about divergent or convergent evolution of the trait. Therefore appears a possibility to explain more completely the reciprocal effects, which are nothing more but the vector sum of maternal and paternal effects.

Figure. Paternal effect

The following formula can be used for measurement of reciprocal effects (r): r = (b - a) / (B - A), where A and B —are values of the character for the initial forms; a —the same for hybrid ♂A x ♀B; b —for reciprocal hybrid ♂B x ♀A. Then positive value of r (r > 0) will correspond to “paternal” effect, negative (r < 0)—to “maternal”. Absolute value of r (│r│) will provide relative measure of the effect in units (B - A).

What characters can be classified as the “new” ones, as those being in evolutionary process? In humans “new”, evolutionary young characters are all social, psychological characters related to the large hemisphere cortex of the brain, to the brain functional asymmetry (first of all, apparently, abstract thinking, spatial vision, humor and other creative abilities). In agricultural animals and plants, these are evidently all economically valuable characters, which are selected by man artificially and according to its goal. In animals these characters are early maturity, productivity of meat, milk, eggs, wool etc. Consequently, one could expect that all economically valuable characters are connected with “paternal effect”, i.e. the character of father's breed or line dominates over the mother's one.

The “paternal effect” was observed on brooding instinct, early maturity of daughters, egg laying capacity, and live weight
“Maternal effect” was observed on the weight of eggs.

The “paternal effect” was observed on the characters for which the selection took place: the vertebra number (selection for long body), the length of small intestine (selection for best food utilization) and growth dynamics (selection for early maturity).
The “maternal effect” was observed for the weight of newborn piglets.

The “paternal effect” was observed on milk yield and fat production (amount of fat).
Small “maternal effect” is observed for the percentage of fat in the milk.

Large father's contribution to the egg yield of daughters was explained by the fact that in hens the female is heterogametic and the male—homogametic, therefore its single X-chromosome the hen receives from its father (Morley, Smith, 1954). If so, it should be expected that in mammals everything must be vice versa, since their males are heterogametic, i.e. greater contribution must be observed, notwithstanding the fact, whether an “old” or “new” character is inherited. According to new theory, disregarding the gamete pattern of sexes, in all cases the “paternal effect” for evolving (selected) characters must exist.

Back to Sexual Dimorphism—Forms

 More about “Paternal Effect”:

Existence of the “Paternal Effect” in the Inheritance of Evolving Characteristics. Geodakyan V. A. Doklady Biological Sciences, 1979, v. 248, N 1-6, p. 1084–1088. Translated from Doklady Akademii Nauk, Vol. 248, No. 1, pp. 230-234, September, 1979.
Evolutionary Logics of Sex Differentiation. Reaction Norm, Sexual Dimorphism, "Paternal Effect". Geodakian V. A.



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