Theory of Asynchronous Evolution


IntroductionChromosomesTheory of Sex | Asymmetry | Questions | FAQ | Articles | Glossary | Site Map |


Other Theories

Theories of Sexual Dimorphism

Sexual dimorphism is a general biological phenomenon widely distributed among dioecious animals and plants. It is observed for a vast number of characters. Therefore the theory, which claims to interpret it, should also be general one and cover all the characters, which show sexual dimorphism. The existing theories do not satisfy this requirement; therefore they have weak explaining and zero predicting ability. They tried to explain the mechanisms: how could sexual dimorphism arise and be maintained? They did not put the questions revealing its regularities. What is sexual dimorphism? What is its evolutionary significance? What is its contribution and meaning? Is it related to other phenomena and how?

Many theories tried to explain the phenomenon of sex. As it was already mentioned, dioecism involves two different fundamental phenomena: mating and sex differentiation. It turns out, that many existing theories and representations analyze mainly the effects of mating and therefore can explain advantages of sexual ways of duplication compare to asexual ones. The Evolutionary Theory of Sex considers the effects of sex differentiation and therefore explains the advantages of dioecious species over hermaphrodites.

Darwin’s theory of sexual selection

Darwin’s theory of sexual selection was the matter of controversy even then. Many authors thought it to be the weakest point of evolution theory. Male competition has never been controversial. But the theory of female choice was rejected by many evolutionists.

There are a number of weak points in the theory of sexual selection.

1.       Sexual dimorphism is often observed for such characters, which are with great difficulty related to sexual selection (e.g., leaf number and shape, branching pattern in plants). According to the theory of sexual selection sexual dimorphism should promote preference in either struggle for the female, or choice by the female. Consequently, at best the theory can be applied only to the animals and characters, which provide such advantages.

2.       Sexual selection can help strong, better-equipped or more attractive male in the struggle for female, but it cannot maintain sexual dimorphism for these characters. Thus it is unclear why these characters are inherited only by male offspring.

3.       Interpretation of the same phenomenon needs different logics. For example, in birds larger size of males is explained by preference in the struggle for female and larger size of females—by advantage of laying large eggs. But it is unclear why in the first case no large eggs and in the second no struggle for female are needed. It is still more difficult for the theory to explain large size of females in some mammals, such as bats, rabbits, flying squirrels, spotted hyenas, dwarf mongooses, some whales and seals.

4.       Other obstacle for the theory of sexual selection is the dependence of sexual dimorphism on the reproductive structure of population (monogamy, polygamy, panmicsy). In this respect two regularities of sexual dimorphism for size are mentioned: (a) sexual dimorphism is more often found and is more pronounced in polygamous species than in monogamous ones, and (b) sexual dimorphism increases with body weight. There is no satisfactory explanation of these phenomena.

5.       It is also difficult to explain in terms of the theory existence of marked sexual dimorphism in monogamic species with sex ratio 1:1. Darwin believed that a male preferred by females started earlier to reproduce, which provided some advantages. Besides, the females, which were first ready to reproduce, seemed to be better mating partners. Such argument seems to us unconvincing. Each species has optimal reproduction time established in the course of evolution. Deviations both towards earlier or later onset of reproduction are disadvantageous and are eliminated by stabilizing natural selection.

6.       One more interpretation of initiation of sexual dimorphism is ecological sex differentiation mainly concerned with nutrition. E.g., differentiation according to dimensions of the food objects used. In any situation the food with dimensions typical for the given species is exhausted earlier, therefore the individuals consuming smaller or larger objects get certain advantages. And if sex differences in size initially existed, the ecological food differentiation will promote their increase. Sometimes such sexual dimorphism is related only to food organs (e.g., beak size of woodpeckers). Such interpretation is also unsatisfactory: (a) such process can at best explain an increase or maintaining of sexual dimorphism) rather than its initiation; (b) it is absolutely unclear why such differentiation, which is basically usual disruptive selection, should be sex-linked. If sexual dimorphism exists for body or beak sizes, it seems most likely that distributions of males and females will be overlapped to a great extent. Then small and large animals ought to be specialized regardless of their sex and according only to their body or beak sizes.

Weakness of the Darwinian Theory treating sexual dimorphism as consequence of sexual selection is a result of a methodological mistake: the wide phenomenon cannot be treated as consequence of the narrow mechanism.

Theories of the evolution of a sex ratio

The existing theories of evolution of a sex ratio are neither capable to explain the known facts in this area, nor especially to predict new facts. They luck generality and unable to explain all complex of the problems connected to a sex ratio, observable deviations of its values, and also dependence of its change on various factors, such as age of parents, life conditions, nutrition, starvation, presence of wars, climate etc.

Weakness of existing theories is caused by that they do not lean on uniform evolutionary logic of the phenomena. As a rule, they proceed from erroneous representations that for a tertiary sex ratio an optimum is always the proportion 1 : 1, and that primary and secondary sex ratio are constants characteristic for a given species and independent of the environmental conditions.

Theories of High Male Mortality

The existing theories are not capable to explain the phenomenon of the raised males’ death rate. Usually on a question “why females live longer than males?” biologists answer: “because mothers are more necessary to the posterity, than fathers”. But at some species mothers take care of posterity, at another species fathers do, and at some species there is no care at all. And of course plants do not take care of their offspring. However, the lowered viability of males is observed everywhere.

For animals some theories explain the raised death rate of males, as a result of their bigger or smaller sizes, brighter coloring or the “risky” behavior connected to getting the food, fight for females, fights with predators, and for humans—with dangerous professions (seamen, military, pilots, etc.). However different animal species have different sexual dimorphism on these attributes (for example, some species have larger males, while others—females) or is absent at all while the raised death rate of males is observed almost for all species. Not clear as well that these distinctions (sexual dimorphism), as a rule, grow with age, and the difference in death rate, on the contrary, decreases, it is maximal for the young and smoothes out with age. These reasons allow rejecting the majority of the listed reasons as main or important. Hence, the given group of theories cannot explain the problem as a whole.

Deserves serious discussion the theory of gene imbalance, which tried to explain higher male death rate due to its heterogametic constitution, absence of the second X-chromosome in a man’s set. Really, male can receive the defective (lethal, semi-lethal) recessive gene only from one parent and the gene will manifest itself. Contrary, female should receive the defective gene from both parents. If the theory of chromosomal or gene imbalance is right, then first the different sex mortality should depend upon a share of hemizigous (nonpaired) genes in a particular genome. Drosophila has a few autosomes compare to human, so this share is relatively more for Drosophila. The whole genome of the haploid hymenoptera males is in a hemizigous state. Therefore according to the theory maximal difference in death rate can be expected for hymenoptera, much smaller—for Drosophila and even less for humans. However, as far as it is known, nobody noted such distinctions. Further, as the son receives the X-chromosome only from mother, higher correlation in longevity between the son and mother should be observed, than between the son and father. But as it has been shown, that such difference in correlation is also absent. And at last, the most important—for species with chromosomal structure of Abraxas type with homogametic males, the females should have higher death rate.

Observations and experiments on Abraxas species (birds, butterflies, moth, some kinds of fishes, etc.) do not leave doubts that, for many of them despite of heterogametic chromosomal constitution of females, the males also have higher death rate. McArthur and Baillie after analyzing data on many species with Abraxas gamety type came to a conclusion, that heterogametic constitution lowers viability, but it cannot be considered as a sole or main cause of different sex mortality. And as heterogametic and homogametic males, as a rule, possess higher level of the basic metabolism, than females, authors thought, that other theory of different death rate of sexes, namely metabolic which explains high male death rate by a high level of their basic metabolism is more comprehensible. Certainly, there is a close relationship between level of metabolism and death rate, in particular between heart beat frequency and longevity. It is natural and clear. But the replacement of unclear “high male death rate” to also unclear “a high level of metabolism” doesn’t resolve the question. These theories simply establish correlation between different attributes, characteristic for one sex or another: the theory of chromosomal imbalance—between longevity and gamety type, the metabolic theory—between longevity and level of metabolism. But they do not explain evolutionary sense, logic and expediency of this phenomenon. Why, despite of a huge variety of species, males have higher death rate (or metabolism)? Hardly is it casual. It is possible to tell, that the raised metabolism is a way to provide males with higher death rate (sensitivity). But what is itspurposeor evolutionary sense?

The chromosomal imbalance and metabolic theories establish correlation between different attributes, characteristic for one sex or another: the theory of chromosomal imbalance—between longevity and gamety type, the metabolic theory—between longevity and level of metabolism. But they do not explain evolutionary sense, logic and expediency of this phenomenon. Why, despite of a huge variety of species, males have higher death rate (or metabolism)? Hardly is it casual. It is possible to tell, that the raised metabolism is a way to provide males with higher death rate (sensitivity). But what is its “purpose” or evolutionary sense?

Congenital Malformations of the Heart and Major Blood Vessels

Rokitansky (1875) explained congenital heart diseases as breaks in heart development at various Ontogeny stages. Spitzer (1923) treats them as returns to one of the Phylogeny stages. Krimsky (1963), synthesizing two previous points of view, considers congenital heart diseases as a stop of development at the certain stage of Ontogeny, corresponding to this or that stage of the Phylogeny. Hence these theories can explain atavistic heart diseases only (feminine and neutral, according to our classification), and all group of men’s defects does not find an explanation.

Brain Asymmetry and Handedness

Initially, when brain asymmetry was considered only human phenomenon, attempts were made to explain it as the consequence of the unique characteristics of mankind: speech, right handedness, self-awareness. But it turned out that brain asymmetry also exists in other vertebrates. Despite this, purely “human” theories on the specialization of the hemispheres continue to appear—verbal-nonverbal, temporal-spatial, analytical-synthetical, sequential-integral (perception), abstract-concrete. Lateralization was seen as a means of duplicating the informational capacity of the brain, although, as it turned out, it can increase reliability but in no way capacity. The left hemisphere was linked with the setting of goals, the right one—with its realization, with inductive and deductive thinking, numerical and analogous processing of information, Western technicalism and Eastern mysticism, etc. Theories of handedness exist—initially naive, explain human right-handedness as due to the right side position of the liver, displacing the body's weight, or to the heart being on the left side, compelling the shield to be carried in the left hand and the sword in the right. The left hand was reserved for excretory-hygienic purposes and the right for eating, and handshaking. Left-handedness was considered to be the result of birth trauma, generally as pathological, and so forth. Not one of the existing theories can explain all the facts consistently or predict new ones.




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


  www Site

html hit counter
html hit counter code