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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 its “purpose”
or 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.
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