|
The Evolutionary Theory of
Sex:
Sexual Dimorphism—Forms
According
to the evolutionary theory of sex sexual dimorphism consists
from two components: reproductive sexual
dimorphism (RSD) and earlier unknown evolutionary
sexual dimorphism (ESD) predicted by the theory.
Reproductive sexual
dimorphism is permanent. It involves primary and
secondary sexual traits different in the male and female sex
that have direct relation to reproduction (gametes, gonads,
genitals, androgen-estrogen ratio, and all traits determined
by them: reaction norm, mammary glands, beard in man, lion’s
mane, heel of cock, etc.). These are fundamental
species-specific traits. According to the theory, genes for
these characters should be common for both sexes. Since
there are no genetic differences, the reproductive sexual
dimorphism is hormonal; i.e., phenotypic. Its function is to
set up programs for the two sexes.
Modificational sexual
dimorphism is temporary and is limited to Ontogeny.
It appears as a result of female sex changes due to its
broader reaction norm. It precedes evolution of any trait.
The wider the reaction norm for the trait, the higher the
associated sexual dimorphism. The purpose of modificational
sexual dimorphism is to protect the female sex from
selection, until new genes appear after been tested in the
male genome. An example of such dimorphism is adaptations of
females living in the Arctic: thick layer of subcutaneous
fat, short legs, and high mineralization of skeleton.
Evolutionary sexual dimorphism
appears as a consequence of asynchronous evolution of any
trait as a result of any type of selection—natural, sexual,
or artificial. It represents the “distance” between sexes.
The vector of sexual dimorphism (from the female form of the
trait to the male one) corresponds to the direction of trait
evolution. The female form of the trait indicates the past
state, while the male form indicates the future state.
Any population that exists in the
stable environment for a long time has only reproductive
sexual dimorphism. Evolutionary sexual dimorphism is absent,
and modificational sexual dimorphism is only of a
variational nature, since the phenotypic variance of the
male sex in a stabilizing environment is greater than that
of the female sex.
In changing environment the
evolutionary sexual dimorphism is a sum of new
information ( In ) that has already
entered the male sex (but did not yet enter the female), and
old information ( Io ) that is
still retained in the female sex (but has already been lost
by the male sex). It should be pointed out that when two
populations interbreed (races or ethnic groups), the common
information undergoes mixing after the first cross, while
the new and the old information remain segregated throughout
the period of sexual dichrony. This view easily explains
differences of interspecies, interracial or interethnic
reciprocal hybrids
associated with the direction of crosses.
Sexual Dimorphism—“Last News of the Evolution”
In relation to
sex the characters of the organisms can be divided into
three groups. The first group includes the characters which
show no difference between males and females. Among these
are the majority of specific characters (number of organs,
extremities, plan and general structure of the body and many
others). There is no sexual dimorphism for these characters
in the norm. It is observed only at some pathological
conditions, and expressed in different frequency of some
congenital anomalies in males and females. The idea as to
classifying congenital defects of development into
“atavistic” (regressions or interruption of development) and
“futuristic” ones (search for new pathways) permits in some
cases to follow in such sexual dimorphism general trends
predicted by the theory. For example, among 2000 newborns
with one kidney there were twice as much boys, while among
4000 newborns with three kidneys there were 2.5-fold more
girls. Is it accidental or this fact implies a certain
evolutionary tendency of oligomerization of multiple organs?
Note that some worms have in each body segment a pair of
specialized excretory gland—metanephridia.
Consequently occurrence of three kidneys can be considered
as an “atavistic” trend, and of one kidney—as a “futuristic”
one. Other example: congenital hip luxation occurs 4–5 times
more often in girls than in boys. It should be noted that
infants with this defect better than normal ones run on all
fours and climb the trees. The third example—anencephaly is
also twice more often observed in girls.
A special
study has been conducted on
congenital defects of the heart and main blood vessels.
The second
group of characters are those which are presented in both
sexes but are differently pronounced or/and are met in the
population with different frequency depending on sex. These
are quantitative characters such as height, weight, size and
proportion, many morpho-physiological and ethologo-psychological
characters. Sexual dimorphism on such characters
can be determined as the difference between mean
values of this character for males and females of the
population. Such "populational" sexual dimorphism can show
direction of the character evolution.
For example, the evolution
of most vertebrates is accompanied by enlargement,
whereas most insects and arachnids become smaller.
Therefore, vertebrate males should be larger than females,
whereas male insects and arachnids should be smaller than
females. It is exactly so (Fig.).
The phylogenetic rule of sexual
dimorphism was successfully checked up on a large
group (173 species) of
lower Crustacean
(Geodakian, Smirnov, 1968). Males in all cases are smaller
than females. According to the rule we can assume
that this group has a common evolution trend of size
reduction. In fact, it is well-known that morphologically
more primitive forms of
Crustacean are larger. Inside the group we can pick
out arrays of forms with sequential size and number of
extremities reduction, and specialization according to
phylogenetic preemptivity and smaller male size.
The same reasoning is
applicable to separate species and populations. For example,
dog's ancestors (wolf, fox, and jackal) seem to have average
size as compared to large (mastiff, St. Bernard
weighing up to 70 kg) and small (Chihuahua— 2.5-3 kg, Toy
Terrier up to 400g) dogs. The theory predicts larger-sized
males in large breeds and females in small ones.
The same tendencies should
be observed inside smaller taxa, say in mammals females of
small forms are larger than males, while large forms has
larger males. For example African savanna elephant males
weight up to 6.5 ton, but females up to 3.5 ton only. Small
forms—some bats, flying squirrels, spotted hyenas, dwarf
mongooses, rabbits and others frequently has larger females.
This rule is also valid
for plants. For example, in poplar female specimens have
more elongated leaves, the male ones more rounded ones.
Leaves of ginkgo female tree have even edges and are
smaller, of male ones larger and cut. As known, poplar
phylogenetic ancestors had narrow (like willows) leaves,
while gingko ancestors—the uncut ones.
The third group is the
characters inherent to one sex only. These include all
primary and secondary sexual characters (internal and
external sex organs, mammary glands, beard in man, mane in
lion), and many economically valuable characters. Sexual
dimorphism for these characters is genotypic, because they
are absent in the phenotype of one sex. Still information
about these characters is written in the genotype of both
sexes. If these traits undergo evolution, the genotypical
sexual dimorphism on them should exist and it can be
revealed in the form of
reciprocal effects. In humans all social, psychological
characters related to the large hemispheres cortex of the
brain, and to their asymmetry (primarily, speech, abstract
thinking, spatial imagination, humor and other creative
abilities) can be attributed to the “new” characters.
Thus the interpretation of
sexual dimorphism as a phylogenetic “distance” between the
sexes, as evolutionary “news” having already arrived to
males, but not to females is applicable to all characters of
humans, animals and plants for which sexual dimorphism is
observed. Only in case of specific characters the regularity
is manifested in pathological fields, of populational ones
in the norm, while in case of sex characters as the
“paternal effect”.
If
there is sexual dimorphism according to a certain trait,
then with age this trait will change, as a rule, from the
female form to the male one. So, the female form of a trait
is more characteristic of the initial, juvenile stage, while
the male form is more characteristic of the definitive stage
(mature, adult).
More about Sexual Dimorphism:
Sexual
dimorphism.
Geodakyan V. A. In: Evolution and morphogenesis. (Mlikovsky
J., Novak V. J. A., eds.), Academia, Praha, 1985, p.
467–477.
|