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The Evolutionary Theory of
Sex:
Dimorphism and Dichronism in Phylogenesis
On phylogenetic time scale when
stable environment starts changing, only male's sex trait
variation changes and evolution begins (Figure).
At the initial, divergent phase (II), the trait
changes in males only. Evolution trajectory divides into
male and female branches. The genotypic sexual dimorphism
appears and improves in a series of generations. The
duration of the divergent phase or sexual dichrony
(SDC) equals the lagging time of the female sex or leading
time of the male sex (Figure).
This temporal “distance” is necessary to test the new traits
in the male sex. However, the divergence of sexes cannot
continue indefinitely, otherwise it would result in
reproductive isolation. A mechanism of genotypic sexual
dimorphism relaxation is turned on; i.e., outflow of
information from the male sex to the female sex; that is,
evolution of the female sex begins. This is the parallel
stage (III), when the trait evolves in both sexes at the
same rate. The genotypic sexual dimorphism remains the same.
The third, convergent stage of evolution (IV) begins when
males are no longer affected by the ecological differential,
whereas females are still under the effect of genotypic
sexual dimorphing. The genotypic sexual dimorphism decreases
and then vanishes, the dimorphic trait becomes monomorphic
(stable), and the evolution of the trait ends.
Figure
Ordinate: Stages of the trait’s
evolution.
Abscissa: A and B — initial and
new value of the trait.
The evolution of any character passes
through the sexual dimorphism stage, therefore
sexual dimorphism is a consequence of any type of
selection: natural, sexual or artificial.
Sexual
dimorphism on any trait exists only during its evolution. It
appears when evolution starts, exists when it goes and
disappears with its end. The absence of sexual dimorphism
means that the trait is stable. So, the presence or absence
of sexual dimorphism can serve as a criterion of trait’s
evolution. If we consider vector of sexual dimorphism as a
“compass” one can say that its arrow is pointing from female
sex norm to the male one. So, the trait is changing from
female form towards male form. This is “Phylogenetic rule
of sexual dimorphism”. Phase of the evolution
process can be determined based on variations of the sexes.
If the variation of a trait is larger in males — the
phase of its evolution is divergent; if variations are
equal – the phase is parallel; if variation is higher in
females — the phase is convergent. This is
“Phylogenetic rule of variation”.
Continue to :
Sexual Dimorphism—Forms
More about Sexual Dimorphism:
Evolutionary Chromosomes And Evolutionary Sex Dimorphism.
Geodakyan V. A. Biology Bulletin, 2000, v. 27, № 2,
p. 99–113. Translated from Izvestija Akademii Nauk, Serija
Biologicheskaya, No. 2, pp. 133-148, 2000.
Evolutionary Logics of Sex Differentiation. Reaction Norm,
Sexual Dimorphism, "Paternal Effect".
Geodakian V. A.
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