Theory of Asynchronous Evolution

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The Evolutionary Theory of Sex: Dimorphism and Dichronism in Phylogeny

On the phylogenetic time scale when stable environment starts changing, the new trait appears only in males and the evolution begins (Figure). This is initial, divergent phase (II). 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 subsystem.

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; the information starts flowing from males to the females and the evolution of females begin. 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 it’s an arrow pointing from the female to male norm and so, the trait is changing from female form towards the male one. This is “Phylogenetic rule of sexual dimorphism”. Phase of the evolution process can be determined based on dispersions of the sexes. If the dispersion of a trait is larger in males — the phase of its evolution is divergent; if dispersions are equal – the phase is parallel; if dispersion is higher in females — the phase is convergent. This is Phylogenetic rule of dispersion”.

 

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.

 

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

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