THE DYNAMICS OF THE EVOLUTION OF SEX - WHY THE SEXES ARE, IN FACT, ALWAYS 2

Darwinian evolution is normally modeled as the outcome of the interact ion between reproduction, variation and selection. These interactions- may become very complex, especially when considering that selection ma y act on several genes simultaneously. A numerical simulation model wa s developed, based on a Jive-step stochastic evolutionary algorithm, w hich-in addition to reproduction, variation and selection included exp licitly mate selection and phenotypic transformations. The model graph ically illustrates:the dynamic interactions affecting a multi-gene sys tem. The results emphasize that biological evolution is an irreversibl e process showing strong genetic drift. Simulations showed :: that ase xual reproductive strategies were very stable in evolutionary competit ions and consistently out-bred sexual ones; Bisexuality succeeded in r eplacing asexuality only in changing environments and could be maintai ned only if non-random mate selection mechanisms were at work. Simulat ions showed that in trisexual organisms, mate selection mechanisms and natural selection do not work efficiently. Thus, reproductive strateg ies with more than two sexes are improbable. Polyploidy may improve ev olutionary success among monosexuals as it reduces the exhaustion of t he variability of the gene pool due to continuous selection pressure. I postulate, that sex evolved under changing selection pressures, allo wing the emergence of sexual selection, which allowed sex to stabilize , thanks to the strong irreversible dynamics in the evolution of compl ex organisms.