Phenotypic evolution and parthenogenesis.

Using phenotypic selection models directly coupled to the polygenic system, examines the validity of the assertion that obligate parthenogenesis is an evolutionary dead end. An equilibrium level of genetic variance is attained under obligate parthenogenesis when the input via mutation is balanced by the output via selection, reached within a few hundred generations. The rate of phenotypic evolution of well-established parthenogens may often approach or even exceed that under obligate bisexuality. The consequences of periodic sex are also examined. By exploiting hidden genetic variance released by sex, cyclical parthenogenesis periodically allows much higher rates of phenotypic evolution that can ever be attained under obligate bisexuality. In the long run, rates of phenotypic evolution are approximately independent of the frequency of sex for populations that have attained their equilibrium levels of genetic variance. Cyclical parthenogenesis is rare not because of any inherent disadvantages, but because of the extremely stringent requirements necessary for the transition to and maintenance of such a complex life cycle. The possibility that occasional backcrosses between parthenogens and their bisexual parental species may play an important role in organic evolution is explored. -from Authors