Phenotypic Evolution and Parthenogenesis

Abstract

Using phenotypic selection models directly coupled to the polygenic system, we examine the validity of the common assertion that obligate parthenogenesis is an evolutionary dead end. As is true for bisexual organisms, an equilibrium level of genetic variance is attained under obligate parthenogenesis when the input via mutation is balanced by the output via selection. We show that for most parthenogens this equilibrium will be reached within a few hundred generations. When the ability of ameiotic parthenogens to utilize the dominance and epistatic components of genetic variance and possible elevated mutation rates and reduced levels of environmental sensitivity in parthenogens are accounted for, it becomes clear that the rate of phenotypic evolution of well-established parthenogens may often approach or even exceed that under obligate bisexuality. We also examine the consequences of periodic sex. By exploiting hidden genetic variance released by sex, cyclical parthenogenesis periodically allows much higher rates of phenotypic evolution than 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. It is suggested that 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, and techniques are suggested for the future analysis of parthenogenetic systems.