One explanation for the success of sexual reproduction is that sex increases the efficacy of natural selection. This hypothesis originally dates back to August Weismann (1886) and several theories explaining the maintenance of sex, such as the Fisher-Muller hypothesis, Muller’s ratchet, and the deterministic mutation hypothesis, are based on Weismann’s principle. Recombination and segregation lead to fitness variance among offspring which then offers a wider target for natural selection. Consequently, adaptation to changing environments is accelerated and population mean fitness will increase in each selection round. Most evidence for the enhanced efficacy of natural selection by sex and recombination focuses on the increased adaptation rate of recombining compared to non-recombining populations. A faster response to selection is seen as an indicator for an increased genetic variance in fitness by sex. Here, we aimed at empirical support to validate Weismann’s principle in natural subpopulations with variable degrees of sex. We investigated whether low levels of sex are associated with increased fitness variance and mean in parthenogenetic biotypes of the planarian flatworm Schmidtea polychroa. Parthenogenetic S. polychroa are triploid and reproduce clonally with occasional sexual reproduction. By-products and measures of occasional sex are the local presence of tetraploids and elevated levels of genotypic diversity. We correlated the proportion of tetraploids and genotypic diversity with fitness attributes of six genetically differentiated locations within one meta-population. Results revealed strong, positive correlations with variance and with mean offspring number produced during a 5-week period. This finding indicates that occasional sex in parthenogenetic S. polychroa leads to an increase in fitness variance at the local population level, which in turn increases mean fitness, which can be interpreted as a signature of a more effective natural selection. Consequently, occasional sex may prevent the extinction of parthenogenetic subpopulations and explain the success of this particular type of parthenogenesis in this species.