Understanding how new phenotypes evolve is often difficult because the intermediate stages in the transition from the ancestral to the derived phenotype remain elusive. Here we suggest and provide evidence for a mechanism that may result in the transition from sexual reproduction to parthenogenesis. In many sexually reproducing invertebrates, a small proportion of unfertilized eggs hatch spontaneously, because errors during meiosis can result in the production of diploid eggs (“tychoparthenogenesis”). We hypothesized that in marginal populations where some proportion of females fail to find mates, tychoparthenogenesis may result in the loss of males through a positive feedback loop. Because unmated females produce daughters only, the sex ratio in the population becomes more females biased, resulting in a yet larger proportion of females not finding mates. Thus, the strength of selection for tychoparthenogenesis increases as the proportion of offspring produced by tychoparthenogenesis increases in the sexual population. We tested whether such a positive feedback loop may occur in sexually reproducing stick insects of the genus Timema. Across a large number of populations, we found that the rate of tychoparthenogenesis (i.e., the proportion of unfertilized eggs that hatched) increased as the sex-ratio became more female-biased. Furthermore, female-biased sex-ratios and high rates of tychoparthenogenesis occurred in marginal, low-density populations in which a large proportion of adult females were indeed not mated. Our results suggest a simple mechanism through which parthenogenesis can evolve in a sexually reproducing population.