Facultative parthenogenesis may be a temporary phase between sexual reproduction and obligate asexuality in stick insects like Timema douglasi. Three genetic lineages of these insects were found to have females capable of both sexual and asexual reproduction, with varying tendencies towards each. Parthenogenesis led to complete loss of genetic diversity in one generation. The transition to obligate parthenogenesis did not require a change in the underlying mechanism, but rather a gradual increase in frequency. The researchers suggest that facultative parthenogenesis could be rare in animals due to a trade-off between the efficiency of sexual and asexual reproduction.