We review transitions between hermaphroditism and dioecy in animals and (mainly flowering) plants. Although hermaphroditism and dioecy represent two end states in a sex-allocation continuum, both vary in major ways among clades. However, drawing on sex-allocation theory and distinguishing between self-fertilization and outcrossing, we recognize five broad paths to dioecy and two broad paths to hermaphroditism. Which path is taken likely depends on the starting state (especially in terms of the mating system), as well as the ecological setting or genetic context of the transition. Androdioecy may have been more important in some transitions to dioecy than current theory would suggest, and gynodioecy may often be an evolutionary end point in itself rather than a step between hermaphroditism and dioecy. Transitions between environmental and genetic sex determination may also play an important role in sexual-system evolution. Further theory is required to address these possibilities. Detailed empirical work is also greatly needed, especially in animal clades that vary in their sexual system.

中文翻译：

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动植物雌雄同体与雌雄同体之间的进化转变

我们回顾了动物和（主要是开花）植物中雌雄同体和雌雄异体之间的转变。尽管雌雄同体和雌雄同体代表了性别分配连续体中的两种最终状态，但两者在进化枝之间存在重大差异。然而，根据性别分配理论并区分自体受精和异型杂交，我们认识到雌雄同体的五种主要途径和雌雄同体的两条主要途径。采取哪条路径可能取决于起始状态（特别是在交配系统方面），以及过渡的生态环境或遗传背景。在某些向雌雄异株的转变中，雄雌花性可能比当前理论所暗示的更为重要，而雌雄异株本身可能往往是进化的终点，而不是雌雄同体和雌雄异体之间的一步。环境和遗传性别决定之间的转变也可能在性系统进化中发挥重要作用。需要进一步的理论来解决这些可能性。还非常需要详细的实证研究，特别是在性系统不同的动物分支中。