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the merging of male and female sex cells belonging to a single bisexual individual (see HERMAPHRODITISM). Self-fertilization is rarely observed in nature. In the process of evolution, most organisms acquired adaptations that made self-fertilization impossible and ensured cross-fertilization. These adaptations increased the genetic variation of the off spring, promoting, in turn, the elaboration of new adaptations and the development of more viable offspring. Self-fertilization is sometimes observed in hydras, flatworms, certain annelids, mollusks, and fishes. Among plants it is observed in many algae, fungi, and flowering plants. In flowering plants self-fertilization is a result of self-pollination.