winter, and then will surface from their burrows to mate once, and establish new nests. Partial voltinism The term partial voltinism is used to refer to two different (but not necessarily exclusive) situations: An organism wherein generations overlap in time, and so are not completely reproductively isolated. For example, in bees of the subfamily Halictinae, one generation is produced in the early summer and one in the late summer, but males produced in the early summer may also mate in the late summer. (More commonly) a population where the voltinism is mixed, because of genetic variation (e.g., in the hybrid zone between a univoltine and a bivoltine area) and/or because environmental stimuli do not induce bivoltinism in all individuals (or across all years). For example, far-northern populations of the green-veined white butterfly Pieris napi are mostly univoltine, but some individuals may avert diapause and produce an additional generation under warm conditions. Evolution The number of breeding cycles in a year is under genetic control in many species and they are evolved in response to the environment. Many phytophagous species that are dependent on seasonal plant resources are univoltine. Some such species have the ability to diapause for a large part of the year, typically during a cold winter. Others that bore in wood or other low-grade, but plentiful, food material may spend nearly the entire year feeding, with only brief pupal, adult and egg stages to complete a univoltine life cycle. Yet other species that live in tropical regions with little seasonality may be highly multivoltine, with several generations feeding on constantly growing vegetation (such as some species of Saturniidae), or continually renewed detritus, such as Drosophila and many other genera of flies with a life cycle of just a week or two. See also Semelparity and iteroparity References Reproduction