Why is cleistogamy a selected reproductive strategy in Impatiens capensis (Balsaminaceae)?

Cleistogamy (self‐fertilization in closed flowers) differs from chasmogamy (open‐pollinated fertilization) mainly in sustaining selfing. Why numerous species develop both of the reproductive modes on the same individuals has long puzzled biologists. In a novel hypothesis presented here, I propose that cleistogamy could be a means by which inbred lines are created and maintained in natural populations; these lines would continuously experience self‐improvement via natural selection and via crosses among lines at the chasmogamous flowers to benefit the populations. Supporting evidence for the hypothesis was found in Impatiens capensis where cleistogamous ovules were fertilized proportionately less (56%) than chasmogamous ovules (67%) in natural populations, but crosses among cleistogamous progeny in the greenhouse led to a nearly 10% increase of fertilized chasmogamous ovules. I established a novel fitness model specific to the cleistogamous species to further examine how various aspects of the mating system affect plant performance. A low inbreeding depression (0.07) was consequently found for the surveyed natural populations of I. capensis, suggesting that the individual‐level percentage of cleistogamy and the population‐level selfing rate may have evolved in the direction of reducing the overall inbreeding depression. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 75, 543–553.