AbstractCompetition among relatives can modify the genetic structure of plant populations; in turn, competitive outcomes can depend on the genetic relatedness of the individuals competing. The offspring from individual parents exhibit a continuum of genetic relatedness, depending on parental reproductive systems. Competition among relatives may have evolutionary significance as a selection pressure; sibling competition, for example, has been invoked to explain the evolution of sexual systems, seed packaging within fruits, seed dispersal dimorphisms, and germination behavior.Density‐dependent fitness consequences of sibling competition have been documented in a population of the annual grassSporobolus vaginiflorus. This species produces seeds matured in cleistogamous spikelets within leaf sheaths along tillers in autumn: when seeds along the tillers of a maternal parent (i.e. a sibship) germinatein situthe following spring in close proximity to one another, sibling competition results in a high‐density zone centered around the original senescent parent. Both intra‐ and intersibship interactions can occur within a population. Although fitness is much reduced for siblings inside the zone of competition, potential seed rain and net primary productivity per unit area are significantly higher relative to outside the zone. This annual is functionally analogous to a perennial ramet producer with a phalanx growth strategy and the unit of selection may be the sibling group. It is not yet known whether sibling competition is a significant selection pressure in other species, but indirect evidence suggests it may be relatively wides