The mating system provides a description of the distribution of mating unions in a population; in hermaphrodites such as most flowering plants, this amounts to the rate of self-fertilization versus outcrossing. Selfing has evolved numerous times in flowering plants, probably because selfers transmit an extra copy of their genes to their seed progeny, and because they can reproduce in the absence of mates. Nevertheless, selfed progeny tend to be less fit and selfing detracts from an individuals outcrossing potential. As a result, most plants avoid selfing via a number of molecular, morphological or phenological adaptations. The mating system also has important implications for the genetic structure of individuals and populations: individuals show reduced heterozygosity, reduced effective recombination and reduced effects of genetic conflict; and populations of selfers tend to be more genetically differentiated from one another than outcrossers.

Key concepts

• The mating system describes and quantifies the distribution of mating unions in a population; in hermaphroditic plants, the mating system typically refers to the selfing or outcrossing rates.
  
• Self-fertilization confers an automatic selective benefit, because selfed progeny carry two copies of a parent's genome rather than just one. An important possible cost of self-fertilization is inbreeding depression, where selfed progeny have lower fitness than their outcrossed counterparts.
  
• Self-fertilization can be costly because pollen used up in selfing is unavailable for competition to sire outcrossed progeny; this is known as pollen discounting.
  
• Self-fertilization may be advantageous when mates or pollinators are scarce, because it provides reproductive assurance by allowing uniparental reproduction.
  
• Inbreeding depression can be caused by either overdominance, where heterozygotes have an advantage over both corresponding homozygotes, or partial dominance, where fitness is reduced on one homozygote by the expression of deleterious recessive alleles.
  
• Plants avoid self-fertilization through molecular self-incompatibility reactions, the spatial separation of the sexual functions within and among flowers (herkogamy or dicliny), or the temporal separation of the sexual functions (dichogamy).