Abstract
Accumulation of deleterious mutations has important consequencesfor the evolution of mating systems and the persistence of smallpopulations. It is well established that consanguineous mating canpurge a part of the mutation load and that lethal mutations canalso be purged in small populations. However, the efficiency ofpurging in natural populations, due to either consanguineousmating or to reduced population size, has been questioned.Consequences of consanguineous mating systems and small populationsize are often equated under 'inbreeding' because both increasehomozygosity, and selection is though to be more efficient againsthomozygous deleterious alleles. I show that two processes ofpurging that I call 'purging by drift' and 'purging by nonrandommating' have to be distinguished. Conditions under which the twoways of purging are effective are derived. Nonrandom mating canpurge deleterious mutations regardless of their dominance level,whereas only highly recessive mutations can be purged by drift.Both types of purging are limited by population size, and sharpthresholds separate domains where purging is either effective ornot. The limitations derived here on the efficiency of purging arecompatible with some experimental studies. Implications of theseresults for conservation and evolution of mating systems arediscussed.