The Effect of Inbreeding on Seed Survivorship in Phlox

Cross-fertilizing plants typically carry deleterious genes which when combined through inbreeding cause a reduction in viability The specific number of deleterious genes responsible for the elevated level ofmortality in products of inbreeding is unknown in most cases. However, there is a substantial body of information on the number oflethal equivalents per zygote. A lethal equivalent is a group of genes which would on average cause one death if dispersed in different individuals and made homozygous (Morton et aI., 1956). The mean number oflethal equivalents per zygote varies from nearly 0 to 5 in herbaceous species and from nearly 0 to 10 in woody species and ferns (cf. Levin, 1984;