Let p(x) denote the frequency of mature plants with x heterozygous lethal alleles.

The probability that outcrossing in the population as a whole produces a zygote with x heterozygous lethal alleles (before mutation) is therefore

The magnitude of identity disequilibrium can be measured by the ratio of the variance to the mean number of heterozygous lethals in mature plants, which is expected to be I under identity equilibrium (with a Poisson distribution of number of lethal alleles per plant).

Equilibrium values of the mean number of lethal alleles per plant, the inbreeding depression, the secondary selfing rate, and the mean fitness as a function of the primary selfing rate are depicted by dashed lines in figures 2 and 3 for different values of the genomic mutation rate to lethals.

6.2), and the equilibrium mean number of lethal alleles per plant is 2n[Mu]/h.

This occurs because there is then too little opportunity for selection to limit the number of lethal alleles to even a very high equilibrium value; in other words, a high genomic mutation rate can overwhelm selection against completely recessive lethals even under an intermediate primary selfing rate, such that the mean number of lethal alleles increases toward infinity.

This corresponds to a buildup of identity disequilibrium in mature plants, hence a greater opportunity for selection and a decreased equilibrium number of lethal alleles as the threshold selfing rate is approached.

Figure 5 demonstrates that for completely recessive lethal mutations the mean number of lethal alleles maintained at low selfing rates increases sharply as the number of loci is increased, confirming that the blow up of the mean number of completely recessive lethals per plant in the Kondrashov model is caused by the assumption of an infinite number of loci.

For completely recessive mutations under random mating, the equilibrium number of lethal alleles per individual is approximately 2n[([Mu]/[Epsilon]).sup.1/2], which increases toward infinity (in proportion to the square root of the number of loci) as [Epsilon] approaches zero.

If the inbreeding depression is initially very high, then increasing the primary selfing rate above the threshold apparently causes the small fraction of surviving selfed zygotes to become sufficiently frequent relative to outcrossed zygotes to permit substantial selection against lethal alleles at individual loci.