The actual and effective number of gametophytic self-incompatibility a
lleles maintained at mutation-drift-selection equilibrium in a finite
population subdivided as in the island model is investigated by stocha
stic simulations. The existing theory founded by WRIGHT predicts that
for a given population size the number of alleles maintained increases
monotonically with decreasing migration as is the case for neutral al
leles. The simulation results here show that this is not true. At migr
ation rates above Nm = 0.01-0.1, the actual and effective number of al
leles is lower than for an undivided population with the same number o
f individuals, and, contrary to WRIGHT's theoretical expectation, the
number of alleles is not much higher than for an undivided population
unless Nm < 0.001. The same pattern is observed in a model where the a
lleles display symmetrical overdominant selection. This broadens the a
pplicability of the results to include proposed models for the major h
istocompatibility (MHC) loci. For a subdivided population over a large
range of migration rates, it appears that the number of self-incompat
ibility alleles (or MHC-alleles) observed can provide a rough estimate
of the total number of individuals in the population but it underesti
mates the neutral effective size of the subdivided population.