The effective population size (N-e) depends strongly on mating system
and generation time. These two factors interact such that, under many
circumstances, N-e is close to N/2, where N is the number of adults. T
his is shown to be the case for both simple and highly polygynous mati
ng systems. The random union of gametes (RUG) and monogamy are two sim
ple systems previously used in estimating N-e, and here a third, lotte
ry polygyny, is added. Lottery polygyny, in which all males compete eq
ually for females, results in a lower N-e than either RUG or monogamy.
Given nonoverlapping generations the reduction is 33% for autosomal l
oci and 25% for sex-linked loci. The highly polygynous mating systems,
harem polygyny and dominance polygyny, can give very low values of N-
e/N when the generation time (T) is short. However, as T is lengthened
, N-e approaches N/2. The influence of a biased sex ratio depends on t
he mating system and, in general, is not symmetrical. Biases can occur
because of sex differences in either survival or recruitment of adult
s, and the potential for a sex-ratio bias to change N-e is much reduce
d given a survival bias. The number of juveniles present also has some
influence: as the maturation time is lengthened, N-e increases.