THE INFLUENCE OF MATING SYSTEM AND OVERLAPPING GENERATIONS ON EFFECTIVE POPULATION SIZE.

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