ON FACTORS AFFECTING THE FIXATION OF CHROMOSOMAL REARRANGEMENTS AND NEUTRAL GENES: COMPUTER SIMULATIONS

Computer simulation models were developed to determine which conditions are favorable for the stochastic fixation of chromosomal mutations within small isolated demes. The models incorporated biological parameters of litter size, age-dependent mortality, overlapping generations, potentially varying sex ratios, and fertility reduction due to meiotic problems when the mutation was present in a heterozygous condition. Results of the models indicated that random processes alone may adequately explain the frequency of fixation of chromosomal mutations under the conditions of a) the number of initial founders is small (5 or 10); b) there is relatively little fecundity reduction due to meiotic problems; c) the number of offspring per mating is high. Furthermore, results of our models imply that fixation of a mutation is unlikely when the number of karyotypic rearrangements is low and when there is high survivorship of individuals from one breeding period to the next. However, if bottlenecks involve larger numbers of individuals (20 or more), few offspring per mating, or substantial reductions in fecundity, then it is unlikely that fixation by random processes will be adequate to explain the situations observed in nature. A significant conclusion of these simulations is that when population size is reduced to five or ten individuals, the extinction rate may exceed 40% or 30%, respectively.