FISSION AND THE GENETIC VARIANCE AMONG POPULATIONS - THE CHANGING DEMOGRAPHY OF FORKED FUNGUS BEETLE POPULATIONS

Random fission events affect population structure because populations necessarily become smaller at the time of splitting and may remain sma ller in future generations, which creates variance in population sizes and a smaller effective population size. When fissioned populations g row rapidly to regain a population size equal to other populations, th e standardized genetic variance among populations becomes approximatel y F(st) = (1 + pi(fis)/(4Nm + 1 + pi(fis), where pi(fis), is the propo rtion of populations that have, in the last generation, arisen from fi ssion events. Random fission events therefore must be common for the f ission process to have much effect on genetic population structure. On the other hand, if populations do not split into equal halves and the fissioned halves are slow to grow to full population size, fission ca n play a larger role in determining the degree of differentiation amon g groups. Nonrandom fission along familial lines can also cause signif icant increases in the differentiation of populations. Measurements of fission rates and fissioned population sizes in forked fungus beetles allow the proportion of new fissioned populations to be estimated as 6%, with unequal splitting of the populations. Smaller population size s created by fission events persist in fungus beetle populations for g enerations after the fission events, which thereby increases the varia nce in population size among beetle populations. As a result of these effects of fission, the genetic variance among populations of fungus b eetles is significantly greater than it would be without random popula tion fission events.