Mc. Whitlock, FISSION AND THE GENETIC VARIANCE AMONG POPULATIONS - THE CHANGING DEMOGRAPHY OF FORKED FUNGUS BEETLE POPULATIONS, The American naturalist, 143(5), 1994, pp. 820-829
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.