Small population processes.

Abstract Some forest tree populations have small geographical distributions or occur in fragmented populations, and the distribution of more species is becoming fragmented as land use increases. In fragmented populations the genetic variability within populations is reduced, and the variance between populations increases. In small populations, homozygosity increases because of inbreeding. This may have deleterious consequences because of inbreeding depression. The rates of inbreeding and drift depend on the genetically efficient population size. Even populations with large census size can be genetically small if only part of the individuals participate in reproduction, or if there is clonal reproduction. A short bottleneck will have minor effects on overall variation, whereas a long term reduction in population size will deplete variability. Similarly, once variability is depleted due to a long term bottleneck, the recovery of variation by mutation takes a long time after the increase of population size. Because of the long time to reach equilibrium between mutation and drift, the current population sizes do not always correlate positively with the level of variability. For neutral loci such as molecular markers, the distributions of allele frequences depend mainly on the balance between mutation, migration and drift, but the level and distribution of quantitative variation is more likely to be affected as well by selection.