Drift is the process by which gene frequency changes erratically in a population. Random sampling of gametes at reproduction is the main cause of drift, and therefore the population size of a species is most important, that is often measured by the effective size. Homozygosity and inbreeding may increase by drift. The significance of drift has become clear in studies of population genetics and evolution at the molecular level by the expansion of genome data. The effects of drift are estimated by statistical analyses of molecular data on polymorphisms and divergence among related species. In particular, the interplay of drift and selection is most important in understanding the evolution of interaction systems at various levels. Epigenetic mechanisms have significant influences on the interplay. Drift, selection, environmental factors, and epigenetics are thought to work together here.

Key Concepts:

• Random sampling of gametes at reproduction is the major cause of drift in a finite population.
  
• Interplay of drift and selection is essential for evolution of complex systems, and the two become inseparable.
  
• Robust genetic systems enhance the effects of drift.
  
• Epigenetics gives opportunities for drift and selection to respond to environmental changes.