Genetic drift can be defined as the change in frequency of a gene due to random sampling or events(1). Within a population there can be many different alleles and gene types. The number of different genes will not be the same in the following generation due to recombination of genes, mutations and natural selection processes(1). This effect (the drift) may make some genotypes or alleles more common in a population or less common than in the previous population.
The process of genetic drift can only have an effect on the genes present within the existing population. Genes cannot become more common in absolute number without migration of individuals into the population: if a gene or allele is lost, it cannot be regained (2). This effect causes a net decrease in the gene variation of a population; a gene can become more common in a population at the expense of other gene types disappearing(2). If a population is small and/or has a low amount of genetic diversity to begin with before significant genetic drift takes place, through bottlenecks or the founder effect, it can become extremely vulnerable to disturbances and random events(1). Allele frequency variation due to drift becomes more prominent in smaller populations as well.
Figure 1. (The data photo present on this page) Drift and allele frequencies. Simulated allele frequencies in replicate populations. Drift happens faster in small populations (top panel) than larger ones (middle panel). Selection at linked sites (bottom panel) also increases the speed of change, but is not identical to a small population size (top panel). Retrieved from http://evolution.berkeley.edu/evosite/evo101/IIID2Genesdrift.shtml
1. Masel, J.; Current Biology. Science Direct October 25, 2011. Volume 21, Issue 20, Pages R837–R838 http://www.sciencedirect.com/science/article/pii/S0960982211008827
2. University of California Museum of Paleontology and the National Center for Science Education. Understanding Evolution: Evolution 101. http://evolution.berkeley.edu/evosite/evo101/IIID2Genesdrift.shtml