In a very large population (interbreeding group), random drift has little effect on long-term gene frequencies. One human eye color gene has two alleles, B and b, leading to brown eyes (BB or Bb) or blue eyes (bb). Imagine a human couple in which both parents are heterozygous for eye color (Bb). If they have only one child, that child has a 3/4 chance of having brown eyes and a 1/4 chance of having blue eyes. If they have lots of children, chances are that the children will display the genotypes BB, Bb, and bb in a 1:2:1 ratio. Over generations, if every heterozygous couple has a large family, both B and b alleles will be found in subsequent generations.
However, in a very small population—for example, an endangered species such as the Florida panther—random fluctuations and events can have a huge impact and may even lead to one allele becoming fixed (the only variant that exists for the gene) or lost, further decreasing genetic variation. Returning to the example of a human couple heterozygous for eye color (Bb), if each couple has only one child, there is a chance that their child will have blue eyes (bb). If there are only a few couples in the population, many of them have only one child, and that child has blue eyes, the frequency of the brown allele (B) will decrease and the frequency of the blue allele (b) will increase. Thus, as a result of random assortment and a small population, allele frequencies can change from one generation to another.
There are two main forms of genetic drift: the bottleneck effect and the founder effect.