Genetic drift

Genetic drift is characterized as significant random fluctuations in allele frequency through chance deviation. The degree of fluctuation increases as the population size decreases. This occurs when a species population is too small to ensure that all alleles will be passed on to successive generations. If a certain allele in a small population has an especially low frequency of occurrence to begin with the chance that it will be breed out of the population is high. When fewer gametes are available to form the next generation a allele can become fixed, resulting in loss of genetic variation. Genetic drift is not specific to the usefulness of a gene and can sometimes result in a high population frequency of an undesirable gene and can eventually lead to extinction.
Genetic drift can be caused by the genetic bottleneck effect in which a population experiences a temporary but dramatic decrease in population. If a population drastically decreases in size (for example, by famine, over hunting, habitat loss or natural disaster leading to forced migration) the chances of genetic drift increase drastically because the breeding pool is now limited.
Genetic drift can also occur due to the founder effect. This is when a population originates from an already small population, such as a remote indigenous tribe, where the allelic variation is low to begin with.
A current and relevant example of genetic drift is the cheeta. Caused mainly by the bottleneck effect the cheeta species share 99% of the same genes. This poor variation has lead to low survivorship, poor sperm quality and high suceptibility to disease.
References
Klug, Cummings, Spencer, Palladino. Concepts of Genetics. San Francisco: Pearson Benjamin Cummings, 2009.
Santa Rosa Junior College, presentation on evolution.