With genetic drift, the key word is “random”
Genetic drift occurs when a population experiences random fluctuations in frequencies of genetic traits. The term “random” is key to an understanding of drift. If any heritable variation leads to genetic changes in a population, natural selection has occurred. Drift has occurred if these changes are unrelated to any heritable feature possessed by individuals in the population. Sometimes, as a result of chance events, certain individuals do not reproduce, and the population evolves as a result of drift. While natural selection results from aspects of an organism’s environment exerting “selective pressure” on the individual (e.g., the desert environment favors the spines of the cactus and the long ears of the fox), drift, by definition, is not a result of environmental pressures. Drift is common to small populations, such as those that colonize islands or other isolated habitats, or those that remain after large-scale disruptions (e.g. earthquakes, fire). Think about it: the random loss of 20 iguanas from a large population of 1 million iguanas is bound to result in fewer overall genetic changes than is the random loss of 20 iguanas from an island population of 100 iguanas.
One form of drift is the founder effect, which results when a small part of a population (the “founders”) moves to a new location, and the small variety of genes of the founders mean that the new population is different from the original population. Ellis-van Creveld Syndrome (a form of dwarfism that involves short stature, extra fingers or toes, and possible heart defects) and several other genetic conditions that are typically rare in large populations, are common in the Pennsylvania Amish population in the United States because of drift—specifically, because of the founder effect. This group was founded relatively recently by a few individuals who presumably had genes for these conditions. The population has grown, but has not been subject to the diversifying effects of gene flow (that is, individuals rarely enter the population from another population). Therefore the current population has these conditions in much higher frequency than the population from which they migrated.