Final answer:
The founder effect is a form of genetic drift that occurs when a new population is established by a small number of individuals from a larger population, leading to a different gene pool in the new population, and is a key mechanism behind allopatric speciation.
Step-by-step explanation:
The concept being described is known as the founder effect, a form of genetic drift that occurs when a small group of individuals becomes isolated from a larger population. This small group may establish a new population with a gene pool that differs from the source population. The repercussions of such isolation can lead to an increase in rare alleles and potentially adverse health outcomes, as exemplified by the prevalence of Tay-Sachs disease in certain isolated human populations. This process is a key mechanism behind allopatric speciation, where geographically isolated populations evolve independently leading to species divergence due to different selective pressures, mutations, and genetic drift.