Founder effects are genetic phenomena, such as limited variation, among a population that started when a small group of people previously settled into a new area. The effect has been commonly observed in isolated areas where colonies were settled.
Founder effects are examples of genetic drift, which describes certain genetic characteristics being represented more or less frequently in a limited population than in a larger population. A population that originated in an area with a limited number of people may show less variation than the larger population they previously came from.
For example, a small group of people breaks off from a larger population to settle a new remote area. One or more of the founding couples carries a genetic mutation that causes a rare disease. As breeding occurs within the new settlement, this genetic mutation and rare disease becomes more common than in the original larger population.