1. Founder effect: This occurs when a small group of individuals from a larger population establishes a new population in a new location. The gene pool of the new population is a subset of the original population's gene pool, and may not be representative of it. This can lead to a significant change in allele frequencies in the new population, even if the original population was in Hardy-Weinberg equilibrium.

2. Bottleneck effect: This occurs when a large population undergoes a sudden reduction in size, often due to a catastrophic event like a natural disaster or disease outbreak. The surviving individuals may not represent the genetic diversity of the original population, resulting in a loss of genetic variation. This can lead to a decrease in the population's ability to adapt to changing environments and increase the risk of extinction.