What phenomenon occurs when a small number of individuals separate from their original population and establish a new population?

The phenomenon described is genetic drift, which occurs when a small group of individuals breaks away from a larger population to form a new population. This scenario often leads to changes in allele frequencies, particularly because the new population may not have the same genetic makeup as the original, larger population. The randomness inherent in this sampling process can result in some alleles being overrepresented or underrepresented simply by chance, especially if the founding members do not reflect the original population's genetic diversity.

While gene flow refers to the movement of alleles between populations through migration, it does not specifically involve the establishment of a new population from a small subgroup. Intraspecific competition is about individuals of the same species competing for resources and does not pertain to population establishment. Sexual dimorphism refers to the differences in appearance between males and females of the same species, which is not related to the dynamics of population founding events. Thus, genetic drift is the correct choice, as it accurately captures the random genetic changes that occur when a small population is formed from a larger one.