What phenomenon occurs when a small group from a larger population establishes a new population?

The phenomenon described in the question is the founder effect. This occurs when a small group from a larger population breaks away and establishes a new population in a different area. Because the new population is started by just a few individuals, it may carry a limited genetic diversity compared to the original population. This reduced genetic variance can lead to changes in allele frequencies, which may cause the new population to diverge genetically from the parent population over time.

The founder effect can significantly influence the evolutionary path of the new population, particularly in terms of its adaptation to the new environment and the potential for speciation. This is particularly evident in island populations or isolated environments where such small founding groups might adapt uniquely due to different selective pressures.

In this context, while concepts like gene flow refers to the transfer of genetic material between populations, divergent evolution involves the accumulation of differences between groups, and adaptive radiation describes a rapid diversification from a common ancestor into various forms to adapt to different environments, these do not specifically address the process of a small group establishing a new population directly.