Final answer:
The change in allele frequency due to a small subpopulation migrating to a new area is known as gene flow, which introduces new alleles to the population.
Step-by-step explanation:
The change in allele frequency when a small subpopulation migrates to a new area to start a new population is referred to as gene flow. When individuals from one population move to another, they bring with them different alleles that may not have been present in the new population, thereby changing the allele frequencies of that population. This is different from genetic drift which is a random change in allele frequencies, often happening in small populations.