Final answer:
Genetic diversity in a small population can be lost through the founder effect, which occurs when a new population is started by a small group of individuals, leading to a genetic structure that is not representative of the original population.
Step-by-step explanation:
A third way that genetic diversity can be lost in a small population is through the founder effect. This occurs when a new population is established by a very small number of individuals from a larger population. Since these individuals are unlikely to represent the full genetic diversity of the original population, the new population's genetic structure will differ and align with that of the founding individuals. The founder effect is significant in examples such as the Afrikaner population in South Africa, where certain genetic mutations are prevalent due to the genetic makeup of the initial settlers.