The gene pools of small populations are reduced compared to those of larger populations primarily due to D) Inbreeding.
In small populations, the chances of mating with closely related individuals increase, which can lead to a decrease in genetic diversity. Inbreeding can result in a higher likelihood of hereditary diseases and decreased adaptability to environmental changes. While lack of suitable mates (A), bottleneck events (B), and lack of advantageous traits (C) can also affect genetic diversity, inbreeding is a direct consequence of small population sizes that specifically leads to a reduced gene pool.