Answer:
D. Inbreeding
Step-by-step explanation:
D. Inbreeding is the main reason why the gene pool of small populations is reduced compared to those of larger populations. Inbreeding occurs when individuals within a small population mate with close relatives, leading to an increased frequency of homozygous (identical) alleles and a decreased amount of genetic diversity. This can lead to an increased frequency of genetic disorders and a decrease in the ability of the population to adapt to changing environmental conditions.
A bottleneck event, such as a natural disaster, disease outbreak, or human intervention, can also dramatically reduce the size of a population and result in a reduction of genetic diversity. However, the reduction in genetic diversity in a bottleneck event is generally temporary and can be restored through genetic drift and gene flow from other populations.
Lack of advantageous traits and lack of suitable mates are not directly related to the reduction of gene pool in small populations. Rather, they can result from a reduction in genetic diversity, as the limited gene pool may not contain the variation necessary for the population to evolve and adapt to changing environmental conditions.