What process leads to a change in the gene pool due to a reduction in population size?

The process that leads to a change in the gene pool due to a reduction in population size is genetic drift. Genetic drift is a mechanism of evolution that causes allele frequencies within a population to change randomly over time. This is particularly pronounced in small populations where random events, such as natural disasters or sudden changes in the environment, can have a significant impact on which individuals survive and reproduce.

When a population experiences a reduction in size, such as through a bottleneck effect, the genetic variance is likely to decrease because only a small number of individuals contribute to the next generation's gene pool. This random sampling can lead to alleles being lost or becoming more prevalent purely by chance, rather than by adaptation to the environment, which would be a result of natural selection. Thus, genetic drift can lead to changes in the population’s genetic structure over time, affecting its evolutionary trajectory.

Natural selection, mutation, and gene flow do not specifically explain changes in a gene pool due to a reduction in population size in the same direct and random manner that genetic drift does. Natural selection involves differential survival based on advantageous traits, while mutation introduces new genetic variations, and gene flow refers to the transfer of alleles between populations.