Final answer:
Small populations are more affected by genetic drift because the loss or gain of an individual has a greater impact on the population's gene pool. This can lead to dramatic shifts in allele frequencies due to the bottleneck or founder effects, which reduces genetic diversity and adaptability.
Step-by-step explanation:
Genetic drift is a significant evolutionary force that is particularly influential in small populations due to random chance events affecting allele frequencies. In a small population, the loss or gain of alleles through the deaths or births of individuals can have a large impact on the genetic makeup of the population. For example, if one individual in a population of ten dies without leaving offspring, 1/10 of the gene pool is instantly lost; this is a substantial change. In contrast, in a population of 100, the loss or addition of one individual only accounts for 1% of the gene pool, which is a much smaller effect. Therefore, small populations are more vulnerable to genetic drift because each individual represents a larger fraction of the population's gene pool.
Genetic drift can lead to the bottleneck effect and the founder effect, both of which can drastically reduce genetic variation within a population. These effects are exacerbated in small populations, which can suffer from reduced genetic variance and thus potentially less adaptability to environmental changes. This in turn can increase the chances of extinction. Consequently, genetic drift is more pronounced in smaller populations than in larger ones, influencing the population's genetic structure and evolutionary potential.