Final answer:
Natural selection must be quite strong to counteract genetic drift in small populations, as they are more susceptible to changes in allele frequencies due to their size. Events like the bottleneck effect, caused by disasters, can increase genetic drift by significantly reducing population size and altering genetic structure.
Step-by-step explanation:
The strength of natural selection must be significantly high to overcome the effects of genetic drift in small populations. Small populations are highly susceptible to random fluctuations in allele frequencies due to their limited size. For example, if one individual of a population of 10 individuals dies without leaving offspring, 1/10 of the population's gene pool is suddenly lost. In contrast, the loss of one individual in a population of 100 has a much smaller impact, only 1 percent of the overall gene pool, which means large populations are more buffered against these changes. Genetic drift can also result from natural or human-caused events that lead to a substantial decrease in population size, known as the bottleneck effect. This effect can drastically change the genetic structure of a population by randomly removing individuals regardless of their phenotype or genotype. Subsequently, the genetic makeup of the survivors dictates the genetic structure of the future population. Natural selection can counterbalance genetic drift if it consistently acts on advantageous traits, enhancing their frequency within the population. However, this requires the beneficial traits to be present and sufficiently advantageous to offset the randomness of genetic drift, which is more challenging in small populations due to their reduced genetic variation.