Final answer:
The magnitude of evolution by genetic drift is greater in smaller populations because random events significantly alter allele frequencies, whereas in large populations, genetic drift has a less pronounced impact.
Step-by-step explanation:
All populations evolve every generation by genetic drift, but the magnitude of evolution by genetic drift is greater in smaller populations. This is because genetic drift is the effect of random chance on a population's gene pool, and its impact is inversely related to population size. In a small population, the loss or gain of alleles due to random events such as the death of individuals or mate selection can significantly alter the allele frequencies from one generation to the next.
For example, if a rare allele is present in a small population and the few individuals carrying it do not reproduce or their offspring do not survive, that allele might be completely lost. In a large population, the effect of losing a few individuals on the gene pool is much smaller, so genetic drift has a less pronounced impact on allele frequencies. This principle is further highlighted by scenarios like the bottleneck effect, where a large portion of a population is wiped out by a natural disaster, or the founder effect, where a new population is established by a small number of individuals from a larger population.