Final answer:
Genetic drift causes random changes in allele frequencies and is most important in small populations because each individual represents a larger fraction of the gene pool, and chance events can significantly alter allele frequencies. Larger populations are less affected due to larger sample sizes and the law of large numbers.
Step-by-step explanation:
Genetic Drift and Population Size
Genetic drift is a mechanism of evolution that refers to random changes in allele frequencies in a population. Small populations are particularly vulnerable to genetic drift for several reasons. First, each individual in a small population holds a larger fraction of the population's overall gene pool. Therefore, if an individual fails to reproduce or dies before reproducing, a significant portion of the population's genetic diversity can be lost. This impact is illustrated with the scenario where an individual represents 10% of the gene pool in a population of ten but only 1% in a population of a hundred.
Another reason is related to the sample size of alleles; in smaller populations, the sample size in the next generation is small, so there is a higher likelihood of significant fluctuations in allele frequencies due to chance alone. This is unlike larger populations where the law of large numbers applies, making allele frequencies more stable and predictable over generations.
Events such as natural disasters or the founder effect can also precipitate genetic drift by drastically reducing population size or by establishing a new population with limited genetic diversity. These events can lead to a significant shift in allele frequencies, sometimes even resulting in the loss of alleles from the population entirely.