Final answer:
Genetic drift, which causes random changes in allele frequencies, is more likely to lead to the complete loss of an allele on an island due to smaller population sizes and higher susceptibility to random events affecting allele distribution.
Step-by-step explanation:
Complete loss of an allele over generations due to random factors is more likely on an island compared to the mainland. This is due to genetic drift, which is significant especially in small populations where chance events have a much larger impact on the allele frequencies. On an island, the population size is typically smaller, and events such as natural disasters can disproportionately affect certain allele frequencies, leading to a quicker loss of alleles by chance alone. Genetic drift occurs because the alleles in an offspring generation are a random sample of the alleles in the parent generation, and not all alleles may be passed down to the next due to various random events, such as mortality or mate selection. When genetic drift occurs in a small population, such as on an island, the loss of even one individual can have a significant effect on the population's genetic structure, potentially leading to the loss of an allele.