Final answer:
Each generation is a random sample of the previous generation's gametes, resulting in genetic drift—random changes in allele frequencies across generations, especially in small populations.
Step-by-step explanation:
Each generation is a random sample of the gametes produced by the previous generation, and genetic drift is a change spread across generations. Genetic drift is a mechanism of evolution that refers to random fluctuations in the frequencies of alleles from one generation to the next. It has the greatest effect in small populations, where chance events can lead to significant changes in allele frequencies over time. These random fluctuations can eventually lead to the fixation or loss of alleles in the population. Genetic drift can result in the reduction of genetic variation within populations, and, over time, the divergence between populations.
An example illustrating genetic drift is a population where the alleles B and b exist equally but, due to a random event, more B alleles are passed on to the next generation. This results in a shift in allele frequency and subsequent generations could see allele b being lost entirely, purely by chance. The concept of genetic drift emphasizes that not all evolutionary changes are driven by natural selection; some are the product of random chance.