Which best defines genetic drift?

Genetic drift is best defined as the change in allele frequencies due to random sampling effects. This concept refers to how allele frequencies can fluctuate from one generation to the next purely by chance, especially in small populations. Random events can lead to certain alleles being passed on to the next generation more frequently than others, irrespective of their functional advantages or disadvantages.

For example, if a small population is affected by a natural disaster, the individuals that happen to survive may not represent the overall genetic diversity of the original population, leading to skewed allele frequencies in subsequent generations. This stochastic process can have significant implications for the genetic structure of populations over time, particularly in small or isolated groups, where the effects of random fluctuations can be more pronounced.

The other options refer to mechanisms that drive evolutionary change in different ways. Natural selection involves differential survival based on advantageous traits, migration involves gene flow between populations, and mutation introduces new genetic material, but none of these processes are characterized specifically by the element of random chance that defines genetic drift.