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Genetic drift is a process that involves random changes in allele frequencies within a population, often occurring in small populations. This randomness means that the changes can happen without regard to the fitness of the organisms involved. Factors like random mating, random deaths, or random births can alter the genetic makeup of the population simply by chance, which is the essence of genetic drift.

Random events can include natural disasters, such as a flood or a fire, that eliminate a significant portion of a population regardless of the individuals' traits. These events can lead to certain alleles becoming more or less common purely by chance. Over time, this can result in a population that differs genetically from its ancestral population more than expected through natural selection or other evolutionary processes.

In contrast, the other options refer to mechanisms that do not result in random changes in allele frequencies. Natural selection involves differential survival and reproduction based on advantageous traits. The introduction of new species often leads to competition and can affect allele frequencies, but it's not a random event affecting only the existing population's genetics. Environmental changes affecting survival typically relate to how well certain traits allow organisms to survive rather than causing the random shifts characteristic of genetic drift.