Which of the following describes the reason behind genetic drift?

Genetic drift refers to the random fluctuations in allele frequencies that occur in a population from one generation to the next. This phenomenon is particularly notable in small populations, where chance events can lead to significant changes in the genetic makeup over time. Unlike natural selection, which is driven by environmental pressures and the relative fitness of individuals, genetic drift occurs without regard to the adaptive value of alleles. Thus, option B accurately captures this concept, highlighting the randomness inherent in the process of allele frequency changes that do not favor any particular trait for survival or reproduction.

The other options describe different biological concepts. Environmental pressures relate to selective forces that shape populations through natural selection, while selective survival of the fittest emphasizes the survival advantage of certain traits in relation to their environment. Hybrid vigor pertains to the phenomenon where crossbreeding results in offspring that display improved or enhanced biological functions, especially in terms of health, vitality, or growth compared to their parents. These concepts do not pertain to the random process described by genetic drift, reinforcing why option B stands out as the correct choice.