Mechanism Of Evolution

Genetic drift is the random change in allele frequencies of a population due to chance events, and its effect is especially strong in small populations. When a catastrophe such as a volcanic eruption kills most individuals indiscriminately, the few survivors may carry allele frequencies that differ markedly from the original population purely by chance, an example called the bottleneck effect. Natural selection is incorrect here because selection acts non-randomly, favouring individuals with advantageous traits, whereas the eruption kills organisms regardless of their fitness. Gene flow is wrong as it involves the movement of alleles between populations through migration, which is not occurring in this isolated catastrophe. Directional selection is a form of natural selection that shifts a trait toward one extreme over time, again a non-random process unlike the chance survival described. As explained in NCERT Class 12, Chapter 7 (Evolution), genetic drift can alter the genetic makeup of founder or bottlenecked populations significantly. As a consistency check, since survival was random and the population became small, the resulting frequency change matches the definition of genetic drift rather than any selective mechanism.