Final answer:
The changes in gene frequencies due to environmental selective pressures that enhance a population's fitness are most accurately described by natural selection. This evolutionary mechanism is distinct from genetic drift, gene flow, and genetic variation, and is a key factor in adaptive evolution.
Step-by-step explanation:
Changes in gene frequencies resulting from selective pressures placed on a population by environmental factors that result in a greater fitness of the population to its ecological niche is known generally as natural selection. This process occurs when there are differences in fitness among members of a population, leading to some individuals passing more genes to the next generation than others, which in turn changes allele frequencies. This is different from genetic drift, which is a random change in allele frequencies, gene flow, which involves the transfer of alleles into or out of a population due to migration, and genetic variation, which is the diversity of alleles within a population.
Natural selection, one of the key mechanisms of evolution, tends to increase the frequency of beneficial alleles that confer traits enhancing survival and reproduction. Such adaptive evolution leads to populations becoming more fit for their environment. Meanwhile, genetic drift can lead to random changes that are not necessarily advantageous, and gene flow can introduce new alleles to a population. Allele frequencies can also change due to nonrandom mating, mutations and environmental variances, contributing to overall population genetics.
The correct answer to the student's question is b) Natural selection.