Of the four forces that can force evolutionary change, mutation pressure, gene flow, genetic drift, and selection pressure, which exerts the greatest and most important influence on the direction of evolution? Explain why three of these forces exert minor influence and one exerts a major influence on evolution.
Although mutations do increase variability in genotypes providing the ultimate raw material for evolution and, therefore, constantly exert a slight influence on the genetic equilibrium of a gene pool, mutation pressure rarely determines the direction of evolutionary change. And although gene flow (in many populations) and genetic drift (in small populations) also disturb genetic equilibrium, these forces seldom determine the direction of evolutionary change either. Rather, it is the selection pressure of natural selection that exerts the greatest and most important influence on the direction of evolution. Indeed, even minute selection pressures can produce very significant changes in gene frequencies when measured on a geologic time scale. Larger selection pressures can cause allelic frequencies to shift within the time span of a century or less.
