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.
Selection pressure that produces evolution in a population operates on those structural, physiological, or behavioral phenotypic manifestations of genetically controlled traits that help determine whether an individual is more or less likely to survive to pass on its genes. Genetically controlled traits that increase an organism's fitness, or its probable genetic contribution to succeeding generations, are called adaptations. Thus, adaptations are characteristics that help an organism perpetuate its genes. Adaptations come in a great variety of forms ranging from being genetically uncomplicated to highly complex; from involving in individual cells to whole organ systems; and from being of benefit only under particular circumstances to being of benefit under many different circumstances. Adaptations favoring fitness arise from random mutations to genes, mutations that often have lingered on through many
