Directional selection presents an apparent paradox. By favoring one allele and disfavoring others, directional selection can lead to fixation (a frequency of 1.0) of the favored allele, after which there is no genetic variation at the locus, and its evolution stops. Explain why directional selection no longer operates in populations after the favored allele reaches fixation.

Key Concepts

Directional Selection

Directional selection is a mode of natural selection that favors one extreme phenotype over others, leading to a shift in allele frequencies in a population. This process enhances the reproductive success of individuals with advantageous traits, gradually increasing the frequency of the favored allele while decreasing that of others. Over time, this can result in the fixation of the favored allele, where it reaches a frequency of 1.0.

Fixation of Alleles

Fixation occurs when a particular allele becomes the only variant present at a genetic locus within a population, resulting in a frequency of 1.0. At this point, all individuals in the population carry the same allele, eliminating genetic diversity at that locus. This lack of variation means that there are no alternative alleles for selection to act upon, effectively halting the evolutionary process at that specific gene.

Genetic Variation and Evolution

Genetic variation is essential for evolution, as it provides the raw material for natural selection to operate. Without variation, populations cannot adapt to changing environments or respond to selective pressures. Once an allele is fixed, the absence of alternative alleles means that directional selection can no longer occur at that locus, as there are no competing traits to favor or disfavor, leading to a stagnation in evolutionary change.