Final answer:
Directional selection results in a decrease in genetic variance as it favors an extreme phenotype and reduces the frequency of alleles associated with other phenotypes, leading to reduced diversity in the trait.
Step-by-step explanation:
Directional selection causes a shift in the population's phenotype distribution by favoring an extreme phenotype over other phenotypes. This type of selection often results in a decrease in genetic variance over time, as the alleles associated with the non-favored phenotypes become less common or even disappear from the gene pool. As directional selection continues across generations, the population may become more homogeneous for the favored trait and thus, show reduced genetic diversity.
For example, if long beaks in birds become advantageous due to a change in available food resources, birds with longer beaks will have higher survival and reproductive rates. Over time, the allele for long beaks increases in frequency while those for shorter beaks decline, leading to a population that is primarily long-beaked with diminished variation in beak length. Genetic variance is reduced as the population skews towards this advantageous trait.