A population of tree salamanders is highly variable in color and blends easily with the surrounding, colorful vegetation. After a factory upstream impacts their environment, several species of trees die off and only those with one color of leaf and flower are left. Within several generations, the salamander population has reduced variability in color.

This is an example of directional selection.

Explanation:

Directional selection is a type of positive selection where the evolutionary fitness of one genotype is higher compared to the other ones, thereby favoring the selection of one particular phenotype and shifting allele frequencies towards the selected genotype. In a biological context, the directional selection increases the chances for surviving and for reproducing in those organisms that have the alleles favored by this type of selection. Thus, the directional selection produces the loss of genetic variability and leads to the fixation of the dominant alleles.