Question

The residues (tailings) of mines often contain such high concentrations of toxic metals (e.g., copper, lead) that most plants are unable to grow on them. However, some hardy species (e.g., certain grasses) are able to spread from the surrounding uncontaminated soil into such areas. These plants evolve resistance to the toxic metals while their ability to grow on uncontaminated soil decreases. Because grasses are wind-pollinated, breeding between the resistant and nonresistant populations continues, but offspring of crosses between the two populations are intermediate and have low fitness. This is an example of:

a. Directional selection
b. Disruptive selection
c. Stabilizing selection
d. Balancing selection

Answer 1

The described scenario of plant populations evolving in mine residues with toxic metals is an example of diversifying selection, where distinct phenotypes are favored and intermediates are selected against, leading to increased genetic variance.

Step-by-step explanation:

The scenario described involves plants evolving resistance to toxic metals in mine residues where offspring of crosses between resistant and nonresistant populations are intermediate and have low fitness. This situation best exemplifies diversifying selection, where two or more distinct phenotypes are favored by natural selection, while intermediate phenotypes are selected against. In diversifying selection, the extremes of the phenotypic spectrum are advantageous and thus, have higher fitness and reproductive success compared to those with intermediate phenotypes. Natural selection favors the hardy plants that can tolerate high metal concentrations on one end of the spectrum and those that thrive in uncontaminated soil on the other, but not the plants with intermediate tolerance levels. Such a mechanism increases genetic variance, as the population becomes more diverse, with distinct sub-populations that are adapted to different environmental conditions.