1. A species of lizards living in the desert has a mouth only large enough to eat small crickets. For the size phenotype, the cricket population will therefore experience:

a) disruptive selection

b) directional selection

c) genetic drift

d) stabilizing selection

e) none of the above

b) Directional selection

Explanation:

Direction selection is the phenomenon in which a population's genetic variance changes with respect to environmental changes to select a new phenotype. In this case, the size of cricket, the food source acts as the selection factor for the phenotype, the size of the lizard's mouth.

In order to accommodate the food source, a single phenotype of mouth size changes and the respective allele is favoured. Correspondingly, the allele frequency will shift unilaterally in one direction only.

The increased frequency of the particular allele ensures a higher survival rate of the lizard. The population evolves towards the favoured phenotype.

The correct answer is: b) directional selection

Directional selection is a type of natural selection in which an extreme phenotype is favored over other phenotypes. In this case, phenotype of large crickets will be favored, because they cannot be eaten by lizards. Large crickets are advantageous in surviving and reproduction. This will cause the allele frequency to shift over time in the direction of that large size phenotype.