Answer:
directional selection
Step-by-step explanation:
Selection is of three types -
1. diversifying
2. stabilizing
3. directional
These selections occur in a given environment to either increase, decrease or maintain the genetic variance of the population.
A type of natural selection in which the phenotype of the species of a given population tends to shift away from the mean and towards one extreme phenotype is known as directional selection.
In the given situation, it is given that the extreme phenotype of having thickest leaves is being favored in drier climate and hence, this is an example of directional selection.