Final answer:
Trees in a forest that receive more sunlight and grow taller are subject to directional selection, which favors their taller height due to increased access to sunlight and leads to a change in the population's average height over time.
Step-by-step explanation:
In a forest, trees that get more sunlight and consequently grow taller than other nearby trees are exhibiting a form of directional selection. This is because one particular trait (height, in this case, associated with greater access to sunlight) is being favored over others, leading to a shift in the average height of the trees over time. Directional selection is a mechanism of evolution in which an environmental pressure leads to increased fitness for individuals with certain phenotypes, thus causing a continuous shift in a population's genetic variance toward that phenotype.
Other forms of selection such as stabilizing selection favors an intermediate phenotype and typically reduces variation within a population, while disruptive selection favors extreme phenotypes over intermediate ones, allowing increased genetic variance in a population.