Final answer:
The statement that when natural selection favors one extreme phenotype, it is called directional selection, is true. Directional selection causes a shift in a population's characteristics towards that extreme, contributing to evolution in response to environmental factors.
Step-by-step explanation:
When natural selection acts on a human trait, such as weight or height, and favors one extreme phenotype, this is indeed called directional selection. The statement is true. Directional selection results in a shift of the phenotypic distribution toward one extreme. This type of selection can cause populations to evolve in one particular direction, often in response to environmental changes or pressures. For example, if there is an advantage to being taller in a certain environment, individuals who are taller may have better survival and reproductive success, leading to an overall increase in the average height of the population over several generations.
It's important to understand the different types of natural selection that can influence the evolution of species. Besides directional selection, there is also stabilizing selection, which favors the average phenotype and works against extreme variants. This type lowers variation in the population and tends to maintain the status quo. Lastly, there is disruptive selection (also known as diversifying selection), which favors extreme phenotypes at both ends of the spectrum, leading to a split in the population into two distinct groups over time.