Final answer:
Skin color in humans is a polygenic trait controlled by multiple genes, resulting in a spectrum of phenotypes. It cannot be categorized into discrete racial groups and varies geographically due to directional selection based on sunlight exposure.
Step-by-step explanation:
Skin color in humans is an example of a trait with polygenic inheritance. This means that it's controlled by more than one gene, resulting in a wide range of possible phenotypes—varying from light to dark shades, for example. This wide variation is typical for polygenic traits, which also include other characteristics such as height and eye color.
Biological anthropology has demonstrated that common notions of race based on skin color are biologically inaccurate, as these traits vary along a spectrum and cannot be neatly categorized into racial groups. This invalidates any attempts to classify humans into discrete racial categories based on skin color alone.