Final answer:
Clinal variation refers to the gradual change of a species' traits over a geographic area, exemplified by the latitudinal distribution of red hair which mostly occurs in Northern European populations.
Step-by-step explanation:
Clinal variation refers to the way specific traits of a species change gradually over a geographical area. An example of this can be observed in the distribution of red hair, which is most commonly found in the Northern European population. This is a latitudinal cline, where the frequencies of red hair decrease as you move away from specific latitudes. The genetic explanation for the association of red hair with very light skin could be due to both traits being adapted to regions with lower sunlight, allowing for better synthesis of Vitamin D.