Final answer:
A graded change in a character along a geographic axis is known as a cline, representing the gradual phenotypic variation of species across different environmental conditions, such as temperature changes with latitude or altitude.
Step-by-step explanation:
A graded change in a character along a geographic axis is known as a cline. This biological concept describes a pattern in which there is gradual change in the phenotype (appearance, behavior, or physiology) of a species across an ecological gradient. For instance, a latitudinal cline represents species variations due to changes in temperature from the equator towards the poles, where warm-blooded animals may develop larger body sizes in cooler climates to conserve heat. Conversely, an altitudinal cline could involve flowering plants that bloom at different times as you move up a mountain's slope, influenced by the changing temperature with altitude.
The presence and significance of clines are important in understanding biogeography and evolution. They help reveal how geographic separation between populations can lead to phenotypic variation as a result of the different environmental conditions experienced across a species' range.