Final answer:
Characteristics across population are distributed on a cline, reflecting a geographical gradation of traits. Population variation becomes evident with geographically separated groups, influencing the divergence of species due to differing evolutionary pressures. Examples include size variation in warm-blooded animals related to climate and blooming periods in plants based on altitude.
Step-by-step explanation:
Characteristics are not clumped into a few 'races', but are typically distributed across a cline - a geographical gradation of allele frequencies or phenotypic traits.
When considering population variation, it's important to understand that it refers to the distribution of phenotypes within a population. An example of this can be seen in domestic dogs, where a myriad of fur colors and patterns are observable. In a geographically continuous population, gene flow tends to be high, maintaining a relatively homogeneous gene pool since individuals can move and mate, which keeps allele frequencies similar across a species' range. However, when geographical separation occurs, this hinders the flow of alleles, potentially leading to divergent evolutionary paths for the separated populations.
A classic example of population variation due to geographic separation is the difference in body size of warm-blooded animals closer to the poles compared to those near the equator, due to the need to conserve heat - a latitudinal cline. Similarly, flowering plants may present variation in blooming periods depending on their location on a mountain slope - an altitudinal cline.