Final answer:
Clinal variation among modern human populations is produced by geographic separation and gene flow between populations.
Step-by-step explanation:
Clinal variation among modern human populations is produced by geographic separation and gene flow between populations. When populations are geographically separated, differences in the environment and selection pressures can lead to gradual variation in phenotype along a gradient. Restricted gene flow, on the other hand, can result in abrupt differences and even speciation.