The map above shows B blood group frequencies across eastern Asia and far western Europe. There is a gradual change in the characteristic from one population to the next. This pattern is called a:.

cline

Explanation:

When a measurable gradient is seen in a particular character across population living in a geographical range then this gradient is termed as cline.The clines can either be abrupt or continuous across a given geographical range. Clines arise due to two factors - gene flow as well as selection.In a map the clines are represented by lines that indicate the changes in character from one end to another across a geographic range.Therefore, the B blood group frequency changes in a map across eastern Asia and far western Europe form a pattern which is termed as a cline.