Found this text today on a site on human variation.
The two different forms of phenotype trait distribution are good explained.


Clinal Distribution

Genetically inherited traits often have a clinal distribution. That is to say there is a continuous, progressive gradation moving from one geographic region to another.



The frequency of yellow-brown hair among Australian Aborigines illustrates this trend (see map above). This trait generally becomes more common with distance from the coast.

Such patterns can result when selective pressures differ from one region to another and when people mate mostly with their immediate neighbors. Selective pressures favoring or discriminating against a trait may come from several sources. There may be natural selection resulting from environmental constraints. At the same time, there also may be patterns of culturally defined discriminatory mate selection that vary from region to region.


Discontinuous distribution

Sometimes, the distribution of genetically inherited traits does not follow a pattern of gradual change from one geographic region to another but has a discontinuous distribution.



The frequency of red hair in Britain illustrates this sort of pattern. Note in the map on above that there are several relatively isolated pockets where there is a high frequency of people with red hair.
Such a pattern can result when groups of people migrate into a new area or when there are closed breeding groups that select mates based on such a trait.


Source: http://anthro.palomar.edu/tutorials/biological.htm