At the basis, this means that there is an abiotic (like temperature), or biotic factor (vegetation type) that shows latitudinal/longitudinal variation and that factor has an selective influence on the phenotype in question.

means that particular phenotype is influenced by geographical variance and may be controlled by multiple alleles. Ex: pungency in pepper varies with geographical altitude.

What factors change with latitude? Climate, average temperature, the length of night and day - to name the most obvious. These variables can surely influence a fenotype.

Factors changing with latitude are in two categories. Some factors change systematically across many different vegetation types, while other factors are linked to secondary factors that respond to the basic climatic differences. Some factors changing across a long range are Minimum daily temperature, maximum daily temperature, average daily temperature, lowest temperature January, highest temperature July, etc. Factors varying a bit more with vegetation zone are factors like humidity. So, without knowing the latitudinal range you are looking at, it will be hard to come up with more specific factors.

Now, LONGitude is a different game, because there are less a-priori factors that come to mind that vary with longitude. Fort those to uncover, you would need to know where exactly the samples are taken. In the US for example, you have a east-west humidity gradient east of the Rocky Mountains, while in Europe that pattern is different.

If you are on a continent like Asia or even Europe and in an area with prevalent wind direction the conditions become more "continental" with wind direction.

In Germany with prevailing West winds this means that the further East you go the more arid it will be. This should be a selective force.

Also the range of temperature (difference between hotest and coldest temperature) is wider the further East you go in a West wind area.

To Mrs. Ivan Subrt and all other honored contributors: Sorry, second question was an error (I deleted it), I am interested to know which factors may impact gradually (systematically) a phenotype with the change of geographical longitude. I mean globally. Could this be possible? I would be very thankful if you suggest me any kind of factor that may have this impact. I found s.m.th. that correlates with longitude, and the correlation was stronger when I removed the data from countries that have a high immigration rate like USA ...... strange .... I would be very grateful also if You could suggest me any study or invite others in answering this questions (sorry for being so restrained but it looks so strange to me so I don't dare to believe that it can be true).