Introduction

Geographical patterns of genetic diversity contain information on the evolutionary processes affecting individuals and populations. For example, random patterns, clines, and patchy distributions may result from different evolutionary phenomena (like panmixia, response to a selective gradient, and isolation by distance, respectively), which one may be willing to reconstruct. In some cases, a simple visual inspection of data is sufficient. Figure 8.1 shows two maps of human allele frequencies in Europe based on several tens of population data (Sokal, Harding & Oden, 1989). These maps were obtained by interpolation, and different shades of grey indicate different classes of allele frequencies. The upper distribution refers to an allele of the HLA-B locus, HLA*BW15. There is no need of sophisticated statistical tests to see that its frequencies are distributed in a gradient, with maxima in Scandinavia and Finland, and an area of low values around the Mediterranean. Other cases are, however, ambiguous. One cannot easily tell if the lower part of Fig. 8.1 (allele 1 of the Group-specific component) is a cline extending from the Southwest to the Northeast on which random variation has been superimposed, if it should be regarded as the overlapping of two clinal patterns, or even if it may simply reflect drift and local gene flow, i.e., isolation by distance.