The allelic distribution at an acid phosphatase locus in Norway spruce (Picea abies) along similar climatic gradients. 
The allele frequency distribution at a polymorphic acid phosphatase locus (APH-B) was determined in natural populations of Norway spruce (Picea abies) from a latitudinal transect in Finland, an altitudinal transect in the Austrian Alps, and from different locations of the Swiss range.  The three independent population groups, selected with respect to similar climatic gradients, were studied to detect the forces that cause the geographic variation at the APH-B locus. In almost all of the populations investigated, four alleles (APH-B1 - B4) could be identified at this enzyme locus, however, the alleles b1 and B2, as well as B3 and B4, show a great similarity according to their phenotypic appearance after electrophoresis as well as to their frequency distributions along the different transects.  With the aid of some theoretical considerations and data comparisons, a selective equivalence of the alleles B1 and B2, as well as B3 and B4, could be ascertained, thus reducing the number of alleles that can respond differently to natural selection. After combining the frequencies of the selectively equivalent alleles, similar clinal variation patterns could be observed along the different geographical transects, where-by the frequency of the allele group APH-B1 /B2 markedly increases with latitudes in Finland and towards higher elevations in the Alps.  Correspondingly, the allele group APH-B3/B4 predominates in the southern parts of Finland and in the lowlands and foothills of Austria and Switzerland.  It is therefore concluded that natural selection causes the geographic variation pattern at the APH-B locus and that one or several temperature variables function as an at least predominant selective force.  Possible relationships between this enzyme polymorphism and other tree characters and the physiological role of acid phosphatases in tree adaptation were discussed.