Several large electrophoretic studies in which populations have been sampled in consecutive years (e.g., MOLLER 1967; DAHLE and JORSTAD 1993) or in which age classes have been compared (e.g., JAMIESON 1975) provide support for this requirement, although samples from some areas exhibit substantial Wahlund effects (e.g., JAMIESON and BIRLEY 1989). The highly similar UPGMA trees produced by the two sets of mark- ers (Figure 5) further suggest thato ur comparisons are valid by demonstrating that the extent of variation among populations from different regions is greater than the extent of variation present within regions