Development of electrophoretic techniques has allowed the estimation of allelic frequencies at many isozyme loci in populations of fish. The pattern of allelic frequency divergence in salmonid populations has been used to describe the pattern of genetic exchange and degree of isolation among local geographic units. Our goal is to determine how much genetic exchange among subpopulations will yield observed patterns of allelic divergence. We approach this goal using population genetics theory and a series of computer simulations.The amount of allelic divergence between subpopulations is a function of the absolute number of migrant individuals exchanged, and not the proportion of individuals exchanged. Therefore, some knowledge of population sizes is needed to estimate degree of reproductive isolation from allelic frequency data. Second, statistically significant allelic divergence will often be present even when there is substantial exchange among subpopulations. For example, significant allelic divergence was present in our simulations over 50% of the time with 20 subpopulations exchanging 50 individuals per generation. Third, allelic frequencies estimated from fry should be used with caution when drawing conclusions about the significance of allelic divergence in the reproducing adults.Key words: allelic divergence, reproductive isolation, gene flow, genetic drift, computer simulations

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Information developed during recently completed evaluations of the status of seven species of anadromous Pacific salmonids (Oncorhynchus spp.) in the Pacific Northwest was used to characterize patterns of intraspecific diversity along three major axes: ecology, life history and biochemical genetics. Within the study area, the species' ranges, and therefore the number of distinct ecological regions inhabited differ considerably, with pink and chum salmon limited to the northern areas and chinook salmon and steelhead distributed over the widest geographic range. The species showed comparable differences in the patterns of life history and genetic diversity, with chinook and sockeye salmon and steelhead having the most major diversity groups and pink, chum and coho salmon having the least. Both life history and genetic diversity showed a strong, positive correlation with the extent of ecological diversity experienced by a species, and the correlation between the number of major genetic and life history groups within a species was even stronger (r=0·96; P<0·05). Departures from these general diversity relationships found in some species (especially sockeye and coho salmon and cutthroat trout) can be explained by different interactions with the freshwater environment and, for cutthroat trout, by the occurrence of substantial intrapopulational diversity in life history traits, a hierarchical level not considered in this study.

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