Abstract
Development of electrophoretic techniques has allowcd the estimation of allelic fre-
quencies 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 substantialexchange 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.
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