Reviews in Fish Biology and Fisheries 10: 265–279, 2001.
© 2001 Kluwer Academic Publishers. Printed in the Netherlands.
Patterns of subspeciﬁc anthropogenic introgression in two salmonid
School of Fisheries, University of Washington, Seattle WA 98195; 206/685-8196, USA
Accepted 29 August 2000
Abstract page 265
Detecting introgression 266
Examples of subspeciﬁc introgression
Fixed allelic markers
Quantitative allelic variants
Patterns of subspeciﬁc introgression 269
Genetic substructure at and beyond the threshold of molecular markers
Outbreeding depression and introgression
Concluding observations 275
Key words: genetic substructure, introgression, molecular markers, Oncorhynchus, outbreeding depression, Salmo
Introgressive hybridizations have often been observed between native and introduced trouts of North America
(Oncorhynchus spp.) and Europe (Salmo spp.), including some lineages that have been isolated for more than a
million years. These observations have suggested that introgression is the expected result between introduced and
indigenous conspeciﬁc salmonids. However, an examination of published information reveals a high variability in
such anticipated gene ﬂow. Many studies have noted the relative ease of translocating freshwater over anadromous
salmonids, and this difference has been related to the more complex adaptations of anadromous populations (e.g.,
freshwater and marine residence, smoltiﬁcation, juvenile and adult migration) that obstruct their translocation to
conspeciﬁcally colonized areas. This contrast extends to introgressive capabilities where examples of introgression
among freshwater populations predominate. Despite intensive introductions of non-native salmonids, indigenous
anadromous populations commonly resist introgression. However, within major lineages, anadromous populations
appear to be more susceptible to introgression. Measuring the extent and dynamics of such introgressions remains
challenging because subgroups within major lineages lie on or below the threshold for detection by molecular
genetic markers. These substructures appear to reﬂect the more rapid evolution of directional selection promoting,
for instance, temporal or microgeographic divergence within a population unit deﬁned by genetic markers.
Consequently, management that assumes panmixia within a particular region based on even intensive molecular
genetic analysis will inevitably erode and prevent reformation of this substructure to the detriment of the overall
genetic variability and productivity.