Complex evolutionary history in the brown trout: Insights on the recognition of conservation units

We screened genetic variation in a polytypic organism, whose populations are often distributed into numerous isolated habitats, and integrated the results into a critique of defining "units" of conservation for organisms with highly fragmented populations. Sixteen populations of brown trout Salmo trutta L. across 8 Portuguese river basins were screened for variation at 5 loci (mtDNA and allozymes). Population history based on mtDNA revealed a mosaic pattern driven by past fragmentation and restricted gene flow with little correspondence to major river drainages or recently proposed OCUs on the Iberian Peninsula. Such patterns of variation offer a challenge to conservation strategies that base themselves on defining units of conservation, particularly if such units intend to reflect a hierarchical evolutionary structure. We suggest that geographically mosaic patterns of evolutionary lineages, as well as adaptively significant traits are common characteristics of many freshwater organisms. Thus, large-scale units, even if diagnosed by mtDNA clades, are often too heterogeneous to consider a "unit" of conservation. Alternatively, a bottom-up perspective that prioritizes populations or metapopulations is both more practical and more effective in recognizing and preserving evolutionary diversity.