EFFECTS OF INTRINSIC AND EXTRINSIC FACTORS ON POPULATION FRAGMENTATION IN 3 SPECIES OF NORTH-AMERICAN MINNOWS (TELEOSTEI, CYPRINIDAE)

Geographic patterns of genetic variation (mitochondrial DNA [mtDNA] an d allozymes) were used to examine effects of intrinsic characteristics (e.g., vagility, habitat specificity, and reproductive behaviors) and extrinsic factors (e.g., climatic and geological history) on populati on fragmentation. The three species of cyprinid fishes examined (Tiaro ga cobitis, Meda fulgida, and Agosia chrysogaster) occupied similar hi storical ranges within the lower Colorado River drainage, but differ i n intrinsic characteristics conducive to population fragmentation. Rel ationships among populations were similar across species, reflecting c ommon historical influences, but results indicate the distribution of variation among species is strongly affected by intrinsic characterist ics. Variation within two species (T. cobitis and M. fulgida) is subdi vided among populations, suggesting little gene flow among rivers. In contrast, similarity of A. chrysogaster populations throughout the Gil a River drainage supports the hypothesis that levels of gene flow are high for this species. Levels of mtDNA divergence were much higher tha n expected for both T. cobitis and A. chrysogaster suggesting long-ter m isolation of geographic regions. These results indicate that both lo ng-term and short-term extrinsic factors have shaped basic patterns of variation within these fishes; however, the intrinsic characteristics of each species have strongly affected the population genetic structu re of these fishes.