International Journal of Molecular Ecology and Conservation, 2025, Vol.15, No.3, 123-133 http://ecoevopublisher.com/index.php/ijmec 128 In Africa, Quaternary climate fluctuations caused the expansion and contraction of lake water systems, which once promoted the convergence and divergence evolution of catfish populations in the Great Lakes of East Africa: when the lake level was high, catfish from different lakes mixed with each other through spillover rivers, and when the lake level was low, they were trapped in isolated water bodies and evolved separately, leaving behind complex genetic lineages. A similar example is the Asian inland river catfish. When the river network changed during the last glacial period, different tributaries may have been temporarily connected, causing catfish species to exchange genes in adjacent basins, resulting in the "most recent shared ancestor" phenomenon currently observed in genetics, that is, the gene lineages of catfish in adjacent basins are mixed rather than completely corresponding to geographical isolation (Watanabe and Nishida, 2003; Fang et al., 2022). 4.3 Genetic exchange driven by human activities Human activities have become a dominant factor affecting catfish gene flow in modern times. On the one hand, river damming and water conservancy projects directly change the connectivity of freshwater ecology. Large dams block the migration channels of migratory fish, limiting or even interrupting the genetic exchange between upstream and downstream populations. For example, the construction of the Ituango Dam on the Magdalena River in Colombia is believed to gradually cause genetic separation between the upstream and downstream populations of the Medellin catfish in the river (García-Castro and Márquez, 2024). It can be seen that the barriers set up by humans on rivers are becoming new isolation mechanisms, exerting differentiation pressure on the originally connected catfish populations. Similarly, habitat fragmentation caused by projects such as river straightening and wetland reclamation will also reduce the migratory reproduction opportunities of catfish and reduce gene flow. On the other hand, the intentional or unintentional introduction and release of species by humans have changed the pattern of catfish gene flow. Globally, catfish are often introduced into new areas as aquaculture and fishery resources due to their strong adaptability and rapid growth. For example, African catfish were introduced to many Asian countries for breeding in the second half of the 20th century and established wild populations in local water bodies. A study in Bangladesh revealed evidence of hybridization between alien African catfish and native walking catfish (Clarias batrachus) (Parvez et al., 2022). This suggests that the invasive African catfish has genetically introgressed with the native walking catfish, resulting in the "dilution" of the gene pool of the original species. Similarly, the introduction of North American catfish into European waters poses a risk of cross-species hybridization. In addition, large-scale seedling release and breed improvement programs are also changing the genetic exchange pattern of catfish. In order to increase production, many farms will crossbreed strains from different regions. 5 Ecological and Evolutionary Significance of Catfish Population Structure and Gene Flow 5.1 Relationship between population differentiation and local adaptation Populations with distinct genetic structure often contain their own unique genetic variation, which may form local adaptation under long-term environmental selection. Populations with limited gene flow are more likely to accumulate allele combinations adapted to the local environment, thereby improving adaptability to specific habitats. For example, geographically isolated populations of Magur catfish live in wetlands with different ecological conditions such as temperature and pH, and it is speculated that they have produced local differences in growth and reproductive cycles. On the contrary, populations with frequent gene flow have their genotype frequencies constantly recombined between different environments, and strong foreign gene mixing may dilute the accumulation of local beneficial mutations, making it difficult to consolidate local adaptation. This may be a disadvantage in the case of rapid environmental differentiation, but in the case of frequent environmental changes, high gene flow can ensure that the population maintains a high genetic diversity. Cubry et al. (2022) pointed out in their review that when there is environmental heterogeneity, limited gene exchange is conducive to directional selection and genetic differentiation of different populations along their respective environmental gradients; however, if there is still moderate gene flow between heterogeneous environments, it will help to increase the speed of adaptation through the introduction of new alleles. Therefore, gene flow and local adaptation are a balanced relationship: a small
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