International Journal of Molecular Ecology and Conservation, 2025, Vol.15, No.3, 123-133 http://ecoevopublisher.com/index.php/ijmec 129 amount of gene flow may provide raw materials for genetic variation while not destroying the adaptive combination, thereby achieving a "balanced genetic structure". 5.2 Evolutionary consequences of hybridization and gene penetration When previously isolated populations re-contact through gene flow, hybridization will occur, and its eco-evolutionary consequences depend on the degree of hybridization and kinship. For hybridization between different populations within the same species (mixing of gene pools), it can often increase the heterozygosity of offspring and produce hybrid vigor effects. Hybridization can quickly combine beneficial genes from different populations and improve overall adaptability. Therefore, the moderate introduction of foreign genetic components in populations with declining fishing may increase genetic diversity and alleviate inbreeding depression. However, hybridization may also have negative consequences. If the gene flow introduces alleles that are not adapted to the local environment, it may cause offspring to be maladapted, which is the so-called hybrid disadvantage or foreign gene load. In addition, for species that have evolved independently for a long time or deeply differentiated populations, hybridization will cause genetic pollution and threaten the purity of the local gene pool. The study of Parvez et al. (2022) is an example: after the alien African catfish invaded Bangladesh, it hybridized with the local walking catfish in large quantities, resulting in the erosion of the latter's genetic composition, and purebred local catfish became increasingly difficult to identify. In view of this, when there is an invasion of alien catfish, isolation and removal measures should be taken in time to protect the genetic integrity of the local population. 5.3 Speciation and phylogenetic evolution The dynamic balance between gene flow and isolation plays a decisive role in the speciation process of catfish. When a population is isolated from other populations for a long time and the gene flow approaches zero, reproductive isolation may occur after sufficient generations of mutation and selection accumulation, thereby differentiating into new species (Huey et al., 2006; MacGuigan et al., 2022). In rivers where catfish are widely distributed, many "subspecies" or "populations" are actually in the incipient speciation stage. The presence of gene flow often delays the completion of speciation. In a high gene flow environment, even if adaptive differences occur, they may merge again due to hybridization (fusion species phenomenon). On the contrary, once gene flow is blocked, each population will quickly accumulate differences under drift and different selection pressures, and the process of speciation will accelerate. For catfish, watershed separation provides a natural "speciation workshop": many sister species are distributed in adjacent but independent river systems, presumably due to geographical isolation. At the same time, in the same watershed, we also observed a "population-species continuum", such as some Pseudoplatystoma catfish populations with partial reproductive isolation, which is in the intermediate state between subspecific differentiation and speciation. Waldbieser et al. (2023) found that there are multiple inversion differences in the genomes of channel catfish and blue catfish. These differences may have restricted hybrid exchange during the speciation process of the two, and contributed to species isolation at the genetic level. 6 Case study of Genetic Structure and Gene Flow in Catfish 6.1 Population structure of catfish in the Mekong River in Asia The giant catfish (striped catfish) in the Mekong River basin is an important aquaculture and fishing species in Southeast Asia. Vu et al. (2020) used SNP typing to compare the genetic structure of wild and farmed striped catfish in Thailand, Cambodia and Vietnam, and found that the downstream population can be divided into two main genetic groups: the wild population in Ubon Ratchathani, Thailand is a separate group with the highest genetic diversity, while the wild population in Dong Thap Province, Vietnam and Phnom Penh, Cambodia, clusters with the local farmed population in another group, reflecting the limited gene exchange between regions. The study also found that the genetic diversity of the farmed population was significantly lower than that of the wild population, with an average heterogeneity of only about half that of the wild population. This suggests that inbreeding or genetic drift may have occurred in the past breeding process, narrowing the gene pool of the farmed strain. This case highlights the possible hidden genetic differentiation between catfish populations in different
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