International Journal of Marine Science, 2025, Vol.15, No.2, 53-64 http://www.aquapublisher.com/index.php/ijms 58 for various reasons, such as differences in spawning timing, behavioral differences, or gametic incompatibility, which reduces the chance of successful fertilization between species. Post-zygotic barriers, such as sterility or low fertility in hybrid offspring, are mainly caused by genomic differences or developmental incompatibilities, which further restrict gene flow. Ecological and genetic factors, including mate choice and the need to adapt to different habitats, can influence these barriers, ultimately causing species to remain separate even if they occasionally hybridize (Rodríguez et al., 2018; Potkamp and Fransen, 2019; Cutter, 2023). 6 Selection and Adaptive Evolution 6.1 Positive selection of genes related to sex determination, vision and immune function Analysis of the grouper genome revealed that many genes related to biological functions are affected by positive selection. In the blue-spotted grouper (Epinephelus coioides cyanopodus), some genes related to immunity, growth, reproduction and even sex determination show characteristics of positive selection, which help this grouper adapt to the complex coral reef environment and develop rapidly. In addition, the expansion of the immune gene family of grouper is higher than that of many other bony fishes, and immune function plays a vital role in grouper's adaptation to environmental pressure and maintenance of diversity (Cao et al., 2022). A similar immune gene expansion phenomenon has also been seen in the brown-spotted grouper (Epinephelus coioides fuscoguttatus), which further supports the role of immune genes in adaptive evolution (Yang et al., 2021). 6.2 Local adaptation to depth, temperature, and salinity Groupers have significant genetic diversity and unique population structure, which is due to the racial isolation of groupers in different marine environments in history. There is a species of grouper (Epinephelus coioides coioides) that has unique genetic characteristics in waters close to the deep sea. Researchers believe that this may be due to population isolation caused by the drop in sea level during the glacial period. These phenomena show that different grouper populations can adapt to different depths, temperatures, salinity and other environments. Their ability to promote phylogenetic diversification can help them effectively cope with climate change (Cao et al., 2014; Chen et al., 2025). After the glaciers retreated, groupers expanded their populations, and this historical time also reflects their adaptation to different regional environments (Cao et al., 2014). 6.3 Detection of selective sweeps and adaptive traits We can find some evolutionary laws of grouper from genetic research on grouper. In the blue-spotted grouper, Cao et al. (2022) found some special genes and expansions of gene families that help adapt to the environment, and they concluded that the genes of the blue-spotted grouper have undergone adaptive changes. In 2021, Yang et al. studied the genes of the brown-spotted grouper and discovered important gene regions that control growth, which are essential for the survival and artificial reproduction of grouper. These genetic data help scientists find important areas and support research on how groupers adapt to the environment and form new species. 6.4 Parallel/convergent evolution between coral reef and deep-sea lineages The diversity of groupers is influenced by ecological and geographical factors. Studies have shown that parallel evolution and convergent evolution exist in lineages inhabiting similar environments. Rapid radiation and adaptive divergence are associated with important geological and climatic events, such as global cooling and crustal movement, which provide repeated opportunities for ecological specialization in coral reef and deep-sea environments (Qu et al., 2014; Qu et al., 2017). Similar ecological traits have repeatedly appeared in different evolutionary branches, suggesting that parallel evolution or convergent evolution has played an important role in shaping the diversity of groupers, especially in response to environmental heterogeneity and biogeographic barriers. 7 Ecological and Behavioral Drivers 7.1 Reproductive biology (hermaphroditism) Groupers have a unique reproductive mode, especially that they are hermaphrodites, that is, they can change from female to male. This reproductive mode is related to their social structure, helping to shape the mating system and population structure, which in turn affects genetic diversity and adaptive evolution, and also promotes the
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