International Journal of Aquaculture, 2024, Vol.14, No.3, 126-138 http://www.aquapublisher.com/index.php/ija 132 5 Evolutionary Mechanisms Underlying Aquatic Biodiversity 5.1 Speciation processes Speciation, the process by which new species arise, is a fundamental mechanism driving aquatic biodiversity. In freshwater environments, speciation can be influenced by various factors, including geographic isolation and ecological differentiation. For instance, the parallel speciation of freshwater forms from ancestral amphidromous forms in Rhinogobius goby fish across the Ryukyu Archipelago, highlighting the role of ecosystem size in predicting speciation probability (Prates and Singhal, 2020). Additionally, the Amazon basin's diverse ecological conditions, such as differences in water types, can drive speciation through local adaptations in sensory systems, which affect interbreeding chances between populations (Borghezan et al., 2021). 5.2 Adaptive radiation Adaptive radiation, where a single ancestral species rapidly diversifies into multiple new species, each adapted to a different ecological niche, is another key mechanism. The Amazon's high freshwater organism richness is a prime example, where distinct water types (black, white, and clear) create varied ecological niches that drive the evolution of sensory systems and contribute to fish diversity (Borghezan et al., 2021). Similarly, the historical biogeography of cycads, which expanded from Laurasian origins to Gondwana, showcases how adaptive radiation can lead to significant biodiversity through vicariance and niche differentiation over geological timescales (Coiro et al., 2023). 5.3 Environmental and ecological drivers Environmental and ecological factors play crucial roles in shaping aquatic biodiversity. Temperature, for instance, has been identified as a significant driver of fungal community composition in freshwater streams, with thermal preferences influencing biodiversity patterns along latitudinal gradients (Seena et al., 2019). In marine environments, environmental conditions such as temperature and anthropogenic stressors like pollution and fishing pressure have been shown to influence the biogeographic patterns of metazoans, protists, and bacteria (Holman et al., 2021). Furthermore, in Lake Lugu, water depth and associated abiotic variables significantly affect the biomass, species richness, and community composition of bacteria, diatoms, and chironomids, demonstrating the importance of both abiotic and biotic interactions in structuring aquatic communities (Zhao et al., 2019). In summary, the evolutionary mechanisms underlying aquatic biodiversity are multifaceted, involving speciation processes driven by geographic and ecological factors, adaptive radiation in response to diverse ecological niches, and environmental drivers that shape community composition and species richness across different aquatic habitats. Understanding these mechanisms is crucial for conserving aquatic biodiversity in the face of ongoing environmental changes. 6 Fossil Record and Molecular Data 6.1 Contributions of the fossil record The fossil record provides critical insights into the origins and diversification of aquatic biodiversity. Fossil evidence reveals the historical biogeography and phylogenetic patterns of various aquatic species, offering a temporal framework to understand how current biodiversity patterns have evolved (Figure 2). For instance, the study of tropical reef fishes highlights the importance of fossil data in identifying areas of species creation and demise, particularly in biodiversity hotspots like the Indo-Australian Archipelago (IAA) (Cowman et al., 2017). Fossils help trace the lineage of species, revealing the historical processes that have shaped current biodiversity distributions. This historical perspective is essential for understanding the long-term dynamics of species richness and endemism in marine environments. Cohuo et al. (2020) investigates the fossil records and species distribution models of four ostracode species from Lake Petén Itzá, focusing on periods spanning the last interglacial and glacial epochs. The analysis reveals that these species, particularly Cypria petenensis, exhibited varying abundance and relative frequencies, with notable occurrences aligning with significant climatic events. The complementary patterns observed between cores PI-2
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