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- Contrasting evolutionary histories from the warm Atlantic to the cold Pacific - overlooked seagrass diversityPublication . Tavares, Ana Isabel Magalhães; Serrão, Ester; Pearson, Gareth; Nakahoka, MasahiroMarine connectivity and genetic differentiation among populations and species across different sides of oceans are hypothesized to be driven by contrasting processes in tropical versus cold and temperate regions. Tropical species occupy restricted warm ocean ranges where patterns of genetic differentiation might be driven more by oceanic connectivity and spatial habitat continuity and stepping stone processes. In contrast, the genetic structure across the ranges of species occupying cold and temperate habitats might be more influenced by the past climate-driven range shifts that led to demographic effects (e.g., bottlenecks, founder events, local extinctions, and expansions) with strong consequences for evolution and extant genetic diversity. There is well supported evidence for the role of Quaternary ice ages that had crucial effects on present distributions of species genetic diversity. Recurrent glacial-interglacial cycles led to a series of events shaping the phylogeographic patterns of marine species, leaving evidence that the present geographical genetic structure of northern hemisphere species has been strongly affected by historical vicariance. Seagrasses of the genera Halodule and Phyllospadix are distributed along coastal shores on both sides of the tropical Atlantic and temperate Pacific, respectively. They are therefore excellent models to study the impact of past and future climatic fluctuations and physical oceanographic connectivity on the patterns of genetic diversity and differentiation along the distributional ranges of both tropical and temperate regions. Additionally, these species they offer valuable insights into past range shifts and contemporary connectivity dynamics. This thesis aims to understand the population genetic history of seagrasses in biogeographical settings with contrasting oceanographic and seascape characteristics.