Towards a comprehensive view on evolutionary refugia in West African rainforests

West African rainforests are amongst the world’s premier biodiversity hotspots. Over time, the size and distribution of these rainforests have changed significantly due to fluctuations in climate. However, compared to other biodiversity hotspots, our understanding of potential West African rainforest refugia remains relatively limited. Studies from different disciplines have provided valuable insights into refugia location, age and evolutionary role. Fossil pollen data and comparative biogeography studies have revealed cycles of rainforest contraction and expansion linked to aridification and have used these patterns to identify large-scale refugia. Phylogeographic studies mostly corroborated the existence and locality of large-scale refugia, but also unearthed the putative presence of microrefugia; smaller isolated regions that jointly form larger refuge zones (macrorefugia). Moreover, some refugia seem to harbour deep evolutionary lineages, potentially reflecting long-term stability, whereas others may have been stable during more recent aridification cycles. Phylogeographic splits are congruent across species, but asynchronous and frequently align with both climatically unstable regions and landscape features, such as rivers. These temporal and biogeographic aspects have been further explored using demographic and Species Distribution Modelling (SDM). Across various rainforest taxa, these methods show that populations and suitable habitat retracted and expanded, promoting macro-evolutionary change. Climatic fluctuations thus appear to have played an important role in shaping West African biodiversity. Overall, research shifted from identifying refugia to quantifying their role in driving evolutionary change. However, the scarcity of studies linking detailed changes in demography with SDM limits our ability to make general statements regarding refugia dynamics, predict species response to climate change and prioritise future climate refugia. Palaeoclimatic cycles of aridification led to the fragmentation and contraction of West African rainforests. Several refugia (e.g. Mount Nimba, Cape Three Points, Cape Palmas and Niger Delta) have been repeatedly identified using pollen and biogeographic data, but phylogeographic analyses point to the presence of microrefugia within and beyond these regions. Some microrefugia may have sustained species persistence since the Miocene (i.e. Fouta Djallon and Ghana), while others supported species persistence during more recent aridification cycles (Pleistocene), suggesting refugia may have operated at different time-scales. Refugia are not the sole driver of diversification, but played a key role in driving diversification of rainforest-dependent species in combination with other factors like river barriers. Future research would benefit from investigating a broader range of taxa, exploring lesser-known regions (e.g. Fouta Djallon or Niger Delta) and adopting landscape demography approaches to achieve a comprehensive understanding refugia dynamics in varying environments.