Abstract
The Eastern Gulf Coastal Plain (EGCP) is a small region within the North American Coastal Plain (NACP), a recognised biodiversity hotspot. The EGCP has been described as ‘one of the most important hotspots of speciation and endemism’ in the United States, yet remains an anomaly relative to other regions with high biodiversity and endemism. In contrast to most biodiversity hotspots, the EGCP is homogeneous in elevation, coastal and geologically young. We here provide a novel synthesis of the origins and diversification of the EGCP biota. We review published phylogenies of range‐restricted endemic vascular plant species in the EGCP to catalogue the geographic locations of their closest relatives. We find the EGCP to represent a crossroads where lineages from diverse locations have converged and, in some cases, diversified. To better understand mechanisms leading to species diversity, we review fine‐scale phylogeographic and population genetic analyses across crown eukaryotes in the EGCP and create ecological niche models of overlapping sister species. We find that geographic barriers to dispersal (often rivers) have likely led to high species diversity in this region due to repeated bisection of the EGCP in times of cyclical inundation (e.g., following glaciation). This work has implications for understanding the formation of biodiversity hotspots at fine scales more generally and acts as a regional test case of an anomalous hotspot in a coastal lowland, showing that such areas can be as crucial for diversification and species richness as mountains or islands.