<strong>[u]Aim[/u]</strong> The biotic mechanisms by which land bridges influence the formation of regional floras remain poorly understood. We tested the hypothesis that some land bridges have biased the migration of species between landmasses according to their biological traits, and that this relative spatial isolation has caused some lineages to diversify more than others. <strong>[u]Location[/u]</strong> The Strait of Gibraltar Floristic Region, a major biodiversity hotspot of the Mediterranean Basin. <strong>[u]Methods[/u]</strong> We compiled the angiosperm flora of the study region to examine patterns of narrow endemism and species disjunctions between southern Iberia and northern Morocco. We focused on species that occur in the western portion of the Mediterranean Basin (n = 566) but not further east in this region, in order to test for the specific effect of the Strait of Gibraltar. Using phylogenetic supertrees, we tested whether species' life-history traits were related to their probability of occurring on both sides of the strait. We looked at patterns of narrow endemism in different families and computed birth–death model estimates of local diversification within the region. <strong>[u]Results[/u]</strong> Species with a short life cycle and propagules dispersed by wind or externally on animals were disproportionately likely to occur on both sides of the strait. Different plant lineages exhibited disparate distribution and endemism patterns across the strait. Some families have experienced disproportionately high rates of local diversification, and these families were systematically characterized by a low migration rate across the strait. We detected no difference of overall rates of local diversification between the southern Iberian and northern Moroccan parts of the study region. <strong>[u]Main conclusions[/u]</strong> Our results indicate that the Strait of Gibraltar has biased species migration between northern Morocco and southern Iberia and that reduced migration through the strait may have triggered local speciation in certain plant families. This pinpoints the fundamental interplay between species migration and evolutionary diversification in the construction of hotspots of biodiversity and narrow endemism.< Réduire