Speciation has been considered as a primary process contributing to species diversity, but its contribution to the diversity of local communities has not been fully appreciated. Based on the theory of classic island biogeography, we derived a model for the number of endemic species as a function of the processes of immigration, speciation, and extinction. The model shows that species endemism on an island is proportional to speciation rate but decreases with the sum of immigration and extinction rates (i.e., the species turnover rate). The model predicts that the contribution of immigration to species richness in local communities decreases with time, while the contribution of speciation to local richness increases with time. It further shows that only when the speciation rate is larger than half of the extinction rate can new species added from speciation eventually surpass those added from immigration. We conclude that, although the model leads to an apparent positive relationship between percentage endemism and species diversity on an island, this positive endemics-diversity relationship is not necessarily driven by speciation.