Whether microevolution on small islands differs from that on larger landmasses is a key question in biology with substantial implications for species conservation. However, due to the difficulties faced in producing adequately replicated samples and in controlling for confounding variables, prior attempts to examine evolutionary questions relating to habitat area and population size have produced equivocal results. Here we show, using experimental design criteria that reduce the potential for such confounding, that bird species on larger landmasses have higher rates of molecular evolution. The study involves a global dataset of 48 independent contrasts for the cytochrome b gene encompassing all possible paired sister species comparisons (from seven orders and 17 families) that were available at the time of dataset assembly. A more rapid evolutionary tempo in larger areas has important ramifications for biodiversity conservation because it indicates a new imperative, beyond that of simply maintaining preexisting genetic diversity, for securing large areas for threatened species. This result suggests that the trend of confining species to limited refugia is likely to be slowing the tempo of microevolution. That effect might constrain the potential for adaptive shifts in response to changing environments such as those associated with global warming.