Some of Charles Darwin’s most important observations in formulating his theory of evolution were of the Galapagos finches,

| There is increasing evidence for the existence of unique ecosystems that are dominated by locally adapted microbiota which harbour distinct lineages and biological capabilities, much like the macrobiota of Darwin’s Galapagos Islands. As a primary example of such a system, we highlight key discoveries from the Cuatro Ciénegas basin in Mexico. We argue that high microbial endemism requires a combination of geographical isolation, long-term continuity and mechanisms for reducing the intensity of horizontal gene transfer (HGT). We also propose that strong phosphorus limitation has an important role in microbial diversification by reducing the intensity of HGT. P e r s P e c t i v e s NATuRE REvIEWS | microbiology vOluME 6 | july 2008 | 559 © 2008 Macmillan Publishers Limited. All rights reserved.