Mini-review and Opinion If everything is everywhere, do they share a common gene pool? Linda K. Medlin Alfred Wegener Institute, Am Handelshafen 12, D27570 Bremerhaven, Germany Received 18 January 2007; received in revised form 10 August 2007; accepted 6 September 2007 Abstract Marine phytoplanktons are highly dispersed with large population sizes and are often considered to be homogenous over their entire range. Thus, using this definition, one would predict that everything is everywhere for these microbes. However, recent molecular analyses have shown both spatial and temporal compartmentalisation in phytoplankton communities, thus calling into question the idea that everything is everywhere, especially if they do not share a global gene pool. Examples are present to document the range of biogeography that has been reported in the phytoplankton and a hypothesis as to how this relates to species evolution on a geological time scale is provided. © 2007 Published by Elsevier B.V. Keywords: Biogeography; Genetic diversity; Gene pools; Phytoplankton The concept that ‘everything is everywhere’ at the microbial level is gaining popularity (Finlay and Clark, 1999; Finlay, 2002) rarely being rigorously tested (Pommier et al., 2007). In the ‘everything is everywhere’ hypothesis, Finlay proposed that organisms below 1 μm have the possibility to be distributed everywhere and the environment will select if they can survive. Although some morpho-species (a species defined by morpho- logical criteria) may be everywhere, their gene pools may or may not be disjunct. Without a global gene pool, ‘everything cannot be everywhere’. In order to falsify this hypothesis, populations of a cosmopolitan species would need to be compared for gene flow between and among population sampled along its entire range. Empirical evidence from microalgae studied at and below the morpho-species level shows that ‘everything is not everywhere’. Although most of these studies have not sampled their species over their entire range, they have been sampled extensively at a local range and populations have been found to be fragmented at a local range and by inference, one would assume that the global population is fragmented. Among the diatoms, genera are very distinct and narrowly defined (Round et al., 1990), and most diatomists would not miss the presence of any genus (Medlin, 1991; Round, 1996). At the Helgoland Time Series Site in the German Bight, a new planktonic genus appeared four years ago, reported elsewhere only from Narragansett Bay, USA thus having a limited global distribution (Kühn et al., 2006). The diatom genus Eunophora is endemic to Australia and New Zealand (Vyverman et al., 1998) and is so distinct morphologically that it is unlikely to have been missed elsewhere. Much of the confusion about the distribution and biogeography of the diatoms stems from using European floras to identify taxa in the southern hemisphere. Thus the appearance of cosmopolitan taxa is an artefact of using inappro- priate floras to identify taxa in very different parts of the world. Mann (2007) has re-examined all of the diatom species reported in Finlay et al. (2002) to be cosmopolitan and found that each one had had part of its cosmopolitan range misidentified, thus refuting their claim to be a cosmopolitan species. Vyverman (2007) presented experimental evidence that there are real limitations to a universal dispersal of diatoms because of dessication intolerances. Among the protists, cryptic species are becoming widely recognised (Knowlton, 1993). Cryptic species can be most easily defined as a complex of morphologically identical or nearly identical strains that be separated into distinct groups genetically and thus, can be termed sibling species. The apparently cosmo- politan diatom, Skeletonema costatum, now contains six semi- cryptic species (Zingone et al., 2005; Sarno et al., 2005) each with Gene xx (2007) xxx – xxx + MODEL GENE-36014; No of Pages 4 www.elsevier.com/locate/gene Abbreviations: rDNA, ribosomal DNA; ITS, internal transcribed spacer regions. E-mail address: lkmedlin@awi-bremerhaven.de. 0378-1119/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.gene.2007.09.011 ARTICLE IN PRESS Please cite this article as: Medlin, L.K., If everything is everywhere, do they share a common gene pool? Gene (2007), doi:10.1016/j.gene.2007.09.011