INTRODUCTION

Biogeography, or the spatial distribution of biological diversity, has been studied since Darwin and Wallace in the 1800s. Their studies, and most later studies, concentrated on macroscopic organisms, mostly animals and plants, and many differences between species were observed, often correlated with geographical isolation. The theoretical basis for these differences was established later and is still being refined. The theories of island biogeography have been extremely influential in many fields of biology (Bell et al., 2005). Critical to biogeographical studies are comparable organisms from different locations with quantifiable diversity, often sequence diversity.

Microbial biogeography

More recently, micro-organisms have been studied (Finlay, 2002), especially with the advent of molecular tools. Studies using enrichment cultures indicated that identical micro-organisms were present wherever they were collected (Smith et al., 1991); however, this is clearly biased due to the relatively small number of micro-organisms that can be cultivated (Pace, 1997). The advent of small-subunit (SSU) rRNA gene sequence analysis indicated that ‘everything is everywhere’, particularly for spore-forming bacteria (Roberts & Cohan, 1995). It was unclear whether this indicated that there was so much dispersal of these spore-forming organisms that they were identical throughout the world or whether it was general for bacteria due to their extremely large population sizes. For the most part, however, only one gene, generally the SSU rRNA gene, was investigated. Extremophiles are thought to have more barriers to dispersal than mesophilic organisms.