Philosophical Transactions of the Royal Society B: Biological Sciences
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Eukaryotic microbes, species recognition and the geographic limits of species: examples from the kingdom Fungi

John W Taylor

John W Taylor

Department of Plant and Microbial Biology, University of CaliforniaBerkeley, CA 94720-3102, USA

[email protected]

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,
Elizabeth Turner

Elizabeth Turner

Department of Plant and Microbial Biology, University of CaliforniaBerkeley, CA 94720-3102, USA

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,
Jeffrey P Townsend

Jeffrey P Townsend

Department of Ecology and Evolutionary Biology, Yale UniversityNew Haven, CT 06520-8106, USA

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,
Jeremy R Dettman

Jeremy R Dettman

University of TorontoMississauga, Ontario, Canada L5L 1C6

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and
David Jacobson

David Jacobson

Department of Plant and Microbial Biology, University of CaliforniaBerkeley, CA 94720-3102, USA

Stanford UniversityStanford, CA 94305, USA

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Published:https://doi.org/10.1098/rstb.2006.1923

    The claim that eukaryotic micro-organisms have global geographic ranges, constituting a significant departure from the situation with macro-organisms, has been supported by studies of morphological species from protistan kingdoms. Here, we examine this claim by reviewing examples from another kingdom of eukaryotic microbes, the Fungi. We show that inferred geographic range of a fungal species depends upon the method of species recognition. While some fungal species defined by morphology show global geographic ranges, when fungal species are defined by phylogenetic species recognition they are typically shown to harbour several to many endemic species. We advance two non-exclusive reasons to explain the perceived difference between the size of geographic ranges of microscopic and macroscopic eukaryotic species when morphological methods of species recognition are used. These reasons are that microbial organisms generally have fewer morphological characters, and that the rate of morphological change will be slower for organisms with less elaborate development and fewer cells. Both of these reasons result in fewer discriminatory morphological differences between recently diverged lineages. The rate of genetic change, moreover, is similar for both large and small organisms, which helps to explain why phylogenetic species of large and small organisms show a more similar distribution of geographic ranges. As a consequence of the different rates in fungi of genetic and morphological changes, genetic isolation precedes a recognizable morphological change. The final step in speciation, reproductive isolation, also follows genetic isolation and may precede morphological change.

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