Do most microscopic organisms have a cosmopolitan distribution, if so, why?

I have had a compound microscopic for a little over a year and many species I end up finding have a cosmopolitan distribution, why is this? I’m sorry if this is a stupid question.

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Did no-one ever tell you that there is no such thing as a stupid question, only stupid answers.

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Or stupid people that don’t ever ask questions.

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Yup, that’s right.

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I’ve also wondered that too, but no idea @blastcat

These papers are worth reading as they directly addresses this issue:

There are quite a few papers on this subject, and evidence of strong endemism in certain places:

The debate over cosmopolitanism seems to be very much dependent on what species definition you’re using, which is itself an enormously complicated and non-inuttive subject. The old schoolboy ‘biological species concept’ (ie. if it can produce fertile offspring it’s the same species) is still taught as though it’s the ‘rule’, but it has largely been abandoned in scientific circles due to having far too many exceptions, and recognizing that it doesn’t really apply to asexually reproducing organisms, a strategy employed by many microbiota.

This issue is compounded by the increasing use of genetic studies that reveal differences that are completely invisible to the naked eye, resulting in a vast amount of ‘cryptic’ biodiversity.

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here is a book …
Biogeography of Microscopic Organisms (cambridge.org)

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I hope that genetic approach will bust this myth about widespread cosmopolitan species as it happened in small flatworm Giratrix hermaphroditus → Is ‘everything everywhere’? Unprecedented cryptic diversity in the cosmopolitan flatworm Gyratrix hermaphroditus https://onlinelibrary.wiley.com/doi/10.1111/zsc.12507

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Thanks, Earthknight, Cooperj, and Sumie-dh for providing us a starting point on learning more about this interesting topic.

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Yes, thank you!

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