within nominal species. We do not believe there is yet any convincing evidence
for a biogeography of such variation in asexual protists. In the case of bacteria
too, similar or identical genotypes occur worldwide (Glo ̈ckneret al., 2000;
Hagstro ̈met al., 2000, Massanaet al., 2000). At the present time, the findings of
identical rDNA genotypes in specific (and similar) habitat types worldwide is
strong evidence supporting cosmopolitan distribution of microbial eukaryotes.‘Biogeography’ of microbes?
Some recent studies indicate that unicellular organisms may demonstrate ‘bio-
geography’. We do not subscribe to the view that microbial eukaryotes have
‘biogeography’ in the classical sense of, for example, geographically restricted
tropical birds and mammals. It is unquestionably true that the distributions of
microbes may show some structure and patterns (Greenet al., 2004; Horner-
Devineet al., 2004; Noguezet al., 2005) that vary over time, and may demonstrate
‘at least some geographical differentiation’ (Greenet al., 2004), but this is rather
weak evidence for biogeography when compared with the stunning geograph-
ical patterns demonstrated by macroscopic organisms with real biogeography
in established biogeographic regions. We resort to the key characteristic that
free-living microbes thrive wherever their preferred habitat requirements are
realized. Interestingly, Horner-Devineet al.(2004) provide evidence that this is
so, using their example of bacteria. They discovered that habitats that are
similar in environmental characteristics, are also similar in bacterial composi-
tion (i.e. even specific bacteria thrive wherever their habitat requirements are
realized). On the other hand, if a microbial habitat becomes so degraded that it
can no longer sustain a particular microbial community, the pattern of ‘bio-
geography’ may simply be ephemeral.General validity
Finlay and Fenchel (2004) revealed a cosmopolitan-biogeography transition in
the region 1–10mm for aquatic microbial eukaryotes, but is it in the right place?
The data set used was the biggest available, but additional data, particularly for
the 1–10mm size range, would be especially useful. Unfortunately, the absence
of any centralized, accessible source of data makes it extremely laborious to
collect the information that would indicate the extent of geographical distribu-
tion of individual taxa.
The two study sites used in the investigations by Finlay and Fenchel (2004)–
Niva^88 in Denmark and Priest Pot in the UK – are situated at roughly the same
latitude in the northern hemisphere, and it is possible that the macrofauna
would present a different picture if the field sites had been placed elsewhere,
such as an area with a large number of endemic species. The existence of a large
pool of undiscovered small tropical organisms cannot be ruled out, but the fact
that Finlay and Fenchel (2004) found a substantial fraction of the global180 B. J. FINLAY AND G. F. ESTEBAN