Phylogeography and the ‘Everything is Everywhere’ hypothesis
In order to test the EiE hypothesis, samples from distant geographic
locations that exhibit similar climate conditions have to be collected
and analysed using phylogenetic methods. Specifically, the EiE
hypothesis can be tested in two non-exclusive ways: (i ) by the
comparison of geographic localities of populations representing
individual species; and (ii ) by the analysis of phylogenetic
relationships of species collected in different zoogeographic realms,
but in localities characterised by similar climate conditions. In the
first case, multiple populations representing the same species are
required, whereas in the second analysis species may be represented by
single populations. Correspondingly, the EiE hypothesis predicts that:
(i ) populations of the same species will be found in
geographically distant and isolated localities, provided that the
localities are characterised by similar climate conditions; and
(ii ) closely related species will likely differ in geographic
localities in which they were collected (i.e. clades will be
geographically diversified). If, however, geographic distributions ofMilnesium species are not subject to the EiE hypothesis, then:
(i ) populations representing the same species will be found
exclusively in a single zoogeographic realm; and (ii ) groups of
closely related species will share the same zoogeographic zone but not
necessarily the same climate zone.
Given that our samples came from nine different zoogeographic realms,
with many localities similar to each other in terms of climate
conditions, we were able to test the EiE hypothesis by the two ways
described above. Specifically, we mapped the zoogeographic origin at the
level of the zoogeographic realm as well as climate type, of analysed
populations onto the molecular phylogeny. Moreover, we utilised RASP 4
(Yu, Blair, & He, 2020) to statistically determine the area of origin,
as well as the climate type for the main clades. The analysis was
carried out on the consensus tree obtained with the BEAST (see Results)
with the Bayesian Binary MCMC using the default settings. Zoographic
realms were specified according to Holt et al. (2013), whereas main
climate categories follow Peel, Finlayson, & McMahon (2007). The RASP
analysis provided information on the geographic origin of the major
clades, which allowed for an identification of ‘inclusion species’, i.e.
species that were found in a different zoogeographic realm than the
majority of species in a given clade (such species suggest ancient LDD).
In parallel, a comparison of the geographic origin of multiple
populations representing a single species allowed for an identification
of recent LDD if such populations were found in more than one
zoogeographic realm.
As the mode of reproduction was hypothesised to influence the dispersal
abilities (Maynard Smith, 1978), this information (dioecy or
parthenogenesis) was also mapped on the phylogenetic tree. The mode was
tested for with PCM and individual culturing whenever possible (Rebecchi
& Nelson, 1998); mode of reproduction was classified as unknown if
culturing was not possible and if a population comprised only females
but there were below ten adult specimens available, as in such a case
the probability of not detecting males was too high to identify
parthenogenesis. Although this does not allow for the distinction
between facultative and obligatory parthenogenesis was, even facultative
parthenogenesis should increase dispersal potential, thus it should be
taken into consideration when investigating biogeography.