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.