Introduction
The distribution of animals across the World has been studied since the nineteenth century, and one of the first results of global biogeographic analyses was the division of the Earth into zoogeographic realms (Wallace, 1876). However, such studies were conducted primarily on large, easily observable organisms, mainly vertebrates (Sclater, 1858). In contrast, for microscopic forms of life, the ‘Everything is Everywhere but environment selects’ hypothesis (EiE) was postulated (Baas-Becking, 1934; Fenchel & Finlay, 2004; Foissner, 2006): because of their small size, <1 mm, such organisms were believed to have unlimited long distance dispersal (LDD) abilities, and therefore they should not show any biogeographical patterns, dwelling wherever the environment is suitable. This hypothesis was argued to be especially applicable to groups with dormant stages, which can be easily dispersed over long distances (Fontaneto, Barraclough, Chen, Ricci, & Herniou, 2008; Incagnone, Marrone, Barone, Robba, & Naselli-Flores, 2015). Earlier studies, which were based on morphological characters, seemed to confirm the EiE hypothesis across different taxonomic groups (e.g. Fenchel, Esteban, & Finlay, 1997; Martiny et al., 2006; Heino et al., 2010). However, with the development of molecular tools, many species of microscopic animals that were previously thought to be cosmopolitan, were demonstrated to actually be species complexes (Fontaneto, Kaya, Herniou, & Barraclough, 2009; Cesari, Bertolani, Rebecchi, & Guidetti, 2009), with respective species that often exhibit limited geographic ranges. Therefore, recently, evidence against the EiE hypothesis in microscopic animals started to accumulate (e.g. Fontaneto et al. 2008; Baltanás & Danielopol, 2013; Garraffoni & Balsamo, 2017; Worsaae, Kerbl, Vang, & Gonzalez, 2019; Gąsiorek, Vončina, Zając, & Michalczyk, in review), undermining the hypothesis or at least its universal application to all micrometazoan species.
One of the groups of microscopic animals (up to ca. 1 mm) that have the ability to withstand hash environmental conditions are limnoterrestrial tardigrades (e.g. Hengherr & Schill, 2018). These ubiquitous invertebrates inhabit a wide variety of environments across the planet (e.g. Nelson, Bartels, & Guil, 2018). Many tardigrade species have been considered cosmopolitan, but the knowledge on the distribution of particular species is usually extremely fragmentary (Gąsiorek et al., 2019b). In recent years, the discussion on whether tardigrades do support or undermine the EiE hypothesis has been debated (e.g. Pilato & Binda, 2001; Guil, 2011), but only a limited number of geographically and/or taxonomically restricted studies on this topic have been conducted so far (e.g. Jørgensen, Møbjerg, & Kristensen, 2007; Guil Sanchez-Moreno, & Machordom, 2009; Cesari, McInnes, Bertolani, Rebecchi, & Guidetti, 2016; Morek, Stec, Gąsiorek, Surmacz, & Michalczyk, 2019a; Gąsiorek et al. 2019b; Gąsiorek, Vončina, Degma, & Michalczyk, 2020; Morek & Michalczyk 2020). Nevertheless, these initial studies already indicate that many species may not be as widely distributed as previously assumed. Importantly, in recent years, many species complexes and pseudocryptic species have been detected (e.g. Bertolani, Rebecchi, Giovannini, & Cesari, 2011; Stec, Morek, Gąsiorek, & Michalczyk, 2018; Morek et al., 2019a), further questioning the putative cosmopolitan distribution of tardigrade species. Moreover, the impact of geological events on the distribution of these animals has been hypothesised to be important (McInnes & Pugh, 1998; Guidetti, McInnes, Cesari, Rebecchi, & Rota-Stabelli, 2017), which could not be the case if tardigrade species were generally cosmopolitan. In parallel, anthropogenic dispersal has to be taken into consideration, as it is likely to obscure the natural distribution of tardigrade species (Gąsiorek Vončina, & Michalczyk, 2019a; Morek et al. 2019b) most likely by artificially broadening geographic ranges of at least some species, which may provide false evidence in favour of the EiE hypothesis.
One of the tardigrade genera with a long history of biogeographic records is Milnesium Doyère, 1840. Importantly, the genus was erroneously considered monotypic for many decades and, as a consequence, the type species, Milnesium tardigradum Doyère, 1840, was reported from numerous localities throughout the globe. Therefore,M. tardigradum was regarded cosmopolitan for over 170 years, but this view has been challenged by its integrative redescription by Michalczyk, Wełnicz, Frohme, & Kaczmarek (2012ab) and further questioned by Morek et al. (2019b). Moreover, the most recent phylogenetic analysis of the genus Milnesium (Morek & Michalczyk, 2020), despite the limited number of analysed populations (34) and species (25), suggested that the dispersal capability ofMilnesium species is limited and that the geographic origin of species is a better predictor of a phylogenetic position of any givenMilnesium species than the morphological characters traditionally used in the apochelan taxonomy.
Importantly, however, with small sample size, the detection of the same species in multiple localities is not likely, especially in species-rich genera in which species are not common/abundant. Thus, even if species are widespread and their geographic distributions conform to the EiE hypothesis, analyses based on small sample size may produce artefactual patterns. In other words, the correlation of geography with phylogeny reported by Morek & Michalczyk (2020) could be an artefact, resulting from undersampling, falsely suggesting low dispersal abilities ofMilnesium species and prematurely rejecting the EiE hypothesis to explain the geographic distribution of species within this genus. Therefore, to verify the biogeographic conclusions stemming from Morek & Michalczyk (2020), we considerably enlarged the phylogeneticMilnesium dataset (from 34 in Morek & Michalczyk, 2020 to 127 populations herein), originating from nine zoogeographic realms (compared to six in Morek & Michalczyk, 2020). Moreover, in order to understand what geological or historical events may have shaped the diversification and distribution of Milnesium , we dated our phylogeny using molecular clock analysis for the first time in Eutardigrada Richters, 1926.