Geology-based and ecological processes of divergence between and within species of wingless darkling beetles

[Aim]: Discerning the relative role of geographical and ecological factors in promoting diversification is central to our understanding of the origin and maintenance of biodi-versity. We explore the roles of geology and ecological tolerance in the diversification of a group of wingless beetles with low dispersal potential. [Location]: Western Mediterranean (Iberian Peninsula and North Africa).
[Taxon]: Darkling beetles (Tenebrionidae: Misolampus).
[Methods]: We sequenced nine gene fragments from the mitochondrial and nuclear genomes in all extant Misolampus species to reconstruct their phylogeny, evaluate species boundaries and potential contact zones and estimate divergence times. We modelled species distributions for different time periods to infer ecological preferences and assess the effects of climatic changes since the last interglacial. We used a time-stratified process-based biogeographical model to estimate ancestral areas of origin and the evolution of geographical ranges.
[Results]: The palaeoclimatic model projections show contractions of favourable areas during the last interglacial period and mid-Holocene, and wide stretches of suitable areas during the last glacial maximum. Analyses of ancestral bioclimatic preferences reveal ecological adaptations in isolated lineages within three species. The phylog-eny of Misolampus is strongly supported and unveils deep divergences within the six species. Two well-supported clades were recovered, one distributed in North Africa- Balearic Islands and another in the Iberian Peninsula. The divergence between the North African and Iberian clades occurred during the early Miocene. The geographical ranges of species are dynamic, shaped by biotic (e.g. life history, demography, species interactions) and abiotic (e.g. tectonic events, climatic variables) factors that vary through space and time. This variation ultimately dictates differential patterns of population viability, connectivity and growth through time in dif-ferent sections of species ranges, with important consequences for species diversification processes (Bellard et al., 2012; Gouveia et al., 2014; Thuiller et al., 2005; Wiens, 2011). Following climatic changes, species can respond by contracting their ranges into cli-matic refugia and/or dispersing to areas where their environmen-tal preferences are maintained (niche tracking), by adapting to new environmental conditions in situ (niche evolution), and often by undergoing extinction in environmentally unsuitable areas (Kozak & Wiens, 2006; Yesson & Culham, 2006a). The fragmentation of ancestral distributions because of tectonic or climatic changes pro-motes allopatric speciation, sometimes associated with niche evolution, whereas demographic expansion of populations tracking climatic changes can lead to range fusion and admixture between formerly isolated population groups. Discerning the relative role of biotic and abiotic factors in promoting diversification is central to our understanding of the origin and maintenance of biodiversity (Wiens & Graham, 2005).The ability of different taxa to track climatic conditions or evolve new adaptations depends on life-history traits like dispersal. Dispersal limitation can result in geographical isolation and ecolog-ical specialization, by promoting species persistence in small areas, which could lead to some resilience to changing climatic conditions and might accelerate the consolidation of a species' ecological niche even at the population level (Baselga et al., 2011). Usually, local adaptation to extrinsic ecological factors is initiated by the presence of a barrier to gene flow. Additionally, dispersal limitation may lead to ecological specialization and reinforce restrictions to gene flow, promoting speciation even in the face of future demographic pro-cesses such as population contractions, fragmentations or expan-sions (Wiens, 2004).Addressing the question of how much lineage diversification is affected by range expansions/contractions through historical biogeography is often tackled by reconstructing ancestral range shifts using phylogenetic analyses (Ree & Smith, 2008 ; Ronquist & Sanmartín,2011). However, the question often remains partially an-swered because of the lack of information on the timing of range shifts regarding the speciation process itself (Recuero et al., 2012; Recuero & García-París, 2011). Robust, accurate estimates of di-vergence times between species and intraspecific lineages are key to consider evolutionary processes at different temporal scales that lead to diversification, which is controlled by multiple extrin-sic ecological factors and intrinsic organismal traits (Wiens, 2004). Therefore, assessing the temporal scale of demographic processes such as range contractions, fragmentations and expansions and their impact on lineage divergence provides decisive evidence to reconstruct the evolutionary history of species. Within a temperate region such as the Western Mediterranean, the influence of demographic processes under climatic oscillations has been well studied, providing evidence for two major evolutionary patterns. On the one hand, allopatric fragmentation, or demographic contraction into refugial areas during glacial periods have allowed species to persist as isolated populations over long peri-ods of time (Gutiérrez-Rodríguez et al., 2017 ; Hewitt, 2004, 2011; Martínez- Freiría et al.,2020; Martínez-Solano et al., 2006; Recuero & García-París, 2011). On the other hand, during interglacial periods many species have expanded their ranges through newly available colonization routes, sometimes forming secondary contact areas with related species or lineages (Branco et al., 2000; Gómez & Lunt, 2007 ; Gonçalves et al., 2009; Miraldo et al., 2011). These two scenarios involve periods without effective contact or gene flow be-tween isolated populations, which has consequences on the process of lineage divergence. For instance, intraspecific divergence could be either indicative of allopatric cladogenetic events (cladogenesis), or ecological specialization (niche differentiation) (Hewitt, 2011).The Western Mediterranean hunchback darkling beetles Misolampus (Tenebrionidae: Stenochiinae: Misolampinii) are well suited for studies on the contribution of geology-driven processes and ecological divergence to diversification in species groups with limited dispersal capacity. These flightless beetles are ecologically linked to woodlands and live mainly beneath the bark or inside dead logs of Pinus, Quercus and other tree species along a broad altitudinal analyses infer an ancestral range including the Iberian, Betic and Rifean Plates, with subsequent splits followed by dispersal events.
[Main conclusions]: Our results favour a dual role of vicariance and dispersal in driving the historical biogeography and diversification of Misolampus since the early Miocene. We also found evidence for incipient speciation events, underscoring the role of tectonic events and adaptation to local climatic conditions in the diversification of the group.
This study was supported by the Spanish government (MINECO/MIUC/AEI) and the European Fund for Regional Development (FEDER) under grants CGL2015-66571-P (collecting and Museum visits) and PID2019-110243GB-I00/AEI/10.13039/501100011033 (Ministerio de Ciencia, Innovación y Universidades, Spain) (molecular analyses) to MGP. Support of the publication fee was granted by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).