Assessing the role of aridity-induced vicariance and ecological divergence in species diversification in North-West Africa using Agama lizards

Species diversification often results from divergent evolution of ecological or social signaling traits. Theoretically, a combination of the two may promote speciation, however, empirical examples studying how social signal and ecological divergence might be involved in diversification are rare in general and typically do not consider range overlap as a contributing factor. We show that ecologically distinct lineages within the Australian sand dragon species complex (including Ctenophorus maculatus, Ctenophorus fordi, and Ctenophorus femoralis) have diversified recently, diverging in ecologically relevant and social signaling phenotypic traits as arid habitats expanded and differentiated. Diversification has resulted in repeated and independent invasion of distinct habitat types, driving convergent evolution of similar phenotypes. Our results suggest that parapatry facilitates diversification in visual signals through reinforcement as a hybridization-avoidance mechanism. We show that particularly striking variation in visual social signaling traits is better explained by the extent of lineage parapatry relative to ecological or phylogenetic divergence, suggesting that these traits reinforce divergence among lineages initiated by ecologically adaptive evolution. This study provides a rare empirical example of a repeated, intricate relationship between ecological and social signal evolution during diversification driven by ecological divergence and the evolution of new habitats, thereby supporting emergent theories regarding the importance of both ecological and social trait evolution throughout speciation.

At the crossroads of two biodiversity hotspots; the biogeographic patterns of Shimba Hills, Kenya Beryl A. Bwong The Shimba Hills of Kenya (SHK) is geographically located at the cross roads of two major biodiversity hotspots; the Coastal Forests of Eastern Africa (CFEA) and the Eastern Afromontane Biodiversity Region, specifically the neighbouring Eastern Arc Mountains (EAM). Results from old and recent collections of its flora and fauna indicate that Shimba Hills harbour species associated with EAM and CFEA as well as taxa that have affinity with West African Guineo-Congolian forest. However, the link between SHK and these biodiversity hotspots has never been tested appropriately using phylogenetic approaches. Using 16S mtDNA, demographic analysis and species distribution modelling, I sought to understand the phylogeographic affiliation of Shimba Hills with the neighbouring CFEA and EAM using its amphibian assemblage. Three main questions were explored, namely: a) Which are the closest relatives of SHK amphibian populations? b) Do amphibian species currently occurring in SHK have similar phylogeographic patterns to each other? c) Which historical processes, if any, account for the observed patterns of genetic diversity? I found that SHK and indeed the entire study area have a complex biogeographic history and no single pattern can explain the current amphibian assemblage in the area. Shimba Hills are more closely affiliated to the CFEA than to the EAM. Two previously undocumented putative phylogeographic breaks are recovered from the study area; one from the Kenya north coast and another in the Tanga region in Tanzania. Historical habitat stability and connectivity appear to play a significant role in species diversification in the area. Additionally, I also report on some fundamental findings on Shimba Hills amphibians during this study; Using a combination of molecular, morphological, spatial and bioacoustics methods the taxonomic status of the only endemic amphibian from Shimba Hills, Hyperolius rubrovermiculatus, is confirmed and description of a new species from the north eastern Tanzania is proposed. Secondly the taxonomic status of a Callulina rediscovered in Shimba Hills after 50 years is confirmed and I also propose the description of three new species of Callulina from the neighbouring Eastern Arc Mountains in Tanzania. Finally, I took the opportunity to compile the first ever annotated checklist of amphibians of Shimba Hills National Reserve where a new country record for the Ribbon Caecilian (Scolecomorphus vittatus) and other interesting discoveries are discussed. The reserve plus the entire SHK area contains the highest number of amphibian diversity for any known locality in Kenya. Therefore its continued conservation will ensure about 30% of Kenya’s amphibian species are preserved. Key words: Coastal forests, Eastern Arc Mountains, Phylogeography, species distribution modelling, checklist, Amphibians.

Geographical variation in species richness in plant groups is determined by the interplay between historical, evolutionary, and ecological processes. However, the processes underlying the striking disparity in species richness between Asia and the Americas remain poorly understood. Here, we synthesize global phylogenetic and macroecological data on the diversification of Smilacaceae, deciphering potential drivers underlying the species diversity pattern biased toward Asia. We compiled global distributions of all Smilacaceae species, and reconstructed the biogeographic history and niche evolution using a new time‐calibrated phylogeny (eight genes, 135 species). Integrating these data sets, we estimated evolutionary histories and diversification rates for each region, and tested correlations among species diversification, niche evolution, and niche divergence. Smilacaceae probably originated during the Late Cretaceous/Early Palaeocene and began to diversify in middle to low latitudes in Central America and Eurasia during the Late Eocene. Both the Old and New World clades exhibited a steady, albeit slight, increase of species diversification from the Late Eocene to Early Miocene. However, the Old World clade experienced an abrupt increase in net diversification during the Late Miocene. Our findings also revealed that species diversification rates were positively correlated with ecological niche evolution and niche divergence. Niche shifts and climatic niche evolution since the Middle Miocene played crucial roles in species diversification dynamics within Smilacaceae. The high plant richness in Asia may be explained by greater diversification in this region, potentially promoted by heterogeneous environments.

Variation in species richness across regions and between different groups of organisms is a major feature of evolution. Several factors have been proposed to explain these differences, including heterogeneity in the rates of species diversification and the age of clades. It has been frequently assumed that rapid rates of diversification are coupled to high rates of ecological and morphological evolution, leading to a prediction that remains poorly explored for most species: the positive association between ecological niche divergence, morphological evolution and species diversification. We combined a time-calibrated phylogeny with distribution, ecological and body size data for scaly tree ferns (Cyatheaceae) to test whether rates of species diversification are predicted by the rates at which clades have evolved distinct ecological niches and body sizes. We found that rates of species diversification are positively correlated with rates of ecological and morphological evolution, with rapidly diversifying clades also showing rapidly evolving ecological niches and body sizes. Our results show that rapid diversification of scaly tree ferns is associated with the evolution of species with comparable morphologies that diversified into similar, yet distinct, environments. This suggests parallel evolutionary pathways opening in different tropical regions whenever ecological and geographical opportunities arise. Accordingly, rates of ecological niche and body size evolution are relevant to explain the current patterns of species richness in this 'ancient' fern lineage across the tropics.

Natural selection is often assumed to drive parallel functional diversification of the sexes. But males and females exhibit fundamental differences in their biology, and it remains largely unknown how sex differences affect macroevolutionary patterns. On microevolutionary scales, we understand how natural and sexual selection interact to give rise to sex-specific evolution during phenotypic diversification and speciation. Here we show that ignoring sex-specific patterns of functional trait evolution misrepresents the macroevolutionary adaptive landscape and evolutionary rates for 112 species of live-bearing fishes(Poeciliidae). Males and females of the same species evolve in different adaptive landscapes. Major axes of female morphology were correlated with environmental variables but not reproductive investment, while male morphological variation was primarily associated with sexual selection. Despite the importance of both natural and sexual selection in shaping sex-specific phenotypic diversification, species diversification was overwhelmingly associated with ecological divergence. Hence, the inter-predictability of mechanisms of phenotypic and species diversification may be limited in many systems. These results underscore the importance of explicitly addressing sex-specific diversification in empirical and theoretical frameworks of evolutionary radiations to elucidate the roles of different sources of selection and constraint.

If you own a birdbath, chances are you’re hosting one of evolutionary biology’s most puzzling enigmas: bdelloid rotifers. These microscopic invertebrates—widely distributed in mosses, creeks, ponds, and other freshwater repositories—abandoned sex perhaps 100 million years ago, yet have apparently diverged into nearly 400 species. Bdelloids (the “b” is silent) reproduce through parthenogenesis, which generates offspring with essentially the same genome as their mother from unfertilized eggs. Biologists have yet to find males, hermaphrodites, or any trace of meiosis—the process that creates sex cells—challenging the long-held assumption that evolutionary success requires genetic exchange. The genetic variation created by meiosis and fertilization, theory holds, bolsters a species’s capacity to weather shifting environmental conditions or resist rapidly evolving parasites. (During meiosis, the genome splits in two, and chromosome pairs swap bits of their DNA; during fertilization, the sex cells fuse to restore the complete genome.) Many multicellular eukaryotes pass through a sexual and asexual phase in their life cycle. But eschewing sex altogether, a la bdelloids, is not theoretically consistent with a long-lived evolutionary life span or extensive species diversification. In a new study, Diego Fontaneto, Timothy Barraclough, and colleagues developed new statistical techniques for combined molecular and morphological analyses of rotifers to test the notion that species diversification requires sex. The researchers show that, despite an ancient aversion for interbreeding, bdelloids display evolutionary patterns similar to those seen in sexually reproducing taxa. How they have avoided the pitfalls of a lifestyle widely regarded as evolutionary suicide remains an open question. Bdelloids have remained such an enduring enigma in part because biologists are still debating whether species exist as true evolutionary entities. And if they do, what forces determine how they diverge? Traditional taxonomy relies on morphological differences to classify species, but it can’t distinguish whether such differences reflect physical variations among a group of clones or adaptations among independently evolving populations. In the traditional view of species diversification, interbreeding promotes cohesion within a population—maintaining the species—and barriers to interbreeding (called reproduction isolation) promote species divergence. With no interbreeding to maintain cohesion, the thinking goes, asexual taxa might not diversify into distinct species. Fontaneto et al. defined species as independently evolving, distinct populations (or units of diversity) subject to distinct evolutionary mechanisms. They predicted that if factors other than interbreeding—such as niche specialization—controlled species cohesion and divergence, then asexual taxa should diverge along the same lines as sexually reproducing organisms. And if this were the case, they would expect to find genetic and morphological cohesion within independently evolving populations and divergence between them. To detect independently evolving populations, the researchers analyzed marker genes isolated from clones of bdelloids collected from diverse habitats around the world. They constructed evolutionary trees using both mitochondrial and nuclear DNA sequences (the molecular “barcode” cox1and 28S ribosomal DNA sequences, respectively) to identify species within the samples. For the morphological analysis, they measured the size and shape of the rotifers’ jaws (called trophi). The morphological results largely fell in line with traditional taxonomic classifications for most bdelloid species. And species identified as related on the DNA trees typically had similar morphology. The correspondence between the molecular and morphological results suggests that the majority of traditionally identified bdelloid species are what’s known as monophyletic—individuals in the same species assort together on the evolutionary tree and share a common ancestor. Only two of these traditional, monophyletic species showed significant variation in trophi size or shape among the populations; both also showed significant divergence in the DNA trees. Using statistical models to determine the likely origin of the observed DNA tree branching patterns, the researchers show that these distinct monophyletic genetic clusters represent independently evolving entities (rather than variations within a single asexual population). But what caused them to evolve independently? Are they geographically isolated populations that evolved under neutral selection, or did they evolve into ecologically discrete species as a result of divergent selection pressures on trophi morphology? If bdelloids have experienced divergent selection, the researchers explain, they would expect to see high variation in trophi traits between species, and low intraspecies variation (compared to neutral changes). And that’s what they found—bdelloids have experienced divergent selection on trophi size (and to a lesser degree, on trophi shape) at the species level. Altogether, these results show that the asexual bdelloids have indeed experienced divergent selection on feeding morphology, most likely as they adapted to different food sources found in different niches. By showing that asexual organisms have diverged into “independently evolving and distinct entities,” the researchers argue, this study “refutes the idea that sex is necessary for diversification into evolutionary species.” They hope others use their approach to study mechanisms underlying species divergence in sexual taxa to clarify the hazy nature of species and biological diversity.

Adaptive radiation is a fundamental driver in the creation of biodiversity, but the processes underlying radiations across broad spatial scales need to be further explored. Organisms that colonize a heterogeneous landscape can occupy a mosaic of environments, exposing them to complex patterns of genetic drift, natural selection and gene flow creating adaptive divergence at different levels of biological organization. Strong differences in natural selection and reduced migration between populations favours the evolution of ecotypes, which can provide the basis for species diversification when reproductive isolation arises between contrasting ecotypes. Identifying how ecological divergence can lead to the evolution of different forms of reproductive isolation and promote species diversification can reveal how adaptive radiation proceeds. Empirical studies linking patterns of adaptive divergence with phenotypic diversification, and the underlying genetic basis, are rare across expansive landscapes. As a consequence, there exists a gap in our understanding of how diversification proceeds during adaptive radiation across heterogeneous landscapes. Research for my dissertation used a combination of extensive reciprocal transplant experiments, field sampling, common garden experiments and quantitative genetic crossing designs to investigate how the ecotypic diversification of an Australian native wildflower species complex, Senecio lautus, has occurred across a heterogeneous landscape. I focussed on four contrasting ecotypes that occupy coastal sand dunes, rocky headlands, dry sclerophyll woodland and moist tableland rainforest. Multiple populations per ecotype, and their hybrids were reciprocally transplanted into the four environments to identify patterns of adaptation both within and between ecotypes. Each population was phenotyped in the glasshouse and environmental variables were recorded in the field to associated phenotype, environment and fitness across a heterogeneous landscape. Crossing designs and extensive phenotyping in glasshouse experiments were used to estimate the additive genetic variance underlying morphological traits and identify whether genetic correlations have constrained adaptation. Finally, artificial hybridization was used to identify whether genetic incompatibilities have created intrinsic reproductive isolation. My results showed that strong patterns of local adaptation were present between ecotypes, but weaker patterns within ecotypes. Populations exhibited trade-offs in performance when planted into foreign environments, which emerged as ontogeny progressed and natural selection acted against foreign populations. The environment played an important role in determining patterns of adaptation, with ecotypes derived from more disparate environments performing more poorly. Suites of phenotypic traits were associated with fitness and exhibited trade-offs between contrasting environments. As a consequence of adaptive divergence, multivariate phenotypic divergence has occurred along two major axes of differentiation, created by differences in plant shape/size in one direction and leaf shape in the opposite direction. Similarly, divergence in genetic variance occurred in plant size/shape traits and aligned with the divergence in phenotypic mean. Finally, given dramatic phenotypic diversification and strong patterns of adaptation, hybridization showed that negative epistasis created genetic incompatibilities in the F2, but not F3 generation and suggests that intrinsic reproductive isolation is progressing, but lags behind ecological divergence. Overall, ecotypic diversification in S. lautus was likely driven by spatial variation in natural selection creating divergence in suites of phenotypic traits and underlying genetic variance. Local adaptation between contrasting environments was associated with performance trade-offs between environments, which may reduce gene flow and maintain ecotypic divergence. Changes in additive genetic variance underlying phenotypic traits suggests that adaptation has either occurred in the direction of greatest genetic variance, or selection on high standing genetic variation has favoured strong genetic correlations and collapsed genetic variance into a smaller number of dimensions. Finally, genetic incompatibilities appeared to be in the early stages of arising, suggesting that ecotypes are moving towards irreversible speciation during adaptive radiation across a heterogeneous landscape.

Divergence of floral morphology and breeding systems are often expected to be linked to angiosperm diversification and environmental niche divergence. However, available evidence for such relationships is not generalizable due to different taxonomic, geographical and time scales. The Palearctic genus Helianthemum shows the highest diversity of the family Cistaceae in terms of breeding systems, floral traits, and environmental conditions as a result of three recent evolutionary radiations since the Late Miocene. Here, we investigated the tempo and mode of evolution of floral morphology in the genus and its link with species diversification and environmental niche divergence. We quantified 18 floral traits from 83 taxa and applied phylogenetic comparative methods using a robust phylogenetic framework based on genotyping-by-sequencing data. We found three different floral morphologies, putatively related to three different breeding systems: type I, characterized by small flowers without herkogamy and low pollen to ovule ratio; type II, represented by large flowers with approach herkogamy and intermediate pollen to ovule ratio; and type III, featured by small flowers with reverse herkogamy and the highest pollen to ovule ratio. Each morphology has been highly conserved across each radiation and has evolved independently of species diversification and ecological niche divergence. The combined results of trait, niche, and species diversification ultimately recovered a pattern of potentially non-adaptive radiations in Helianthemum and highlight the idea that evolutionary radiations can be decoupled from floral morphology evolution even in lineages that diversified in heterogeneous environments as the Mediterranean Basin.

The role of species divergence due to ecologically based divergent selection—or ecological speciation—in generating and maintaining biodiversity is a central question in evolutionary biology. Comparison of the genomes of phylogenetically related taxa spanning a selective habitat gradient enables discovery of divergent signatures of selection and thereby provides valuable insight into the role of divergent ecological selection in speciation. Tidal marsh ecosystems provide tractable opportunities for studying organisms' adaptations to selective pressures that underlie ecological divergence. Sharp environmental gradients across the saline–freshwater ecotone within tidal marshes present extreme adaptive challenges to terrestrial vertebrates. Here, we sequence 20 whole genomes of two avian sister species endemic to tidal marshes—the saltmarsh sparrow (Ammospiza caudacutus) and Nelson's sparrow (A. nelsoni)—to evaluate the influence of selective and demographic processes in shaping genome‐wide patterns of divergence. Genome‐wide divergence between these two recently diverged sister species was notably high (genome‐wide F ST = 0.32). Against a background of high genome‐wide divergence, regions of elevated divergence were widespread throughout the genome, as opposed to focused within islands of differentiation. These patterns may be the result of genetic drift resulting from past tidal march colonization events in conjunction with divergent selection to different environments. We identified several candidate genes that exhibited elevated divergence between saltmarsh and Nelson's sparrows, including genes linked to osmotic regulation, circadian rhythm, and plumage melanism—all putative candidates linked to adaptation to tidal marsh environments. These findings provide new insights into the roles of divergent selection and genetic drift in generating and maintaining biodiversity.

How diversity arises and what is the relative role of allopatric and ecological divergence are among the most persistent questions in evolution and ecology. Here, we assessed whether ecological divergence has enhanced the diversification of the Neotropical alpine plant complex Espeletia, also known as frailejones. This genus has one of the highest diversification rates ever reported and is distributed in the world’s fastest evolving biodiversity hotspot, the Páramo (Neotropical alpine grasslands at elevations of c. 2800–4700 m). Our goal was to determine whether ecology plays a role in divergence within the Espeletia complex by quantifying genome-wide patterns of ecological divergence. We characterized 162 samples of the three most common and contrasting ecotypes (distinct morphotypes occupying particular habitats) co-occurring in six localities in the northern Andes using Genotyping by Sequencing. Contrasting ecotypes were caulescent cloud forest populations, caulescent populations from wind-sheltered and well-irrigated depressions and acaulescent populations from wind-exposed drier slopes. We found high polymorphism with a total of 1,273 single nucleotide polymorphisms (SNPs) that defined the relationships among nine genetic clusters. We quantified allelic associations of these markers with localities and habitats using 18 different general and mixed-effects statistical models that accounted for phylogenetic distance. Despite that these models always yielded more SNPs associated with the localities, markers associated with the habitat types were recovered too. We found strong evidence for isolation-by-distance (IBD) across populations despite rampant gene flow, as expected for plant groups with limited seed dispersal. Contrasts between populations of different habitat types showed that an isolation-by-environment (IBE) trend emerged and masked the IBD signal. Maximum likelihood estimation of the number of migrants per generation (Nem) among ecotypes confirmed the IBE pattern. This result illustrates the importance of mountains’ environmental variation at a local scale in generating rapid morphological radiations and maintaining multiple adaptations in a fast-evolving ecosystem like the Páramo.

Convergent Evolution, Adaptive Radiation, and Species Diversification in Plants

Disjunct distribution is a key issue in biogeography and ecology, but it is often difficult to determine the relative roles of dispersal vs. vicariance in disjunctions. We studied the phylogeographic pattern of the monotypic Conandron ramondioides (Gesneriaceae), which shows Sino‐Japanese disjunctions, with ddRAD sequencing based on a comprehensive sampling of 11 populations from mainland China, Taiwan Island, and Japan. We found a very high degree of genetic differentiation among these three regions, with very limited gene flow and a clear Isolation by Distance pattern. Mainland China and Japan clades diverged first from a widespread ancestral population in the middle Miocene, followed by a later divergence between mainland China and Taiwan Island clades in the early Pliocene. Three current groups have survived in various glacial refugia during the Last Glacial Maximum, and experienced contraction and/or bottlenecks since their divergence during Quaternary glacial cycles, with strong niche divergence between mainland China + Japan and Taiwan Island ranges. Thus, we verified a predominant role of vicariance in the current disjunction of the monotypic genus Conandron. The sharp phylogenetic separation, ecological niche divergence among these three groups, and the great number of private alleles in all populations sampled indicated a considerable time of independent evolution, and suggests the need for a taxonomic survey to detect potentially overlooked taxa.

In eastern Neotropical South America, the Cerrado, a large savanna vegetation, and the Atlantic Forest harbour high biodiversity levels, and their habitats are rather different from each other. The biomes have intrinsic evolutionary relationships, with high lineage exchange that can be attributed, in part, to a large contact zone between them. The genomic study of ecotypes, i.e. populations adapted to divergent habitats, can be a model to study the genomic signatures of ecological divergence. Here, we investigated two ecotypes of the tree Plathymenia reticulata, one from the Cerrado and the other from the Atlantic Forest, which have a hybrid zone in the ecotonal zone of Atlantic Forest-Cerrado. The ecotypes were sampled in the two biomes and their ecotone. The evolutionary history of the divergence of the species was analysed with double-digest restriction site-associated DNA sequencing. The genetic structure and the genotypic composition of the hybrid zone were determined. Genotype-association analyses were performed, and the loci under putative selection and their functions were investigated. High divergence between the two ecotypes was found, and only early-generation hybrids were found in the hybrid zone, suggesting a partial reproductive barrier. Ancient introgression between the Cerrado and Atlantic Forest was not detected. The soil and climate were associated with genetic divergence in Plathymenia ecotypes and outlier loci were found to be associated with the stress response, with stomatal and root development and with reproduction. The high genomic, ecological and morphophysiological divergence between ecotypes, coupled with partial reproductive isolation, indicate that the ecotypes represent two species and should be managed as different evolutionary lineages. We advise that the forest species should be re-evaluated and restated as vulnerable. Our results provide insights into the genomic mechanisms underlying the diversification of species across savanna and forest habitats and the evolutionary forces acting in the species diversification in the Neotropics.

Aim To investigate how fruit type evolution influences ecological divergence, species diversification, and biogeographic history in tribe Ligustrinae (Oleaceae). Location Temperate and subtropical regions across the Northern Hemisphere. Taxon Tribe Ligustrinae, comprising Syringa and Ligustrum . Methods We reconstructed a robust phylogeny and estimated divergence times using plastome sequences. Ecological niches modelling and ancestral niche reconstruction were performed to infer habitat preferences. Biogeographic analyses and paleodistribution modelling were used to explore historical range dynamics and niche evolution across different fruit types. Results Capsules were primarily associated with humid, temperate regions, while drupes occupied broader ecological niches, expanding into warmer, humid subtropical zones. Biogeographic reconstruction indicated a north‐to‐south expansion of tribe Ligustrinae, coinciding with the transition from capsules to drupes. Paleodistribution models revealed fruit‐type‐specific ecological dynamics and niche expansion patterns over time. Main Conclusions Fruit type evolution in tribe Ligustrinae is closely linked to ecological divergence, lineage diversification, and biogeographic patterns. Drupes and capsules occupy distinct ecological niches and distributions, reflecting coordinated changes between reproductive traits and environmental adaptation. These findings highlight the combined role of fruit morphology and climatic factors in shaping plant diversity, adaptation, and distribution.

The formation and uplift of mountain ranges constitute major geological phenomena that can have long-lasting effects on the evolutionary diversification of living organisms. They provide opportunities for adaptive evolution through an increase of spatial heterogeneity of the landscape, including elevation, and the generation of a wide variety of ecologically diverse biotopes, and affect the migration of organisms and the distribution of species since mountain ranges can act as both biological corridors and ecological barriers. Hence, it should come as no surprise that mountain ranges host a large proportion of the biological diversity on earth (Barthlott et al., 2007; Jenkins et al., 2013). The reviews of Wen et al. (2014) and Luebert and Weigend (2014) included in this Research Topic present accounts of plant diversification processes in two major mountain regions of the world: the Andes and the Qinghai-Tibetan Plateau. Both studies show that plant diversifications have occurred in relatively recent geological times, primarily since the Middle Miocene, and therefore followed the late uplift phases of the high mountain ranges of both the Andes and the Qinghai-Tibetan region during the last 15–20 million years (Garzione et al., 2008; Wang et al., 2008). It is thus likely that the formations of those mountain ranges are at least partially responsible for the observed diversification processes, as proposed in other, more recent studies (e.g., Favre et al., 2015; Sanchez-Baracaldo and Thomas, 2014). Rapid diversification processes are documented for the Paramo clade of the plant genus Hypericum in the northern Andes (Nurk et al., 2013). The high diversity of this group originated recently (2.3–5.6 mya) and its diversification rate is well above the background diversification of Hypericum. This pattern of diversification is also seen in other endemic plant groups of the Paramo flora, and the region has the highest average diversification rate among all biodiversity hotspots in the world (Madrinan et al., 2013). Given the similar ages of these lineages (Luebert and Weigend, 2014) and of the high-elevation environments of the Paramo (Mora et al., 2010), these diversifications may have been triggered by the formation of high-mountain habitats in the northern Andes. Increased speciation rates would have occurred along with mountain uplift and habitat diversification, as observed in other Andean groups such as hummingbirds (Chaves et al., 2011) and butterflies (Despland, 2014), confirming the ideas initially proposed by Simpson (1975). Similar diversifications as those observed in the Paramo ecosystem have also occurred in plants of the Qinghai-Tibetan Plateau (Wen et al., 2014), but more studies are necessary to gain insights into any large-scale pattern (Favre et al., 2015). Mutke et al. (2014) report the distribution patterns of four Andean plant groups to reflect habitat heterogeneity rather than uplift history or barrier effects of mountain ranges, supporting, at least partially, the hypothesis that direct drivers of plant diversification in both the Qinghai-Tibetan Plateau and the Andes include plant-pollinator interactions, local adaptation to diverse environmental conditions and polyploidization (Luebert and Weigend, 2014; Wen et al., 2014). Although not reported in this Research Topic, the occurrence of polyploidization has been shown for the European-centered plant genus Campanula. Polyploid species of this genus are concentrated in the Campanula rotundifolia-complex, a mountain clade of Pliocene origin (Mansion et al., 2012). The significance of plant-pollinator interactions for the isolation of plant populations and plant diversification in mountain ranges, on the other hand, has been shown for three Penstemon species by Kramer et al. (2010). The different studies reported in this Research Topic clearly illustrate the potential effects of mountain uplift and formation on species diversification, at least in two major mountain regions of the world. A synthesis of biological diversification on mountains is, however, still far from being achieved and the potentially high complexity of the involved history, geography and biological processes encourages further research (Hoorn et al., 2013; Favre et al., 2015; Luebert and Weigend, 2014; Wen et al., 2014). Nevertheless, we hope that the collection of papers in this Research Topic will be of interest to scientists and will stimulate development of new studies and syntheses. We sincerely thank the authors and the reviewers for their efforts and contributions that made this Research Topic possible.