Deciphering ancient rapid radiations

A deeper phylogenetic understanding of ancient patterns of diversification would contribute to solving many problems in evolutionary biology, yet many of these phylogenies remain poorly resolved. Ancient rapid radiations pose a major challenge for phylogenetic analysis for two main reasons. First, the pattern to be deciphered, the order of divergence among lineages, tends to be supported by small amounts of data. Second, the time since divergence is large and, thus, the potential for misinterpreting phylogenetic information is great. Here, we review the underlying causes of difficulty in determining the branching patterns of diversification in ancient rapid radiations, and review novel data exploration tools that can facilitate understanding of these radiations.     

Resolving an ancient, rapid radiation in Saxifragales

Despite the prior use of approximately 9000 bp, deep-level relationships within the angiosperm clade, Saxifragales remain enigmatic, due to an ancient, rapid radiation (89.5 to 110 Ma based on the fossil record). To resolve these deep relationships, we constructed several new data sets: (1) 16 genes representing the three genomic compartments within plant cells (2 nuclear, 10 plastid, 4 mitochondrial; aligned, analyzed length = 21,460 bp) for 28 taxa; (2) the entire plastid inverted repeat (IR; 26,625 bp) for 17 taxa; (3) "total evidence" (50,845 bp) for both 17 and 28 taxa (the latter missing the IR). Bayesian and ML methods yielded identical topologies across partitions with most clades receiving high posterior probability (pp = 1.0) and bootstrap (95% to 100%) values, suggesting that with sufficient data, rapid radiations can be resolved. In contrast, parsimony analyses of different partitions yielded conflicting topologies, particularly with respect to the placement of Paeoniaceae, a clade characterized by a long branch. In agreement with published simulations, the addition of characters increased bootstrap support for the putatively erroneous placement of Paeoniaceae. Although having far fewer parsimony-informative sites, slowly evolving plastid genes provided higher resolution and support for deep-level relationships than rapidly evolving plastid genes, yielding a topology close to the Bayesian and ML total evidence tree. The plastid IR region may be an ideal source of slowly evolving genes for resolution of deep-level angiosperm divergences that date to 90 My or more. Rapidly evolving genes provided support for tip relationships not recovered with slowly evolving genes, indicating some complementarity. Age estimates using penalized likelihood with and without age constraints for the 28-taxon, total evidence data set are comparable to fossil dates, whereas estimates based on the 17-taxon data are much older than implied by the fossil record. Hence, sufficient taxon density, and not simply numerous base pairs, is important in reliably estimating ages. Age estimates indicate that the early diversification of Saxifragales occurred rapidly, over a time span as short as 6 million years. Between 25,000 and 50,000 bp were needed to resolve this radiation with high support values. Extrapolating from Saxifragales, a similar number of base pairs may be needed to resolve the many other deep-level radiations of comparable age in angiosperms.     

Recent and rapid speciation with limited morphological disparity in the genus Rattus

Recent and rapid radiations provide rich material to examine the factors that drive speciation. Most recent and rapid radiations that have been well-characterized involve species that exhibit overt ecomorphological differences associated with clear partitioning of ecological niches in sympatry. The most diverse genus of rodents, Rattus (66 species), evolved fairly recently, but without overt ecomorphological divergence among species. We used multilocus molecular phylogenetic data and five fossil calibrations to estimate the tempo of diversification in Rattus, and their radiation on Australia and New Guinea (Sahul, 24 species). Based on our analyses, the genus Rattus originated at a date centered on the Pliocene-Pleistocene boundary (1.84-3.17 Ma) with a subsequent colonization of Sahul in the middle Pleistocene (0.85-1.28 Ma). Given these dates, the per lineage diversification rates in Rattus and Sahulian Rattus are among the highest reported for vertebrates (1.1-1.9 and 1.6-3.0 species per lineage per million years, respectively). Despite their rapid diversification, Rattus display little ecomorphological divergence among species and do not fit clearly into current models of adaptive radiations. Lineage through time plots and ancestral state reconstruction of ecological characters suggest that diversification of Sahulian Rattus was most rapid early on as they expanded into novel ecological conditions. However, rapid lineage accumulation occurred even when morphological disparity within lineages was low suggesting that future studies consider other phenotypes in the diversification of Rattus.     

Timing and tempo of early and successive adaptive radiations in Macaronesia

The flora of Macaronesia, which encompasses five Atlantic archipelagos (Azores, Canaries, Madeira, Cape Verde, and Salvage), is exceptionally rich and diverse. Spectacular radiation of numerous endemic plant groups has made the Macaronesian islands an outstanding area for studies of evolution and speciation. Despite intensive investigation in the last 15 years, absolute age and rate of diversification are poorly known for the flora of Macaronesia. Here we report molecular divergence estimates and rates of diversification for five representative, putative rapid radiations of monophyletic endemic plant lineages across the core eudicot clade of flowering plants. Three discrete windows of colonization during the Miocene and early Pliocene are suggested for these lineages, all of which are inferred to have had a single colonization event followed by rapid radiation. Subsequent inter-archipelago dispersal events into Madeira and the Cape Verdes took place very recently during the late Pliocene and Pleistocene after initial diversification on the Canary Islands. The tempo of adaptive radiations differs among the groups, but is relatively rapid compared to continental and other island radiations. Our results demonstrate that opportunity for island colonization and successful radiation may have been constrained to discrete time periods of profound climatic and geological changes in northern African and the Mediterranean.     

Phylogenetic analyses of Gammaridae crustacean reveal different diversification patterns among sister lineages in the Tethyan region

The Gammaridae shows the greatest disparity in species diversity and distribution pattern in the Amphipoda, with some genera ranging from the Palearctic to Nearctic, while others are limited to the Mediterranean region or ancient Tethyan margins. Here we present the first molecular phylogenetic analysis of the Gammaridae to investigate its evolutionary history using four genetic markers and a comprehensive set of taxa representing 198 species. The phylogenetic results revealed that the Gammaridae originated from the Tethyan region in the Cretaceous, and split into three morphologically and geographically distinct lineages by the end of the Paleocene. Diversification analysis combined with paleogeological evidence suggested that the Tethyan changes induced by sea‐level fluctuation and tectonic uplift triggered different diversification modes and range expansions for the three lineages. The Gammarus lineage underwent an early rapid radiation across Eurasia and North America, then declined towards modern species. Pontogammarids maintained stable diversification with restricted distributions around the Tethyan basin, whereas sarothrogammarids experienced evolutionary stasis by stranding on the ancient Tethyan margins. Our findings suggest that environmental changes have played an important role in the diversification of Gammaridae lineages, which could be an opportunity to promote adaptive radiations in new habitats, or constraints resulting in evolutionary relicts.     

Rapid lineage accumulation in a non-adaptive radiation: phylogenetic analysis of diversification rates in eastern North American woodland salamanders (Plethodontidae: Plethodon)

Adaptive radiations have served as model systems for quantifying the build-up of species richness. Few studies have quantified the tempo of diversification in species-rich clades that contain negligible adaptive disparity, making the macroevolutionary consequences of different modes of evolutionary radiation difficult to assess. We use mitochondrial-DNA sequence data and recently developed phylogenetic methodologies to explore the tempo of diversification of eastern North American Plethodon, a species-rich clade of woodland salamanders exhibiting only limited phenotypic disparity. Lineage-through-time analysis reveals a high rate of lineage accumulation, 0.8 species per million years, occurring 11-8 million years ago in the P. glutinosus species group, followed by decreasing rates. This high rate of lineage accumulation is exceptional, comparable to the most rapid of adaptive radiations. In contrast to classic models of adaptive radiation where ecological niche divergence is linked to the origin of species, we propose that phylogenetic niche conservatism contributes to the rapid accumulation of P. glutinosus-group lineages by promoting vicariant isolation and multiplication of species across a spatially and temporally fluctuating environment. These closely related and ecologically similar lineages persist through long-periods of evolutionary time and form strong barriers to the geographic spread of their neighbours, producing a subsequent decline in lineage accumulation. Rapid diversification among lineages exhibiting long-term maintenance of their bioclimatic niche requirements is an under-appreciated phenomenon driving the build-up of species richness.     

AN ADAPTIVE RADIATION OF FROGS IN A SOUTHEAST ASIAN ISLAND ARCHIPELAGO

Living amphibians exhibit a diversity of ecologies, life histories, and species‐rich lineages that offers opportunities for studies of adaptive radiation. We characterize a diverse clade of frogs (Kaloula, Microhylidae) in the Philippine island archipelago as an example of an adaptive radiation into three primary habitat specialists or ecotypes. We use a novel phylogenetic estimate for this clade to evaluate the tempo of lineage accumulation and morphological diversification. Because species‐level phylogenetic estimates for Philippine Kaloula are lacking, we employ dense population sampling to determine the appropriate evolutionary lineages for diversification analyses. We explicitly take phylogenetic uncertainty into account when calculating diversification and disparification statistics and fitting models of diversification. Following dispersal to the Philippines from Southeast Asia, Kaloula radiated rapidly into several well‐supported clades. Morphological variation within Kaloula is partly explained by ecotype and accumulated at high levels during this radiation, including within ecotypes. We pinpoint an axis of morphospace related directly to climbing and digging behaviors and find patterns of phenotypic evolution suggestive of ecological opportunity with partitioning into distinct habitat specialists. We conclude by discussing the components of phenotypic diversity that are likely important in amphibian adaptive radiations.     

Mitochondrial genes from 18 angiosperms fill sampling gaps for phylogenomic inferences of the early diversification of flowering plants

The early diversification of angiosperms is thought to have been a rapid process, which may complicate phylogenetic analyses of early angiosperm relationships. Plastid and nuclear phylogenomic studies have raised several conflicting hypotheses regarding overall angiosperm phylogeny, but mitochondrial genomes have been largely ignored as a relevant source of information. Here we sequenced mitochondrial genomes from 18 angiosperms to fill taxon-sampling gaps in Austrobaileyales, magnoliids, Chloranthales, Ceratophyllales, and major lineages of eudicots and monocots. We assembled a data matrix of 38 mitochondrial genes from 107 taxa to assess how well mitochondrial genomic data address current uncertainties in angiosperm relationships. Although we recovered conflicting phylogenies based on different data sets and analytical methods, we also observed congruence regarding deep relationships of several major angiosperm lineages: Chloranthales were always inferred to be the sister group of Ceratophyllales, Austrobaileyales to mesangiosperms, and the unplaced Dilleniales was consistently resolved as the sister to superasterids. Substitutional saturation, GC compositional heterogeneity, and codon-usage bias are possible reasons for the noise/conflict that may impact phylogenetic reconstruction; and angiosperm mitochondrial genes may not be substantially affected by these factors. The third codon positions of the mitochondrial genes appear to contain more parsimony-informative sites than the first and second codon positions, and therefore produced better resolved phylogenetic relationships with generally strong support. The relationships among these major lineages remain incompletely resolved, perhaps as a result of the rapidity of early radiations. Nevertheless, data from mitochondrial genomes provide additional evidence and alternative hypotheses for exploring the early evolution and diversification of the angiosperms.     

Rapid speciation and the evolution of hummingbird pollination in neotropical Costus subgenus Costus (Costaceae): evidence from nrDNA ITS and ETS sequences

We estimate phylogenetic relationships and the biogeographic and pollination history of Costus subgenus Costus (Costaceae) using sequence data from the internal and external transcribed spacer (ITS and ETS) regions of 18S-26S nuclear ribosomal DNA. The African members of the subgenus form a series of lineages basal to a monophyletic neotropical species radiation. The neotropical species have large, showy flowers visited by either euglossine bees or hummingbirds. The hummingbird pollination syndrome is supported as a derived character state from the bee pollination syndrome, and we estimate that it has evolved independently seven or more times in the neotropics. A molecular clock approach suggests that diversification of the neotropical clade has been recent and rapid and that it coincides with dramatic climatic and geologic changes, Andean orogeny, and the closing of the Panama isthmus that occurred in the Pliocene and Pleistocene epochs. We propose a scenario for the diversification of Costus, in which rapid floral adaptation in geographic isolation and range shifts in response to environmental changes contribute to reproductive isolation among close relatives. We suggest that these processes may be common in other recently diversified plant lineages centered in Central America or the Northern Andean phytogeographic region.     

Biogeographical origins and diversification of the exoneurine allodapine bees of Australia (Hymenoptera,Apidae)

Aim Early diversification of allodapine bees occurred in Africa c. 50 Ma. They are most abundant in sub‐Saharan Africa and Australia, and one of the oldest phylogenetic divergences in the tribe involves a split between an African + Malagasy clade and an Australian clade. The historical biogeographical scenario for this has been highly problematic, entailing an Eocene dispersal from Africa to Australia, followed by an unresolved, and apparently rapid, set of bifurcations leading to the Australian ‘exoneurine’ genera. Here we use an expanded taxon set of Australian species to explore the timing and historical biogeography of the exoneurine radiation. Location Australia, Africa, Madagascar. Methods One nuclear gene (F2 copy of elongation factor 1α) and two mitochondrial genes (cytochrome c oxidase subunit I and cytochrome b) were sequenced for 33 Australian exoneurine species from all five genera found on the continent, as well as for an additional 37 species from all non‐parasitic genera in the remainder of the tribe. We used Bayesian inference analyses to study phylogenetic topology and penalized likelihood analyses to infer key dates of divergence within the tribe. We also used lineage‐through‐time (LTT) analyses and Bayesian analyses to explore the tempo of radiations and biogeographical history of the exoneurines. Results Results from the phylogenetic analyses were congruent with previous studies, indicating a single colonization event c. 34 Ma, too late for Gondwanan vicariance models, and too early for a Laurasian dispersal route. In contrast to earlier studies, we show that this colonization event did not result in an ancient rapid radiation. However, LTT patterns indicated a rapid radiation of the temperate‐adapted genera Exoneura and Brevineura, but not of the xeric‐adapted genus Exoneurella, from 10 to 6 Ma. Main conclusions Our results indicate a trans‐oceanic dispersal event from Africa to Australia, most likely via Antarctica, with an accelerated diversification of temperate‐adapted lineages during the major Late Miocene event referred to as the ‘Hill Gap’. This is the first study to link radiations in Australian bee faunal elements to changing climate, and differs from many other plant and insect phylogenetic studies by showing increased radiation of temperate clades, rather than xeric clades, with increasing aridification of Australia.     

The distribution of species diversity across a flora's component lineages: dating the Cape's 'relicts'

The Cape Floristic Region is exceptionally species-rich both for its area and latitude, and this diversity is highly unevenly distributed among genera. The modern flora is hypothesized to result largely from recent (post-Oligocene) speciation, and it has long been speculated that particular species-poor lineages pre-date this burst of speciation. Here, we employ molecular phylogenetic data in combination with fossil calibrations to estimate the minimum duration of Cape occupation by 14 unrelated putative relicts. Estimates vary widely between lineages (7-101 Myr ago), and when compared with the estimated timing of onset of the modern flora's radiation, it is clear that many, but possibly not all, of these lineages pre-date its establishment. Statistical comparisons of diversities with lineage age show that low species diversity of many of the putative relicts results from a lower rate of diversification than in dated Cape radiations. In other putative relicts, however, we cannot reject the possibility that they diversify at the same underlying rate as the radiations, but have been present in the Cape for insufficient time to accumulate higher diversity. Although the extremes in diversity of currently dated Cape lineages fall outside expectations under a constant underlying diversification rate, sampling of all Cape lineages would be required to reject this null hypothesis.     

The Dynamics of Incomplete Lineage Sorting across the Ancient Adaptive Radiation of Neoavian Birds

The diversification of neoavian birds is one of the most rapid adaptive radiations of extant organisms. Recent whole-genome sequence analyses have much improved the resolution of the neoavian radiation and suggest concurrence with the Cretaceous-Paleogene (K-Pg) boundary, yet the causes of the remaining genome-level irresolvabilities appear unclear. Here we show that genome-level analyses of 2,118 retrotransposon presence/absence markers converge at a largely consistent Neoaves phylogeny and detect a highly differential temporal prevalence of incomplete lineage sorting (ILS), i.e., the persistence of ancestral genetic variation as polymorphisms during speciation events. We found that ILS-derived incongruences are spread over the genome and involve 35% and 34% of the analyzed loci on the autosomes and the Z chromosome, respectively. Surprisingly, Neoaves diversification comprises three adaptive radiations, an initial near-K-Pg super-radiation with highly discordant phylogenetic signals from near-simultaneous speciation events, followed by two post-K-Pg radiations of core landbirds and core waterbirds with much less pronounced ILS. We provide evidence that, given the extreme level of up to 100% ILS per branch in super-radiations, particularly rapid speciation events may neither resemble a fully bifurcating tree nor are they resolvable as such. As a consequence, their complex demographic history is more accurately represented as local networks within a species tree.     

Phylogenomics of the hyperdiverse daisy tribes: Anthemideae,Astereae, Calenduleae,Gnaphalieae, and Senecioneae

Asteraceae account for 10% of all flowering plant species, and 35%–40% of these are in five closely related tribes that total over 10 000 species. These tribes include Anthemideae, Astereae, Calenduleae, Gnaphalieae, and Senecioneae, which form one of two enormous clades within Subfamily Asteroideae. We took a phylogenomics approach to resolve evolutionary relationships among these five tribes. We sampled the nuclear and plastid genomes via HybSeq target enrichment and genome skimming, and recovered 74 plastid genes and nearly 1000 nuclear loci, known as Conserved Orthologous Sequences. We tested for conflicting support in both data sets and used network analyses to assess patterns of reticulation to explain the early evolutionary history of this lineage, which has experienced whole‐genome duplications and rapid radiations. We found concordance and conflicting support in both data sets and documented four ancient hybridization events. Due to the timing of the early radiation of this five‐tribe lineage, shortly before the Eocene–Oligocene extinction event (34 MYA), early lineages were likely lost, obscuring some details of their early evolutionary history.     

Diet and Diversification in the Evolution of Coral Reef Fishes

The disparity in species richness among evolutionary lineages is one of the oldest and most intriguing issues in evolutionary biology. Although geographical factors have been traditionally thought to promote speciation, recent studies have underscored the importance of ecological interactions as one of the main drivers of diversification. Here, we test if differences in species richness of closely related lineages match predictions based on the concept of density-dependent diversification. As radiation progresses, ecological niche-space would become increasingly saturated, resulting in fewer opportunities for speciation. To assess this hypothesis, we tested whether reef fish niche shifts toward usage of low-quality food resources (i.e. relatively low energy/protein per unit mass), such as algae, detritus, sponges and corals are accompanied by rapid net diversification. Using available molecular information, we reconstructed phylogenies of four major reef fish clades (Acanthuroidei, Chaetodontidae, Labridae and Pomacentridae) to estimate the timing of radiations of their subclades. We found that the evolution of species-rich clades was associated with a switch to low quality food in three of the four clades analyzed, which is consistent with a density-dependent model of diversification. We suggest that ecological opportunity may play an important role in understanding the diversification of reef-fish lineages.     

Exceptional among-lineage variation in diversification rates during the radiation of Australia's most diverse vertebrate clade

The disparity in species richness among groups of organisms is one of the most pervasive features of life on earth. A number of studies have addressed this pattern across higher taxa (e.g. 'beetles'), but we know much less about the generality and causal basis of the variation in diversity within evolutionary radiations at lower taxonomic scales. Here, we address the causes of variation in species richness among major lineages of Australia's most diverse vertebrate radiation, a clade of at least 232 species of scincid lizards. We use new mitochondrial and nuclear intron DNA sequences to test the extent of diversification rate variation in this group. We present an improved likelihood-based method for estimating per-lineage diversification rates from combined phylogenetic and taxonomic (species richness) data, and use the method in a hypothesis-testing framework to localize diversification rate shifts on phylogenetic trees. We soundly reject homogeneity of diversification rates among members of this radiation, and find evidence for a dramatic rate increase in the common ancestor of the genera Ctenotus and Lerista. Our results suggest that the evolution of traits associated with climate tolerance may have had a role in shaping patterns of diversity in this group.     

Dating the Dipsacales: comparing models, genes, and evolutionary implications

Dipsacales is an asterid angiosperm clade of ca. 1100 species, with most of its lineages occupying temperate regions of the Northern Hemisphere. A recent phylogenetic analysis based on 7593 nucleotides of chloroplast DNA recovered a well-resolved and strongly supported phylogenetic hypothesis, which we use here to estimate divergence times within the group. A molecular clock is strongly rejected, regardless of data partition. We used recently proposed methods that relax the assumption of rate constancy among lineages (local clocks, nonparametric rate smoothing, penalized likelihood, and Bayesian relaxed clock) to estimate the ages of major lineages. Age estimates for Dipsacales varied widely among markers and codon positions, and depended on the fossils used for calibration and method of analysis. Some methods yielded dates for the Dipsacales diversification that appear to be too old (prior to the presumed 125 my [million years] age of eudicots), and others suggested ages that are too young based on well-documented Dipsacales fossils. Concordant penalized likelihood and Bayesian studies imply that Dipsacales originated in the Cretaceous, as did its two major lineages, Adoxaceae and Caprifoliaceae. However, diversification of crown Adoxaceae and Caprifoliaceae mainly occurred in the Tertiary, with the origin of major lineages within these clades mainly occurring during the Eocene. Another round of diversification appears to have occurred in the Miocene. Several radiations, such as Valerianaceae in South America and Dipsacaceae around the Mediterranean, are even more recent. This study demonstrates the wide range of divergence times that can be obtained using different methods and data sets, and cautions against reliance on age estimates based on only a single gene or methodology. Despite this variance, significant conclusions can be made about the timing of Dipsacales evolution.     

EXPLOSIVE RADIATION OR CRYPTIC MASS EXTINCTION? INTERPRETING SIGNATURES IN MOLECULAR PHYLOGENIES

How biodiversity is generated and maintained underlies many major questions in evolutionary biology, particularly relating to the tempo and pattern of diversification through time. Molecular phylogenies and new analytical methods provide additional tools to help interpret evolutionary processes. Evolutionary rates in lineages sometimes appear punctuated, and such "explosive" radiations are commonly interpreted as adaptive, leading to causative key innovations being sought. Here we argue that an alternative process might explain apparently rapid radiations ("broom-and-handle" or "stemmy" patterns seen in many phylogenies) with no need to invoke dramatic increase in the rate of diversification. We use simulations to show that mass extinction events can produce the same phylogenetic pattern as that currently being interpreted as due to an adaptive radiation. By comparing simulated and empirical phylogenies of Australian and southern African legumes, we find evidence for coincident mass extinctions in multiple lineages that could have resulted from global climate change at the end of the Eocene.     

Speciation and radiations track climate transitions since the Miocene Climatic Optimum: a case study of southern African chameleons

Aim The high amount of species diversity concentrated in southern Africa has been attributed to palaeoclimatic factors, and the timing of radiations in some taxa corresponds to global palaeoclimatic trends. Using dwarf chameleons (Bradypodion: Chamaeleonidae) as a model system, we explored the relationship between palaeoclimatic fluctuations and cladogenesis with respect to both temporal and spatial patterns in an effort to understand the process of speciation in southern Africa. Location South Africa, with particular emphasis on the Cape Floristic Region and the Maputaland–Pondoland–Albany hotspot. Methods Mitochondrial sequence data (ND2 and 16S) were used to estimate the timing of major radiations and to examine the number of lineages through time. A dated phylogeny was constructed using Bayesian phylogenetic reconstruction, and a Bayesian relaxed molecular clock was used to estimate divergence times. Spatial data and lineage‐through‐time plots were used to identify geographic regions that underwent diversification in connection with major climatic events. Both parsimony and likelihood optimizations of habitat type on the phylogeny were used to determine whether major habitat shifts have occurred. On a coarse scale (half‐degree grid cells), phylogenetic diversity (sum of the branch lengths linking terminals) was compared with species richness (absolute number of species) to identify areas of conservation importance. Results The complete species phylogeny of dwarf chameleons shows that the timing and mode of diversification exhibit spatio‐temporal patterns that link to phases in the evolution of southern Africa’s climate over the last 14 Myr. Optimizations of habitat on the phylogenetic tree show a progression from closed to open habitats since the Mid‐Miocene, corresponding to the shift from C₃ to C₄ environments, and later with the development of south‐western Africa’s winter‐rainfall regime. These shifts are not simultaneous across the region, with different geographic centres of diversity generated during different time periods. Main conclusions Regions that are prominent centres of chameleon diversification are encompassed by the current biodiversity hotspots as shown by chameleon species richness and phylogenetic diversity. Diversity within the Cape Floristic Region appears to be the result of a Late Pliocene radiation, whereas the diversity encompassed within the Maputaland–Pondoland–Albany hotspot is an aggregate of asynchronous radiation events, probably influenced by lineage losses. Overall, dwarf chameleons have experienced a shift in habitat types, with recent radiations occupying open habitats, and older lineages persisting in relictual forested habitats, corresponding to the continental shift of vegetation types since the Miocene Climatic Optimum.     

Mitochondrial Phylogenomics of Modern and Ancient Equids

The genus Equus is richly represented in the fossil record, yet our understanding of taxonomic relationships within this genus remains limited. To estimate the phylogenetic relationships among modern horses, zebras, asses and donkeys, we generated the first data set including complete mitochondrial sequences from all seven extant lineages within the genus Equus. Bayesian and Maximum Likelihood phylogenetic inference confirms that zebras are monophyletic within the genus, and the Plains and Grevy’s zebras form a well-supported monophyletic group. Using ancient DNA techniques, we further characterize the complete mitochondrial genomes of three extinct equid lineages (the New World stilt-legged horses, NWSLH; the subgenus Sussemionus; and the Quagga, Equus quagga quagga). Comparisons with extant taxa confirm the NWSLH as being part of the caballines, and the Quagga and Plains zebras as being conspecific. However, the evolutionary relationships among the non-caballine lineages, including the now-extinct subgenus Sussemionus, remain unresolved, most likely due to extremely rapid radiation within this group. The closest living outgroups (rhinos and tapirs) were found to be too phylogenetically distant to calibrate reliable molecular clocks. Additional mitochondrial genome sequence data, including radiocarbon dated ancient equids, will be required before revisiting the exact timing of the lineage radiation leading up to modern equids, which for now were found to have possibly shared a common ancestor as far as up to 4 Million years ago (Mya).     

Dynamism and context‐dependency in diversification of the megadiverse plant genus Solanum (Solanaceae)

Explosive radiations—substantial increases in net species diversification—have been considered one of the most intriguing diversification patterns across the Tree of Life, but the subsequent change, movement, and extinction of the constituent lineages make radiations hard to discern or understand as geological time passes. We used the megadiverse angiosperm genus Solanum L. (Solanaceae), with ca. 1200 currently accepted species distributed worldwide in a wide array of habitats, to explore these patterns on a global scale. We synthesized phylogenetic and distributional data for this ongoing radiation to show how dispersal events and past climatic changes have interacted to shape diversification. We find that, despite the vast diversity of Solanum lineages in the Neotropics, lineages in the Old World are diversifying more rapidly. This recent increase in diversification coincides with a long‐distance dispersal event from the Neotropics to regions where major climatic changes were taking place. Two separate groups of Solanum have migrated and established in Australia, but only the arid‐adapted lineages underwent significant increases in diversification rate, as they were able to adapt to the continent's long‐term climatic trend towards seasonally dry and arid biomes (a pattern observed in the diversification of other arid‐adapted groups). Our findings provide a clear example of how successful colonization of new areas and niches can—but does not always—drive explosive diversifications.