On the cover: Close-up of the Tuberculated Gecko (Bunopus tuberculatus), a widely distributed species complex and a successful colonizer of the Arabian Peninsula whose evolutionary history was largely influenced by paleoenvironmental conditions. Photo taken in eastern Jordan by Lukáš Pola.
Gene flow from central to edge populations is thought to limit population growth at range edges by constraining local adaptation. In this study, we explore the thesis that range edges can differ in their dynamics and be either ‘hard’ (e.g. a river) or ‘soft’ (e.g. ecological gradients). We hypothesize that soft edge populations will have smaller effective population sizes than central populations and that gene flow will be greater from the centre to the edge than vice versa. Conversely, we hypothesize that hard edge populations should have similar effective population sizes to central populations and that gene flow will be equal between the two.
Kentucky, West Virginia, and Virginia, USA.
Plethodon kentucki (Caudata: Plethodontidae).
We evaluated landscape suitability using an ecological niche model, then we compared gene flow and effective population sizes between edge and central populations and quantified gene flow between populations. Finally, we characterized landscape genetic variation, testing for isolation by distance and isolation by environment.
We found continuously decreasing habitat quality along soft edges, with hard edges more variable. Additionally, we found that soft edges had lower effective population sizes than central populations and that gene flow was greater from the centre of the range to the soft edges than the reverse. In hard edges, by contrast, we found effective population sizes in edge populations were similar to central populations, with relatively equal gene flow in both directions.
Understanding why species have range limits is central to investigations of the structure of biodiversity, yet the evolutionary dynamics of range edges remain poorly understood. We show that within a single species with a small range, the evolutionary dynamics operating at range boundaries may depend on the nature of the boundary.
Beta-diversity quantifies the change in taxonomic and phylogenetic composition between areas, and is a scalar between local (α) and regional (γ) diversity. Geographic distance, which reflects dispersal limitation, and climatic distance, which reflects environmental filtering, are major drivers of β-diversity. Here, we analyse a comprehensive data set of angiosperms in regional floras across Africa to assess the relationships of β-diversity, and its components, to three major types of environmental variables (current climate, Quaternary climate change and topographic heterogeneity) thought to drive β-diversity.
Africa.
Angiosperms.
Africa was divided into 27 regions. Species lists of angiosperms for each region were collated. The relationships of both taxonomic and phylogenetic β-diversity, and their respective turnover and nestedness components, with geographic and environmental distances were assessed.
This study showed that (1) regions of the lowest β-diversity are located in moist tropical climates, (2) the turnover and nestedness components of β-diversity are negatively correlated with each other, (3) taxonomic β-diversity is higher than phylogenetic β-diversity across Africa, (4) variation in β-diversity of angiosperms is more strongly associated with current climate than with Quaternary climate change and topographic heterogeneity and (5) the variation in taxonomic β-diversity and its turnover component that is independently explained by geographic distance is much larger than that is independently explained by climatic distance for angiosperms in Africa.
The finding that geographic distance explained more variation than climatic distance suggests that dispersal limitation plays a greater role than environmental filtering in shaping angiosperm β-diversity in Africa. Of climatic factors, current climate plays a more important role than Quaternary climate change in shaping angiosperm β-diversity in Africa.
Understanding the impacts of past and future climate change on genetic diversity and structure is a current major research gap. We ask whether past range shifts explain the observed genetic diversity of surfgrass species and if future climate change projections anticipate genetic diversity losses. Our study aims to identify regions of long-term climate suitability with higher and unique seagrass genetic diversity and predict future impacts of climate change on them.
Northeast Pacific.
Analyses considered a timeframe from the Last Glacial Maximum (LGM; 20 kybp) until one Representative Concentration Pathway (RCP) scenario of future climate changes (RCP 8.5; 2100).
Two seagrass species belonging to the genus Phyllospadix.
We estimated population genetic diversity and structure using 11 polymorphic microsatellite markers. We predicted the distribution of the species for the present, LGM, and near future (RCP 8.5, no climate mitigation) using Species Distribution Models (SDMs).
SDMs revealed southward range shifts during the LGM and potential poleward expansions in the future. Genetic diversity of Phyllospadix torreyi decreases from north to south, but in Phyllospadix scouleri the trend is variable. Phyllospadix scouleri displays signals of genome admixture at the southernmost and northernmost edges of its distribution.
The genetic patterns observed in the present reveal the influence of climate-driven range shifts in the past and suggest further consequences of climate change in the future, with potential loss of unique gene pools. This study also shows that investigating climate links to present genetic information at multiple timescales can establish a historical context for analyses of the future evolutionary history of populations.
Competition is often proposed to drive niche segregation along multiple axes in speciose communities. Understanding spatial partitioning of foraging areas is particularly important in species that are constrained to a central place. We present a natural experiment examining variation in habitat preferences of congeneric Southern Ocean predators in sympatry and allopatry. Our aim was to ascertain consistency of habitat preferences within species, and to test whether preferences changed in the presence of the congener.
Southern Hemisphere.
Multiple colonies of both species within the genus Phoebetria (sooty albatrosses).
The two Phoebetria albatrosses breed on islands located from ~37–55°S – sooty albatrosses (P. fusca) in the north and light-mantled albatrosses (P. palpebrata) in the south – with sympatric overlap at locations ~46–49°S. We analysed GPS and PTT tracks from 87 individuals and multiple remotely sensed environmental variables using GAMs, to determine and compare the key factors influencing habitat preference for each species at each breeding colony.
While foraging habitat preferences are consistent in light-mantled albatrosses, there is divergence of preferences in sooty albatrosses depending on whether they are in sympatry with their congener or in allopatry.
This study represents the most comprehensive work on this genus to date and highlights how habitat preferences and behavioural plasticity may influence species distributions under different competitive conditions.
Species have different distribution patterns across the globe and among biogeographical regions. The Nearctic and Palaearctic regions share lineages because of their parallel biogeographic histories and ecological conditions. As the number of phylogenetic studies increases, there are more insights into past exchange events between these two regions and their effects on the current distribution of diversity. However, several groups have not been tested and an overall generalization is still missing. Here, we analyse the biogeographic history across multiple genera of odonates to elucidate a general process of species exchange, vicariance and species divergence between these two regions.
The Holarctic, including the entire Nearctic and the East and West Palaearctic.
14 genera of Odonata (Insecta).
We reconstructed a time-calibrated phylogenetic tree for each genus to determine species relationships and divergence time using 3614 COI sequences of 259 species. Biogeographic ancestral range estimation was inferred for each phylogeny using BioGeoBEARS. Preferred habitat (lotic versus lentic) was established for each species.
Exchange events were not restricted in time, direction or either lentic habitat or lotic habitat. Most genera crossed between both regions only once, and it was mainly across the Beringia, while three diverse anisopteran genera revealed multiple exchanges. Recent exchanges during the Pleistocene were associated with cold-dwelling and lentic species.
Our finding reveals the absence of a generalizable pattern of species exchange and divergence between the Nearctic and Palaearctic regions; instead, we found lineage-specific biogeographic patterns. This finding highlights the complexity of drivers and functional traits that shaped current diversity patterns. Moreover, it emphasizes that general conclusions cannot be formulated based on one single clade.
We studied the niche evolution and diversification modes in transisthmian Alpheus shrimps by examining the interplay between environmental niche divergence and conservatism in allopatric sister species. In a broader perspective, the current study analysed the evolution of climatic niche and the role of the environment in species diversification of Alpheus transisthmian shrimp.
Atlantic and Eastern-Pacific oceans.
Alpheus shrimps (Caridea: Alpheidae).
We assembled georeferenced occurrences for 33 species of Alpheus (with 24 sister species) from a time-calibrated molecular phylogeny. We modelled their ecological niches and assessed niche overlap through pairwise comparisons. Additionally, we performed phylogenetic reconstructions of the ancestral environmental niche, for each niche axis.
Our results demonstrate that thermal tolerances, food availability and hydrodynamic forces were relevant environmental axes in evolutionary processes in transisthmian species of Alpheus. Among the 528 paired comparisons, we found that most niches were divergent, including in 12 clades formed by pairs of sister species (in only two of these clades were the niches fully equivalent). Phylogenetic reconstructions of ancestral niches showed an initial niche conservatism in all axes, with divergences intensifying in the last 12 million years.
We found evidence that confirms the relevance of the environmental changes that occurred in the West Atlantic and East Pacific for niche evolution in transisthmian Alpheus species, as well as for the emergence of some lineages. Our findings provide evidence for different modes of Alpheus species speciation in a period consistent with the closure of the Isthmus of Panama.
Environmental features can act as selection pressures and barriers to gene flow between populations. The genetic structuring of highly mobile but philopatric seabirds creates a paradox, and the role of oceanographic and geographic variables is still poorly understood. In this study, we investigate the influence of environmental and geographic variables in the genetic and phenotypic diversity of a pantropical seabird breeding in islands and archipelagos separated by different geographic distances, up to thousand kilometres, and which differ in environmental characteristics.
Islands and archipelagos in the southwestern (SW) Atlantic Ocean.
Sula dactylatra, Lesson, 1831 (masked booby).
The population structure of the species was accessed through mitochondrial and nuclear DNA. To test Isolation by Environment (IBE) versus by Distance (IBD), sea surface temperature, primary productivity and salinity, as well as isotopic niche based on carbon and nitrogen, and distances between colonies and from the continent, were used. We also tested the correlation between the genetic structure and the morphometry of individuals in each colony.
We uncover the presence of low genetic structure between populations. Nevertheless, differences were identified between inshore and offshore colonies, with the influence of landscape characteristics of these two types of environment. The morphometric and isotopic niche variations are consistent with this segregation.
Environmental variables of coastal and oceanic environments seem to influence the genetic structure of masked boobies, even though it is low in the SW Atlantic Ocean, highlighting the role of environmental heterogeneity in shaping biodiversity.
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