Species delimitation and biogeography of the gnatcatchers and gnatwrens (Aves: Polioptilidae)

An Elevational Phylogeographic Diversity Gradient in Neotropical Birds Is Decoupled from Speciation Rates

AbstractA key question about macroevolutionary speciation rates is whether they are controlled by microevolutionary processes operating at the population level. For example, does spatial variation in population genetic differentiation underlie geographical gradients in speciation rates? Previous work suggests that speciation rates increase with elevation in Neotropical birds, but underlying population-level gradients remain unexplored. Here, we characterize elevational phylogeographic diversity between montane and lowland birds in the megadiverse Andes-Amazonian system and assess its relationship to speciation rates to evaluate the link between population-level differentiation and species-level diversification. We aggregated and georeferenced nearly 7,000 mitochondrial DNA sequences across 103 species or species complexes in the Andes and Amazonia and used these sequences to describe phylogeographic differentiation across both regions. Our results show increased levels of both discrete and continuous metrics of population structure in the Andean mountains compared with the Amazonian lowlands. However, higher levels of population differentiation do not predict higher rates of speciation in our dataset. Multiple potential factors may lead to our observed decoupling of initial population divergence and speciation rates, including the ephemerality of incipient species and the multifaceted nature of the speciation process, as well as methodological challenges associated with estimating rates of population differentiation and speciation.

The Genomic Landscapes of Desert Birds Form over Multiple Time Scales

Spatial models show that genetic differentiation between populations can be explained by factors ranging from geographic distance to environmental resistance across the landscape. However, genomes exhibit a landscape of differentiation, indicating that multiple processes may mediate divergence in different portions of the genome. We tested this idea by comparing alternative geographic predctors of differentiation in ten bird species that co-occur in Sonoran and Chihuahuan Deserts of North America. Using population-level genomic data, we described the genomic landscapes across species and modeled conditions that represented historical and contemporary mechanisms. The characteristics of genomic landscapes differed across species, influenced by varying levels of population structuring and admixture between deserts, and the best-fit models contrasted between the whole genome and partitions along the genome. Both historical and contemporary mechanisms were important in explaining genetic distance, but particularly past and current environments, suggesting that genomic evolution was modulated by climate and habitat There were also different best-ftit models across genomic partitions of the data, indicating that these regions capture different evolutionary histories. These results show that the genomic landscape of differentiation can be associated with alternative geographic factors operating on different portions of the genome, which reflect how heterogeneous patterns of genetic differentiation can evolve across species and genomes.

Empirical and philosophical problems with the subspecies rank

Species‐level taxonomy derives from empirical sources (data and techniques) that assess the existence of spatiotemporal evolutionary lineages via various species “concepts.” These concepts determine if observed lineages are independent given a particular methodology and ontology, which relates the metaphysical species concept to what “kind” of thing a species is in reality. Often, species concepts fail to link epistemology back to ontology. This lack of coherence is in part responsible for the persistence of the subspecies rank, which in modern usage often functions as a placeholder between the evolutionary events of divergence or collapse of incipient species. Thus, prospective events like lineages merging or diverging require information from unknowable future information. This is also conditioned on evidence that the lineage already has a detectably distinct evolutionary history. Ranking these lineages as subspecies can seem attractive given that many lineages do not exhibit intrinsic reproductive isolation. We argue that using subspecies is indefensible on philosophical and empirical grounds. Ontologically, the rank of subspecies is either identical to that of species or undefined in the context of evolutionary lineages representing spatiotemporally defined individuals. Some species concepts more inclined to consider subspecies, like the Biological Species Concept, are disconnected from evolutionary ontology and do not consider genealogy. Even if ontology is ignored, methods addressing reproductive isolation are often indirect and fail to capture the range of scenarios linking gene flow to species identity over space and time. The use of subspecies and reliance on reproductive isolation as a basis for an operational species concept can also conflict with ethical issues governing the protection of species. We provide a way forward for recognizing and naming species that links theoretical and operational species concepts regardless of the magnitude of reproductive isolation.

Common patterns in the molecular phylogeography of western palearctic birds: a comprehensive review

A plethora of studies have offered crucial insights in the phylogeographic status of Western Palearctic bird species. However, an overview integrating all this information and analyzing the combined results is still missing. In this study, we compiled all published peer-reviewed and grey literature available on the phylogeography of Western Palearctic bird species. Our literature review indicates a total number of 198 studies, with the overwhelming majority published as journal articles (n = 186). In total, these literature items offer information on 145 bird species. 85 of these species are characterized by low genetic differentiation, 46 species indicate genetic variation but no geographic structuring i.e. panmixia, while 14 species show geographically distinct lineages and haplotypes. Majority of bird species inhabiting the Western Palearctic display genetic admixture. The glaciation cycles in the past few million years were pivotal factors in shaping this situation: during warm periods many species expanded their distribution range to the north over wide areas of Eurasia; whereas, during ice ages most areas were no longer suitable and species retreated to refugia, where lineages mixed.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10336-021-01893-x.

Why is Amazonia a 'source' of biodiversity? Climate-mediated dispersal and synchronous speciation across the Andes in an avian group (Tityrinae)

Amazonia is a 'source' of biodiversity for other Neotropical ecosystems, but which conditions trigger in situ speciation and emigration is contentious. Three hypotheses for how communities have assembled include (1) a stochastic model wherein chance dispersal events lead to gradual emigration and species accumulation, (2) diversity-dependence wherein successful dispersal events decline through time due to ecological limits, and (3) barrier displacement wherein environmental change facilitates dispersal to other biomes via transient habitat corridors. We sequenced thousands of molecular markers for the Neotropical Tityrinae (Aves) and applied a novel filtering protocol to identify loci with high utility for dated phylogenomics. We used these loci to estimate divergence times and model Tityrinae's evolutionary history. We detected a prominent role for speciation driven by barriers including synchronous speciation across the Andes and found that dispersal increased toward the present. Because diversification was continuous but dispersal was non-random over time, we show that barrier displacement better explains Tityrinae's history than stochasticity or diversity-dependence. We propose that Amazonia is a source of biodiversity because (1) it is a relic of a biome that was once more extensive, (2) environmentally mediated corridors facilitated emigration and (3) constant diversification is attributed to a spatially heterogeneous landscape that is perpetually dynamic through time.