Social selection parapatry in Afrotropical sunbirds

Evolutionary and Ecological Processes Underlying Geographic Variation in Innate Bird Songs

AbstractEcological and evolutionary processes underlying spatial variation in signals involved in mate recognition and reproductive isolation are crucial to understanding the causes of population divergence and speciation. Here, to test hypotheses concerning the causes of song divergence, we examine how songs of two sister species of Atlantic Forest suboscine birds with innate songs, the Pyriglena fire-eye antbirds, vary across their ranges. Specifically, we evaluated the influence of isolation by distance and introgressive hybridization, as well as morphological and environmental variation, on geographic variation in male songs. Analyses based on 496 male vocalizations from 63 locations across a 2,200-km latitudinal transect revealed clinal changes in the structure of songs and showed that introgressive hybridization increases both the variability and the homogenization of songs in the contact zone between the two species. We also found that isolation by distance, morphological constraints, the environment, and genetic introgression independently predicted song variation across geographic space. Our study shows the importance of an integrative approach that investigates the roles of distinct ecological and evolutionary processes that influence acoustic signal evolution.

Climate Cycles, Habitat Stability, and Lineage Diversification in an African Biodiversity Hotspot

The Eastern Arc Mountains of Tanzania and Kenya, a montane archipelago of 13 uplifted fault blocks (sky islands) isolated by lowland arid savanna, are a center of exceptional biological endemism. Under the influence of humid winds from the Indian Ocean, forests and associated species may have persisted in this region since the final uplift of these blocks in the late Miocene. Today, these mountains are inhabited by a remarkable diversity of bird species. To better understand the evolutionary processes behind this diversity, we combined molecular phylogenetic studies of East African montane birds with paleoclimate modeling of its montane forests. Across its largest lowland barrier, the 125 km between the Usambara and Nguru/Nguu Mountains, 10 of the 14 bird lineages exhibited a phylogeographic break. Using Bayesian methods, we established that at least three periods of forest contraction and expansion affected the diversification of Eastern Arc birds. Habitat distribution models suggest that lower-elevation hills may have acted as stepping-stones connecting isolated highlands to allow for the dispersal of montane forest-dependent species across them. Periods of vicariance during paleoclimatic cycles extending back through the Last Glacial Maximum would have then isolated these populations within the highlands they had reached. The broad distribution of neoendemic species across the mountains of East Africa provides evidence of climate cycling as a driver of lineage diversification. The high incidence of narrow-range endemism of paleoendemic species on the Usambara, Uluguru, and Udzungwa Mountains of this region is harder to explain. Our paleoclimate models retrodicted the persistence of montane forest during climate cycles on several Eastern Arc sky islands but not on the Southern Tanzania Volcanic Highlands. Consistent with recent theoretical work, different rates of local extinction rather than increased rates of lineage diversification may explain the pattern of excessive narrow-range endemism on some sky islands over others. Thus, a regional filtering effect is generated, with paleoendemics maintaining populations through time only in areas where habitat persisted, providing a credible explanation for the dramatic variance in levels of endemism among different East African sky islands.

Deep ecomorphological and genetic divergence in Steller's Jays (Cyanocitta stelleri, Aves: Corvidae)

The relationship between ecology and morphology is a cornerstone of evolutionary biology, and quantifying variation across environments can shed light on processes that give rise to biodiversity. Three morphotypes of the Steller's Jay (Cyanocitta stelleri) occupy different ecoregions in western North America, which vary in climate and landcover. These morphotypes (Coastal, Interior, Rocky Mountain) differ in size, plumage coloration, and head pattern. We sampled 1080 Steller's Jays from 68 populations (plus 11 outgroups) to address three main questions using data on morphology, plumage, genetics (mtDNA, microsatellites), and ecological niches: (1) How do phenotypic and genetic traits vary within and among populations, morphotypes, and ecoregions? (2) How do population‐level differences in Steller's Jays compare with other sister species pairs of North American birds? (3) What can we infer about the population history of Steller's Jays in relation to past climates, paleoecology, and niche evolution? We found substantial morphological, genetic, and ecological differentiation among morphotypes. The greatest genetic divergence separated Coastal and Interior morphotypes from the Rocky Mountain morphotype, which was associated with warmer, drier, and more open habitats. Microsatellites revealed additional structure between Coastal and Interior groups. The deep mtDNA split between Coastal/Interior and Rocky Mountain lineages of Steller's Jay (ND2 ~ 7.8%) is older than most North American avian sister species and dates to approximately 4.3 mya. Interior and Rocky Mountain morphotypes contact across a narrow zone with steep clines in traits and reduced gene flow. The distribution of the three morphotypes coincides with divergent varieties of ponderosa pine and Douglas fir. Species distribution models support multiple glacial refugia for Steller's Jays. Our integrative dataset combined with extensive geographic sampling provides compelling evidence for recognizing at least two species of Steller's Jay.

Punctuated evolution in the learned songs of African sunbirds

Learned traits are thought to be subject to different evolutionary dynamics than other phenotypes, but their evolutionary tempo and mode has received little attention. Learned bird song has been thought to be subject to rapid and constant evolution. However, we know little about the evolutionary modes of learned song divergence over long timescales. Here, we provide evidence that aspects of the territorial songs of Eastern Afromontane sky island sunbirds Cinnyris evolve in a punctuated fashion, with periods of stasis of the order of hundreds of thousands of years or more, broken up by evolutionary pulses. Stasis in learned songs is inconsistent with learned traits being subject to constant or frequent change, as would be expected if selection does not constrain song phenotypes over evolutionary timescales. Learned song may instead follow a process resembling peak shifts on adaptive landscapes. While much research has focused on the potential for rapid evolution in bird song, our results suggest that selection can tightly constrain the evolution of learned songs over long timescales. More broadly, these results demonstrate that some aspects of highly variable, plastic traits can exhibit punctuated evolution, with stasis over long time periods.

Primary hybrid zone formation in Tephroseris helenitis (Asteraceae), following postglacial range expansion along the central Northern Alps

Distinguishing between secondary versus primary hybrid zone formation remains a challenging task as, for instance, the time window in which these historical (vicariant) versus contemporary (environmental-selective) processes are distinguishable may be relatively narrow. Here, we examine the origin and structure of a transition zone between two subspecies of Tephroseris helenitis along the central Northern Alps, using molecular (AFLP) and morphological (achene type) data in combination with ecological niche models (ENMs) to hindcast ranges at the Last Glacial Maximum (LGM) and mid-Holocene. Samples were collected over a c. 350 km long transect, largely covered by ice during the LGM. Genetically nonadmixed individuals of subspp. helenitis versus salisburgensis dominated the westernmost versus eastern transect areas, with admixed individuals occurring in between. Clines for achene morphology and outlier loci potentially under climate-driven selection were steep, largely noncoincidental, and displaced to the east of the cline centre for neutral AFLPs. During the LGM, ssp. helenitis should have been able to persist in a refugium southwest of the transect, while suitable habitat for ssp. salisburgensis was apparently absent at this time. Together with patterns of genetic and clinal variation, our ENM data are suggestive of a primary hybrid zone that originated after the species' postglacial, eastward expansion. The observed clinal changes may thus reflect random/nonadaptive processes during expansion and selection on particular loci, and possibly achene type, in response to a long-term, west-to-east climate gradient in the direction of more stressful (e.g., wetter/cooler) conditions. Overall, this study adds to the vast hybrid zone literature a rare example of a hybrid zone caused by primary differentiation within a plant species, underlaid by historical range expansion.

Dispersal Predicts Hybrid Zone Widths across Animal Diversity: Implications for Species Borders under Incomplete Reproductive Isolation

Hybrid zones occur as range boundaries for many animal taxa. One model for how hybrid zones form and stabilize is the tension zone model, a version of which predicts that hybrid zone widths are determined by a balance between random dispersal into hybrid zones and selection against hybrids. Here, we examine whether random dispersal and proxies for selection against hybrids (genetic distances between hybridizing pairs) can explain variation in hybrid zone widths across 131 hybridizing pairs of animals. We show that these factors alone can explain ∼40% of the variation in zone width among animal hybrid zones, with dispersal explaining far more of the variation than genetic distances. Patterns within clades were idiosyncratic. Genetic distances predicted hybrid zone widths particularly well for reptiles, while this relationship was opposite tension zone predictions in birds. Last, the data suggest that dispersal and molecular divergence set lower bounds on hybrid zone widths in animals, indicating that there are geographic restrictions on hybrid zone formation. Overall, our analyses reinforce the fundamental importance of dispersal in hybrid zone formation and more generally in the ecology of range boundaries.

Behavioral Isolation and Incipient Speciation in Birds

Behavioral changes, such as those involved in mating, foraging, and migration, can generate reproductive barriers between populations. Birds, in particular, are known for their great diversity in these behaviors, and so behavioral isolation is often proposed to be the major driver of speciation. Here, we review empirical evidence to evaluate the importance of behavioral isolation in the early stages of avian speciation. Experimentally measured mating preferences indicate that changes in mating behavior can result in premating barriers, with their strength depending on the extent of divergence in mating signals. Differences in migratory and foraging behavior also can play important roles in generating reproductive barriers in the early stages of speciation. However, because premating behavioral isolation is imperfect, extrinsic postzygotic barriers, in the form of selection against hybrids having intermediate phenotypes, also play an important role in avian diversification, especially in completing the speciation process.

The systematics and biogeography of African tailorbirds (Cisticolidae: Artisornis) with comment on the choice of Bayesian branch-length prior when analyzing heterogeneous data

The Long-billed Tailorbird (Artisornis moreaui), one of Africa's rarest birds, has a strikingly disjunct distribution, the origin of which has long puzzled biogeographers. One small population (subspecies moreaui) occurs in sub-montane forest in the East Usambara Mountains, a sky island near the coast of northern Tanzania, and another (subspecies sousae) on Serra Jeci in northwestern Mozambique, 950km away. The African Tailorbird, the putative sister-species of Long-billed Tailorbird, also occurs in the East Usambara Mountains and on Serra Jeci, but in addition occupies all the Eastern Arc Mountain forests between these disjunct sites. Stuart (1981) hypothesized that the two tailorbird distributions could be explained by strong ecological competition, with African Tailorbird populations having eliminated Long-billed Tailorbird populations via competitive exclusion in montane forests between the East Usambara and Serra Jeci. If such competitive exclusion explains these geographic distributions, the co-occurrence of the two species in the East Usambara and at Serra Jeci may be ephemeral, with the status of Long-billed Tailorbird especially in doubt. We sought to (1) determine whether the two species of African tailorbirds are indeed sister-species, and (2) test predictions from Stuart's (1981) competitive exclusion hypothesis using genetic data. Phylogenetic analyses of our seven gene dataset (3 mtDNA, 4 introns; 4784bp) indeed place these two species together in the genus Artisornis. Instead of finding shallow divergence among African Tailorbird populations and deep divergence between Long-billed Tailorbird populations as expected from Stuart's hypothesis, we recover deep genetic divergence and geographic structure among populations of both tailorbird species. This result is consistent with long-term co-existence of the two species at East Usambara and Serra Jeci. Observational data from both the East Usambara and Serra Jeci suggest that the two species have diverged in use of forest canopy strata. From a conservation standpoint, our results suggest that extinction of the Long-billed Tailorbird as a function of competition with African Tailorbird is highly unlikely, and should not be viewed as imminent. Threats to its survival are instead anthropogenic, and conservation measures should take this into account. Finally, our empirical results suggest that mis-specification of the branch-length prior in Bayesian analyses of mitochondrial DNA data can have a profound effect on the overall tree-length (sum of branch-lengths), whereas the topology and support values tend to remain more stable. In contrast, mis-specification of the branch-length prior had a lesser impact on all aspects of the nuclear-only DNA analyses. This problem may be exacerbated when mitochondrial and nuclear DNA analyses are combined in a total evidence approach.