Rethinking the dispersal of Homo sapiens out of Africa

Current fossil, genetic, and archeological data indicate that Homo sapiens originated in Africa in the late Middle Pleistocene. By the end of the Late Pleistocene, our species was distributed across every continent except Antarctica, setting the foundations for the subsequent demographic and cultural changes of the Holocene. The intervening processes remain intensely debated and a key theme in hominin evolutionary studies. We review archeological, fossil, environmental, and genetic data to evaluate the current state of knowledge on the dispersal of Homo sapiens out of Africa. The emerging picture of the dispersal process suggests dynamic behavioral variability, complex interactions between populations, and an intricate genetic and cultural legacy. This evolutionary and historical complexity challenges simple narratives and suggests that hybrid models and the testing of explicit hypotheses are required to understand the expansion of Homo sapiens into Eurasia.     

Long‐term change in a behavioural trait: truncated spawning distribution and demography in Northeast Arctic cod

Harvesting may be a potent driver of demographic change and contemporary evolution, which both may have great impacts on animal populations. Research has focused on changes in phenotypic traits that are easily quantifiable and for which time series exist, such as size, age, sex, or gonad size, whereas potential changes in behavioural traits have been under‐studied. Here, we analyse potential drivers of long‐term changes in a behavioural trait for the Northeast Arctic stock of Atlantic cod Gadus morhua, namely choice of spawning location. For 104 years (1866–1969), commercial catches were recorded annually and reported by county along the Norwegian coast. During this time period, spawning ground distribution has fluctuated with a trend towards more northerly spawning. Spawning location is analysed against a suite of explanatory factors including climate, fishing pressure, density dependence, and demography. We find that demography (age or age at maturation) had the highest explanatory power for variation in spawning location, while climate had a limited effect below statistical significance. As to potential mechanisms, some effects of climate may act through demography, and explanatory variables for demography may also have absorbed direct evolutionary change in migration distance for which proxies were unavailable. Despite these caveats, we argue that fishing mortality, either through demographic or evolutionary change, has served as an effective driver for changing spawning locations in cod, and that additional explanatory factors related to climate add no significant information.     

Isolation‐driven divergence: speciation in a widespread North American songbird (Aves: Certhiidae)

Lineage, or true ‘species’, trees may differ from gene trees because of stochastic processes in molecular evolution leading to gene‐tree heterogeneity. Problems with inferring species trees because of excessive incomplete lineage sorting may be exacerbated in lineages with rapid diversification or recent divergences necessitating the use of multiple loci and individuals. Many recent multilocus studies that investigate divergence times identify lineage splitting to be more recent than single‐locus studies, forcing the revision of biogeographic scenarios driving divergence. Here, we use 21 nuclear loci from regional populations to re‐evaluate hypotheses identified in an mtDNA phylogeographic study of the Brown Creeper (Certhia americana), as well as identify processes driving divergence. Nuclear phylogeographic analyses identified hierarchical genetic structure, supporting a basal split at approximately 32°N latitude, splitting northern and southern populations, with mixed patterns of genealogical concordance and discordance between data sets within the major lineages. Coalescent‐based analyses identify isolation, with little to no gene flow, as the primary driver of divergence between lineages. Recent isolation appears to have caused genetic bottlenecks in populations in the Sierra Madre Oriental and coastal mountain ranges of California, which may be targets for conservation concerns.     

Reconstruction of a beech population bottleneck using archival demographic information and Bayesian analysis of genetic data

Range expansion and contraction has occurred in the history of most species and can seriously impact patterns of genetic diversity. Historical data about range change are rare and generally appropriate for studies at large scales, whereas the individual pollen and seed dispersal events that form the basis of geneflow and colonization generally occur at a local scale. In this study, we investigated range change in Fagus sylvatica on Mont Ventoux, France, using historical data from 1838 to the present and approximate Bayesian computation (ABC) analyses of genetic data. From the historical data, we identified a population minimum in 1845 and located remnant populations at least 200 years old. The ABC analysis selected a demographic scenario with three populations, corresponding to two remnant populations and one area of recent expansion. It also identified expansion from a smaller ancestral population but did not find that this expansion followed a population bottleneck, as suggested by the historical data. Despite a strong support to the selected scenario for our data set, the ABC approach showed a low power to discriminate among scenarios on average and a low ability to accurately estimate effective population sizes and divergence dates, probably due to the temporal scale of the study. This study provides an unusual opportunity to test ABC analysis in a system with a well-documented demographic history and identify discrepancies between the results of historical, classical population genetic and ABC analyses. The results also provide valuable insights into genetic processes at work at a fine spatial and temporal scale in range change and colonization.     

Inter‐island divergence within Drosophila mauritiana,a species of the D. simulans complex: Past history and/or speciation in progress?

Speciation with gene flow may be more common than generally thought, which makes detailed understanding of the extent and pattern of genetic divergence between geographically isolated populations useful. Species of the Drosophila simulans complex provide a good model for speciation and evolutionary studies, and hence understanding their population genetic structure will increase our understanding of the context in which speciation has occurred. Here, we describe genetic diversity and genetic differentiation of two distant populations of D. mauritiana (Mauritius and Rodrigues Islands) at mitochondrial and nuclear loci. We surveyed the two populations for their mitochondrial haplotypes, eight nuclear genes and 18 microsatellite loci. A new mitochondrial type is fixed in the Rodrigues population of D. mauritiana. The two populations are highly differentiated, their divergence appears relatively ancient (100,000 years) compared to the origin of the species, around 0.25MYA, and they exhibit very limited gene flow. However, they have similar levels of divergence from their sibling, D. simulans. Both nuclear genes and microsatellites revealed contrasting demographic histories between the two populations, expansion for the Mauritius population and stable population size for the Rodrigues Island population. The discovery of pronounced geographic structure within D. mauritiana combined to genetic structuring and low gene flow between the two island populations illuminates the evolutionary history of the species and clearly merits further attention in the broad context of speciation.     

Tree dieback and regeneration in secondary Acacia zanzibarica woodlands on an abandoned cattle ranch in coastal Tanzania

Questions: Bush encroachment is a major problem when African savanna ecosystems are used for cattle ranching. How do secondary woodlands develop after ranching is abandoned? What are the patterns and rates of tree mortality and regeneration? Location: Mkwaja Ranch (now part of Saadani National Park) in coastal Tanzania. Methods: Ninety‐seven circular plots (4‐m radius) were set in secondary Acacia zanzibarica woodland along a gradient of tree density. Variables relating to tree and grass layers and soil characteristics of plots were recorded. Seedlings were counted twice in the wet seasons, and resprouts once. Tree flowering and pod production were assessed during the fruiting season, while survival of trees initially present was recorded after 12 and 32 months. Interrelationships among variables were investigated using multiple linear regression, binary logistic regression and mixed effects models. Results: After 32 months, over one‐third of trees in plots had died. Most died after fire, especially on heavy soils, and mortality was significantly related to the tree live biomass ratio and soil conditions. Seed production was very low, especially in denser stands. Numbers of seedlings correlated with soil and grass variables but not with seed rain. Half of trees killed above‐ground produced coppice shoots from the base; in contrast, root suckering was independent of topkill. By the end of the study, no seedlings survived and no resprouts emerged above the grass layer. Conclusions: A. zanzibarica woodlands at Mkwaja Ranch were able to develop because of ranching, and can only persist under intensive grazing. The woodlands do not represent a successional stage towards forest and will probably revert to predominantly grassland vegetation within 10–20 years unless grazing pressure from wild ungulates increases considerably and/or fire regimes change.     

Spread of North American wind-dispersed trees in future environments

Despite ample research, understanding plant spread and predicting their ability to track projected climate changes remain a formidable challenge to be confronted. We modelled the spread of North American wind-dispersed trees in current and future (c. 2060) conditions, accounting for variation in 10 key dispersal, demographic and environmental factors affecting population spread. Predicted spread rates vary substantially among 12 study species, primarily due to inter-specific variation in maturation age, fecundity and seed terminal velocity. Future spread is predicted to be faster if atmospheric CO(2) enrichment would increase fecundity and advance maturation, irrespective of the projected changes in mean surface windspeed. Yet, for only a few species, predicted wind-driven spread will match future climate changes, conditioned on seed abscission occurring only in strong winds and environmental conditions favouring high survival of the farthest-dispersed seeds. Because such conditions are unlikely, North American wind-dispersed trees are expected to lag behind the projected climate range shift.     

Influence of physical environmental characteristics and anthropogenic factors on the position and structure of a contact zone between two chromosomal races of the house mouse on the island of Madeira (North Atlantic, Portugal)

Aim This study aimed to investigate if and how environmental characteristics (physical factors of the natural environment and the human impact on the landscape) influence the position and structure of a contact zone between two chromosomal races of the house mouse (Mus musculus domesticus Rutty 1712) from the island of Madeira. Location The western part of Madeira, a volcanic island in the North Atlantic. Methods Mice were sampled along a south/north‐western transect following the main road, in human‐modified outdoor habitats. Karyotypes of mice were determined using the yeast‐stimulated bone marrow cell method. Trapping sites were characterized in terms of their physical (altitude, temperature, precipitation and soil type) and habitat (human landscape use and occupancy) features. Demographic parameters of mouse populations, based on trapping‐with‐removal techniques, were also analysed (relative abundance, sex‐ratio, juvenile ratio and female fertility ratio), as well as body size (weight and length). Results Four chromosomal zones were identified on the basis of the frequency of two diagnostic rearrangements (Rb(6.7) in race E. Calheta and Rb(7.15) in race A. Cruz). E. Calheta was present in the two southern‐most zones, followed by the contact zone characterized by the presence of two inter‐racial hybrids and the co‐occurrence of mice belonging to the two races. The northern‐most part of the transect was occupied by A. Cruz. Environmental features differed leading us to split the transect into two parts. The southern part is characterized by lower altitude and precipitation, milder temperature, better soil quality supporting vegetable crops and vineyards, and more abundant and evenly distributed human habitats. This southern part is occupied by E. Calheta mice. The north‐western part presents characteristics opposite to those described above with cereals as the main cultivated crop, and it includes the contact zone as well as the zone inhabited by A. Cruz mice. The demographic parameters evaluated in this study did not differ significantly between chromosomal zones. Main conclusions This ecological survey highlights differences in climatic and edaphic features that have moulded the agricultural activities of humans, contributing to a differentiation of their spatial development, and hence the structure of potential habitats for mice. Results are interpreted within the source–sink framework of population dynamics, following which E. Calheta may function as a source and the areas where A. Cruz and the contact zone are located may function as a sink. Our study suggests that the position and chromosomal composition of the contact zone is influenced by the human component underlying broader environmental features. Similar characteristics were most likely present during the historical settlement of Madeira. They may have favoured the independent divergence of the two races and influenced the dynamics of the contact zone.     

Patterns of male residency and intergroup transfer in gray-cheeked mangabeys (Lophocebus albigena)

We describe the movements and fates of 36 collared gray-cheeked male mangabeys (Lophocebus albigena) that resided in seven social groups in Kibale National Park, Uganda. The male mangabeys were captured, radiotagged, and then contacted regularly over a period of up to 8 years. Individuals varied considerably in how tightly they were associated with their groups. "Central" males were usually found in close spatial association with their own groups, but they sometimes visited others. "Peripheral" males were often found several hundred meters away from their own groups. Most adult males were characterized by repeated intergroup transfers, manifested as "visits" and "dispersals." Visits were transfers that did not result in mating. The visits were brief, and ranged from a few minutes to a few days before the male returned to his previous group, moved on to another group, or returned to solitary life. The term "dispersal" refers to a transfer that results in mating, after which the male remains in the new group. Young males began to drift away from their natal groups in early subadulthood, but the timing of first associations with estrous females in other groups was highly variable. Natal dispersers were generally solitary for a month or more, and at least half moved into nonadjacent groups. Secondary dispersal was common. After they immigrated, adult males remained in a group for a median of 19 months before they emigrated again, with the probability of departure being approximately constant in time. Secondary dispersal was usually made into an adjacent group and lasted less than 7 days. Emigration and immigration were not seasonal, and males emigrated singly and apparently independently; however, immigrations were clumped in time, and male dispersal contributed to considerable heterogeneity of group composition. L. albigena dispersal patterns are intermediate between those described for Papio baboons and forest Cercopithecus, and models that predict demographic consequences of dispersal for baboons can be refined to apply to mangabeys.