Genetic drift in range expansions is very sensitive to density dependence in dispersal and growth

Theory predicts rapid genetic drift during invasions, yet many expanding populations maintain high genetic diversity. We find that genetic drift is dramatically suppressed when dispersal rates increase with the population density because many more migrants from the diverse, high‐density regions arrive at the expansion edge. When density dependence is weak or negative, the effective population size of the front scales only logarithmically with the carrying capacity. The dependence, however, switches to a sublinear power law and then to a linear increase as the density dependence becomes strongly positive. We develop a unified framework revealing that the transitions between different regimes of diversity loss are controlled by a single, universal quantity: the ratio of the expansion velocity to the geometric mean of dispersal and growth rates at expansion edge. Our results suggest that positive density dependence could dramatically alter evolution in expanding populations even when its contribution to the expansion velocity is small.     

Fine-scale genetic structure and marginal processes in an expanding population of Biscutella laevigata L. (Brassicaceae)

Evolutionary processes acting at the expanding margins of a species' range are still poorly understood. Genetic drift is considered prevalent in marginal populations, and the maintenance of genetic diversity during recolonization might seem puzzling. To investigate such processes, a fine-scale investigation of 219 individuals was performed within a population of Biscutella laevigata (Brassicaceae), located at the leading edge of its range. The survey used amplified fragment length polymorphisms (AFLPs). As commonly reported across the whole species distribution range, individual density and genetic diversity decreased along the local axis of recolonization of this expanding population, highlighting the enduring effect of the historical colonization on present-day diversity. The self-incompatibility system of the plant may have prevented local inbreeding in newly found patches and sustained genetic diversity by ensuring gene flow from established populations. Within the more continuously populated region, spatial analysis of genetic structure revealed restricted gene flow among individuals. The distribution of genotypes formed a mosaic of relatively homogenous patches within the continuous population. This pattern could be explained by a history of expansion by long-distance dispersal followed by fine-scale diffusion (that is, a stratified dispersal combination). The secondary contact among expanding patches apparently led to admixture among differentiated genotypes where they met (that is, a reshuffling effect). This type of dynamics could explain the maintenance of genetic diversity during recolonization.     

Limiting inbreeding in disjunct and isolated populations of a woody shrub

Pollen movements and mating patterns are key features that influence population genetic structure. When gene flow is low, small populations are prone to increased genetic drift and inbreeding, but naturally disjunct species may have features that reduce inbreeding and contribute to their persistence despite genetic isolation. Using microsatellite loci, we investigated outcrossing levels, family mating parameters, pollen dispersal, and spatial genetic structure in three populations of Hakea oldfieldii, a fire‐sensitive shrub with naturally disjunct, isolated populations prone to reduction in size and extinction following fires. We mapped and genotyped a sample of 102 plants from a large population, and all plants from two smaller populations (28 and 20 individuals), and genotyped 158–210 progeny from each population. We found high outcrossing despite the possibility of geitonogamous pollination, small amounts of biparental inbreeding, a limited number of successful pollen parents within populations, and significant correlated paternity. The number of pollen parents for each seed parent was moderate. There was low but significant spatial genetic structure up to 10 m around plants, but the majority of successful pollen came from outside this area including substantial proportions from distant plants within populations. Seed production varied among seven populations investigated but was not correlated with census population size. We suggest there may be a mechanism to prevent self‐pollination in H. oldfieldii and that high outcrossing and pollen dispersal within populations would promote genetic diversity among the relatively small amount of seed stored in the canopy. These features of the mating system would contribute to the persistence of genetically isolated populations prone to fluctuations in size.     

Genetic load,inbreeding depression,and hybrid vigor covary with population size: An empirical evaluation of theoretical predictions

Reduced population size is thought to have strong consequences for evolutionary processes as it enhances the strength of genetic drift. In its interaction with selection, this is predicted to increase the genetic load, reduce inbreeding depression, and increase hybrid vigor, and in turn affect phenotypic evolution. Several of these predictions have been tested, but comprehensive studies controlling for confounding factors are scarce. Here, we show that populations of Daphnia magna, which vary strongly in genetic diversity, also differ in genetic load, inbreeding depression, and hybrid vigor in a way that strongly supports theoretical predictions. Inbreeding depression is positively correlated with genetic diversity (a proxy for N_e), and genetic load and hybrid vigor are negatively correlated with genetic diversity. These patterns remain significant after accounting for potential confounding factors and indicate that, in small populations, a large proportion of the segregation load is converted into fixed load. Overall, the results suggest that the nature of genetic variation for fitness‐related traits differs strongly between large and small populations. This has large consequences for evolutionary processes in natural populations, such as selection on dispersal, breeding systems, ageing, and local adaptation.     

Genetic connectivity and diversity in inselberg populations of Acacia woodmaniorum,a rare endemic of the Yilgarn Craton banded iron formations

Historically rare plant species with disjunct population distributions and small population sizes might be expected to show significant genetic structure and low levels of genetic diversity because of the effects of inbreeding and genetic drift. Across the globe, terrestrial inselbergs are habitat for rich, often rare and endemic flora and are valuable systems for investigating evolutionary processes that shape patterns of genetic structure and levels of genetic diversity at the landscape scale. We assessed genetic structure and levels of genetic diversity across the range of the historically rare inselberg endemic Acacia woodmaniorum. Phylogeographic and genetic structure indicates that connectivity is not sufficient to produce a panmictic population across the limited geographic range of the species. However, historical levels of gene flow are sufficient to maintain a high degree of adaptive connectivity across the landscape. Genetic diversity indicates gene flow is sufficient to largely counteract any negative genetic effects of inbreeding and random genetic drift in even the most disjunct or smallest populations. Phylogeographic and genetic structure, a signal of isolation by distance and a lack of evidence of recent genetic bottlenecks suggest long-term stability of contemporary population distributions and population sizes. There is some evidence that genetic connectivity among disjunct outcrops may be facilitated by the occasional long distance dispersal of Acacia polyads carried by insect pollinators moved by prevailing winds.     

Pursuing glacier retreat: genetic structure of a rapidly expanding Larix decidua population

One of the greatest threats to the long-term viability of migrating plant species is the loss of genetic diversity due to founder effects. Populations can expand as a response to climate change, but it is uncertain if long-lived plant species can maintain sufficient genetic diversity at the leading edge of migrating populations. This study uses an expanding Larix decidua population investigated along a chronosequence at landscape (350 ha) and local (0.8 ha) scales to test whether accelerated migration as a result of climate warming has the potential to intensify genetic erosion. Nine SSR markers revealed similar genetic diversity among eight sub-populations along the chronosequence (overall H(e) = 0.73; SE=0.04). Sub-populations were not genetically differentiated and all sampled individuals (N=730) formed one major genetic cluster indicating homogenizing gene flow despite spatial genetic structure (SGS) up to 80 m. At the local scale, individuals at the leading edge [early successional sub-population (ESSP), N =140] and a sub-population at equilibrium [late successional sub-population (LSSP), N = 290] revealed high genetic diversity in largest-sized cohorts. SGS among juveniles occurred up to 30 m in LSSP but there was no structure in ESSP. Accordingly, a maximum likelihood paternity assignment revealed local gene dispersal in LSSP (2-48 m) and intermediate-to-long distance dispersal into ESSP (115-3132 m). The findings indicate intensive mixing of the genes in this expanding population instead of founder effects and support the view that genetic diversity can be maintained in a long-lived species during rapid population expansion driven by climate warming.     

Species diversity and population density affect genetic structure and gene dispersal in a subtropical understory shrub

Aims The dispersal of pollen and seeds is spatially restricted and may vary among plant populations because of varying biotic interactions, population histories or abiotic conditions. Because gene dispersal is spatially restricted, it will eventually result in the development of spatial genetic structure (SGS), which in turn can allow insights into gene dispersal processes. Here, we assessed the effect of habitat characteristics like population density and community structure on small-scale SGS and estimate historical gene dispersal at different spatial scales.Methods In a set of 12 populations of the subtropical understory shrub Ardisia crenata, we assessed genetic variation at 7 microsatellite loci within and among populations. We investigated small-scale genetic structure with spatial genetic autocorrelation statistics and heterogeneity tests and estimated gene dispersal distances based on population differentiation and on within-population SGS. SGS was related to habitat characteristics by multiple regression.Important findings The populations showed high genetic diversity (H e = 0.64) within populations and rather strong genetic differentiation (F ′ ST = 0.208) among populations, following an isolation-by-distance pattern, which suggests that populations are in gene flow–drift equilibrium. Significant SGS was present within populations (mean Sp = 0.027). Population density and species diversity had a joint effect on SGS with low population density and high species diversity leading to stronger small-scale SGS. Estimates of historical gene dispersal from between-population differentiation and from within-population SGS resulted in similar values between 4.8 and 22.9 m. The results indicate that local-ranged pollen dispersal and inefficient long-distance seed dispersal, both affected by population density and species diversity, contributed to the genetic population structure of the species. We suggest that SGS in shrubs is more similar to that of herbs than to trees and that in communities with high species diversity gene flow is more restricted than at low species diversity. This may represent a process that retards the development of a positive species diversity–genetic diversity relationship.     

Rapid range expansion increases genetic differentiation while causing limited reduction in genetic diversity in a damselfly

Many ectothermic species are currently expanding their geographic range due to global warming. This can modify the population genetic diversity and structure of these species because of genetic drift during the colonization of new areas. Although the genetic signatures of historical range expansions have been investigated in an array of species, the genetic consequences of natural, contemporary range expansions have received little attention, with the only studies available focusing on range expansions along a narrow front. We investigate the genetic consequences of a natural range expansion in the Mediterranean damselfly Coenagrion scitulum, which is currently rapidly expanding along a broad front in different directions. We assessed genetic diversity and genetic structure using 12 microsatellite markers in five centrally located populations and five recently established populations at the edge of the geographic distribution. Our results suggest that, although a marginal significant decrease in the allelic richness was found in the edge populations, genetic diversity has been preserved during the range expansion of this species. Nevertheless, edge populations were genetically more differentiated compared with core populations, suggesting genetic drift during the range expansion. The smaller effective population sizes of the edge populations compared with central populations also suggest a contribution of genetic drift after colonization. We argue and document that range expansion along multiple axes of a broad expansion front generates little reduction in genetic diversity, yet stronger differentiation of the edge populations.     

Restricted mating dispersal and strong breeding group structure in a mid-sized marsupial mammal (Petrogale penicillata)

Ecological genetic studies have demonstrated that spatial patterns of mating dispersal, the dispersal of gametes through mating behaviour, can facilitate inbreeding avoidance and strongly influence the structure of populations, particularly in highly philopatric species. Elements of breeding group dynamics, such as strong structuring and sex-biased dispersal among groups, can also minimize inbreeding and positively influence levels of genetic diversity within populations. Rock-wallabies are highly philopatric mid-sized mammals whose strong dependence on rocky terrain has resulted in series of discreet, small colonies in the landscape. Populations show no signs of inbreeding and maintain high levels of genetic diversity despite strong patterns of limited gene flow within and among colonies. We used this species to investigate the importance of mating dispersal and breeding group structure to inbreeding avoidance within a 'small' population. We examined the spatial patterns of mating dispersal, the extent of kinship within breeding groups, and the degree of relatedness among brush-tailed rock-wallaby breeding pairs within a colony in southeast Queensland. Parentage data revealed remarkably restricted mating dispersal and strong breeding group structuring for a mid-sized mammal. Breeding groups showed significant levels of female kinship with evidence of male dispersal among groups. We found no evidence for inbreeding avoidance through mate choice; however, anecdotal data suggest the importance of life history traits to inbreeding avoidance between first-degree relatives. We suggest that the restricted pattern of mating dispersal and strong breeding group structuring facilitates inbreeding avoidance within colonies. These results provide insight into the population structure and maintenance of genetic diversity within colonies of the threatened brush-tailed rock-wallaby.     

Determinants of extinction in fragmented plant populations: Crepis sancta (asteraceae) in urban environments

Local populations are subject to recurrent extinctions, and small populations are particularly prone to extinction. Both demographic (stochasticity and the Allee effect) and genetic factors (drift load and inbreeding depression) potentially affect extinction. In fragmented populations, regular dispersal may boost population sizes (demographic rescue effect) or/and reduce the local inbreeding level and genetic drift (genetic rescue effect), which can affect extinction risks. We studied extinction processes in highly fragmented populations of the common species Crepis sancta (Asteraceae) in urban habitats exhibiting a rapid turnover of patches. A four-year demographic monitoring survey and microsatellite genotyping of individuals allowed us to study the determinants of extinction. We documented a low genetic structure and an absence of inbreeding (estimated by multilocus heterozygosity), which suggest that genetic factors were not a major cause of patch extinction. On the contrary, local population size was the main factor in extinction, whereas connectivity was shown to decrease patch extinction, which we interpreted as a demographic rescue effect that was likely due to better pollination services for reproduction. This coupling of demographic and genetic tools highlighted the importance of dispersal in local patch extinctions of small fragmented populations connected by gene flow.     

Genetic characterization of free-ranging Asiatic wild ass in Central Asia as a basis for future conservation strategies

Loss of genetic diversity due to drift and inbreeding reduces a population’s ability to respond to environmental change and may result in inbreeding depression. The Asiatic wild ass (Equus hemionus), regionally also known as Gobi khulan, Turkmen kulan, or Persian onager, has become confined to less than 3% of its historic distribution range. Remaining populations in Central Asia outside of the Mongolian Gobi are small and fragmented. Questions concerning subpopulation status remain disputed and concerns over the viability of these populations have been raised because of small size, past bottlenecks, or recent founder events. We used non-invasive faecal samples to assess the genetic diversity and divergence among Turkmen kulan and Persian onager from five free-ranging and one captive population from Turkmenistan, Kazakhstan and Iran and compared their genetic constitution to the large autochthonous population in the Mongolian Gobi. We observed loss of genetic diversity (drift and inbreeding) in the captive and reintroduced populations as well as in one rapidly declining autochthonous population. Population differentiation and structure using microsatellites and mtDNA based phylogenetic analysis do not support the current separation of the autochthonous populations of Turkmen kulan and Persian onager into different subspecies, but rather suggest a cline with the Iranian population in Bahram-e-Goor at the southern end and the Turkmen population in Badhyz at the northern end falling into two distinct clusters, and the northern Iranian population in Touran being intermediate. We compare our findings to other population genetics studies of equids and discuss the implications of our findings for the future conservation of the Asiatic wild ass in the region.     

Comparison of contemporary mating patterns in continuous and fragmented Eucalyptus globulus native forests

While habitat fragmentation is a central issue in forest conservation studies in the face of broad-scale anthropogenic changes to the environment, its effects on contemporary mating patterns remain controversial. This is partly because of the inherent variation in mating patterns which may exist within species and the fact that few studies have replication at the landscape level. To study the effect of forest fragmentation on contemporary mating patterns, including effective pollen dispersal, we compared four native populations of the Australian forest tree, Eucalyptus globulus . We used six microsatellite markers to genotype 1289 open-pollinated offspring from paired fragmented and continuous populations on the island of Tasmania and in Victoria on mainland Australia. The mating patterns in the two continuous populations were similar, despite large differences in population density. In contrast, the two fragmented populations were variable and idiosyncratic in their mating patterns, particularly in their pollen dispersal kernels. The continuous populations showed relatively high outcrossing rates (86–89%) and low correlated paternity (0.03–0.06) compared with the fragmented populations (65–79% and 0.12–0.20 respectively). A greater proportion of trees contributed to reproduction in the fragmented ( de/d ≥ 0.5) compared with the continuous populations ( de/d  =   0.03–0.04). Despite significant inbreeding in the offspring of the fragmented populations, there was little evidence of loss of genetic diversity. It is argued that enhanced medium- and long-distance dispersal in fragmented landscapes may act to partly buffer the remnant populations from the negative effects of inbreeding and drift.     

Population structure of an endangered species living in contrasted habitats: Parnassia palustris (Saxifragaceae)

In endangered species, it is critical to analyse the level at which populations interact (i.e. dispersal) as well as the levels of inbreeding and local adaptation to set up conservation policies. These parameters were investigated in the endangered species Parnassia palustris living in contrasted habitats. We analysed population structure in 14 populations of northern France for isozymes, cpDNA markers and phenotypic traits related to fitness. Within population genetic diversity and inbreeding coefficients were not correlated to population size. Populations seem not to have undergone severe recent bottleneck. Conversely to pollen migration, seed migration seems limited at a regional scale, which could prevent colonization of new sites even if suitable habitats appear. Finally, the habitat type affects neither within-population genetic diversity nor genetic and phenotypic differentiation among populations. Thus, even if unnoticed local adaptation to habitats exists, it does not influence gene flow between populations.     

When it comes to inbreeding: slower is better

The extent to which genetic diversity is lost from inbred populations is important for conservation biology, evolutionary ecology, and plant and animal breeding. This importance stems from the fact that the amount of genetic diversity a population has is expected to correlate with evolutionary potential. A population's ability to avert extinction during rapidly changing environmental conditions, or the magnitude of response to selection on a trait, depend on the ability of the genome to maintain potentially adaptive genetic variation in the face of random genetic drift. Although a few previous studies have demonstrated that the rate of inbreeding affects the amount of genetic diversity maintained, the elegant work of Demontis et al. , in this issue, clearly demonstrates that slow inbreeding maintains more genetic diversity than fast inbreeding and that the primary mechanism could be balancing selection. In their study, populations that took 19 generations, rather than one generation, to reach the same level of inbreeding maintained 10% higher levels of allelic richness and 25% higher levels of heterozygosity. The use of specifically chosen molecular markers not expected to be neutral makes this study especially noteworthy, as the study provides evidence concerning the mechanisms underlying the maintenance of genetic diversity in the face of inbreeding.     

Small-scale spatial genetic structure in the Central African rainforest tree species Aucoumea klaineana: a stepwise approach to infer the impact of limited gene dispersal, population history and habitat fragmentation

Under the isolation-by-distance model, the strength of spatial genetic structure (SGS) depends on seed and pollen dispersal and genetic drift, which in turn depends on local demographic structure. SGS can also be influenced by historical events such as admixture of differentiated gene pools. We analysed the fine-scale SGS in six populations of a pioneer tree species endemic to Central Africa, Aucoumea klaineana. To infer the impacts of limited gene dispersal, population history and habitat fragmentation on isolation by distance, we followed a stepwise approach consisting of a Bayesian clustering method to detect differentiated gene pools followed by the analysis of kinship-distance curves. Interestingly, despite considerable variation in density, the five populations situated under continuous forest cover displayed very similar extent of SGS. This is likely due to an increase in dispersal distance with decreased tree density. Admixture between two gene pools was detected in one of these five populations creating a distinctive pattern of SGS. In the last population sampled in open habitat, the genetic diversity was in the same range as in the other populations despite a recent habitat fragmentation. This result may due to the increase of gene dispersal compensating the effect of the disturbance as suggested by the reduced extent of SGS estimated in this population. Thus, in A. klaineana, the balance between drift and dispersal may facilitate the maintenance of genetic diversity. Finally, from the strength of the SGS and population density, an indirect estimate of gene dispersal distances was obtained for one site: the quadratic mean parent-offspring distance, sigma(g), ranged between 210 m and 570 m.     

Genetic diversity and population size: island populations of the common shrew, Sorex araneus

Populations of many species are currently being fragmented and reduced by human interactions. These processes will tend to reduce genetic diversity within populations and reduce individual heterozygosities because of genetic drift, inbreeding and reduced migration. Conservation biologists need to know the effect of population size on genetic diversity, as this is likely to influence a population's ability to persist. Island populations represent an ideal natural experiment with which to study this problem. In a study of common shrews (Sorex araneus) on offshore Scottish islands, 497 individuals from 13 islands of different sizes and 6 regions on the mainland were trapped and genotyped at eight microsatellite loci. Previous genetic work had revealed that most of the islands in this study were highly genetically divergent from one another and the mainland. We found that most of the islands exhibited lower genetic diversity than the mainland populations. In the island populations, mean expected heterozygosity, mean observed heterozygosity and mean allelic richness were significantly positively correlated with log island size and log population size, which were estimated using habitat population density data and application of a Geographic Information System.     

The genetic underpinnings of population cyclicity: establishing expectations for the genetic anatomy of cycling populations

Despite extensive research into the mechanisms underlying population cyclicity, we have little understanding of the impacts of numerical fluctuations on the genetic variation of cycling populations. Thus, the potential implications of natural and anthropogenically‐driven variation in population cycle dynamics on the diversity and evolutionary potential of cyclic populations is unclear. Here, we use Canada lynx Lynx canadensis matrix population models, set up in a linear stepping‐stone, to generate demographic replicates of biologically realistic cycling populations. Overall, increasing cycle amplitude predictably reduced genetic diversity and increased genetic differentiation, with cyclic effects increased by population synchrony. Modest dispersal rates (1–3% of the population) between high and low amplitude cyclic populations did not diminish these effects suggesting that spatial variation in cyclic amplitude should be reflected in patterns of genetic diversity and differentiation at these rates. At high dispersal rates (6%) groups containing only high amplitude cyclic populations had higher diversity and lower differentiation than those mixed with low amplitude cyclic populations. Negative density‐dependent dispersal did not impact genetic diversity, but did homogenize populations by reducing differentiation and patterns of isolation by distance. Surprisingly, temporal changes in diversity and differentiation throughout a cycle were not always consistent with population size. In particular, negative density‐dependent dispersal simultaneously decreased differences in genetic diversity while increasing differences in genetic differentiation between numerical peaks and nadirs. Combined, our findings suggest demographic changes at fine temporal scales can impact genetic variation of interacting populations and provide testable predictions relating population cyclicty to genetic variation. Further, our results suggest that including realistic demographic and dispersal parameters in population genetic models and using information from temporal changes in genetic variation could help to discern complex demographic scenarios and illuminate population dynamics at fine temporal scales.     

Pollen dispersal patterns and population persistence in a small isolated population of <Emphasis Type="Italic">Fagus crenata</Emphasis>

The potential of long-distance pollen dispersal and the effects of small population size and population isolation on persistence of Fagus crenata populations were investigated in a small, severely isolated population (the Gofuku-ji population) and two other populations located within 7 km of this population (including 87 adult trees in total). Parentage analysis using 13 microsatellite loci showed that 94 of 100 seedlings derived from seeds collected from the Gofuku-ji population had parent pairs within this population, six had one parent within the population, and four of the six seedlings had alleles that were not detected in any of the three populations, indicating that some pollen is dispersed over distances exceeding 7 km. The estimated expected heterozygosity and effective population size were lower in the Gofuku-ji population than in previously examined large continuous populations. Therefore, levels of genetic diversity within the population may have been reduced by strong genetic drift and limitations of pollen- and seed-mediated gene flow associated with the small size and severe isolation. The contemporary mating pattern estimated at the seedling stage was biased toward outbreeding, which may be explained by possible processes: the level of inbreeding in the adult trees is increased; then, inbreeding frequently occurs but is rarely successful, while outbreeding successfully produces offspring. Additionally, high levels of significant linkage disequilibrium and higher numbers of alleles than expected under mutation–drift equilibrium from analyses of the populations’ evolutionary history suggest that the Gofuku-ji population may have experienced admixture before its severe isolation. Therefore, the persistence of the Gofuku-ji population is being adversely affected by the decrease in population size and severe isolation. Further studies of gene flow via pollen in other populations with various degrees of isolation could enhance our understanding of the effects of population isolation and long-distance pollen dispersal in F. crenata and similar species.     

Relationships between population density,fine-scale genetic structure,mating system and pollen dispersal in a timber tree from African rainforests

Owing to the reduction of population density and/or the environmental changes it induces, selective logging could affect the demography, reproductive biology and evolutionary potential of forest trees. This is particularly relevant in tropical forests where natural population densities can be low and isolated trees may be subject to outcross pollen limitation and/or produce low-quality selfed seeds that exhibit inbreeding depression. Comparing reproductive biology processes and genetic diversity of populations at different densities can provide indirect evidence of the potential impacts of logging. Here, we analysed patterns of genetic diversity, mating system and gene flow in three Central African populations of the self-compatible legume timber species Erythrophleum suaveolens with contrasting densities (0.11, 0.68 and 1.72 adults per ha). The comparison of inbreeding levels among cohorts suggests that selfing is detrimental as inbred individuals are eliminated between seedling and adult stages. Levels of genetic diversity, selfing rates (∼16%) and patterns of spatial genetic structure (Sp ∼0.006) were similar in all three populations. However, the extent of gene dispersal differed markedly among populations: the average distance of pollen dispersal increased with decreasing density (from 200 m in the high-density population to 1000 m in the low-density one). Overall, our results suggest that the reproductive biology and genetic diversity of the species are not affected by current logging practices. However, further investigations need to be conducted in low-density populations to evaluate (1) whether pollen limitation may reduce seed production and (2) the regeneration potential of the species.     

Migration and recovery of the genetic diversity during the increasing density phase in cyclic vole populations

In cyclic populations, high genetic diversity is currently reported despite the periodic low numbers experienced by the populations during the low phases. Here, we report spatio-temporal monitoring at a very fine scale of cyclic populations of the fossorial water vole (Arvicola terrestris) during the increasing density phase. This phase marks the transition from a patchy structure (demes) during low density to a continuous population in high density. We found that the genetic diversity was effectively high but also that it displayed a local increase within demes over the increasing phase. The genetic diversity remained relatively constant when considering all demes together. The increase in vole abundance was also correlated with a decrease of genetic differentiation among demes. Such results suggest that at the end of the low phase, demes are affected by genetic drift as the result of being small and geographically isolated. This leads to a loss of local genetic diversity and a spatial differentiation among demes. This situation is counterbalanced during the increasing phase by the spatial expansion of demes and the increase of the effective migration among differentiated demes. We provide evidences that in cyclic populations of the fossorial water voles, the relative influence of drift operating during low density populations and migration occurring principally while population size increases interacts closely to maintain high genetic diversity.