Sand lizards are an ideal system for this kind of work. The species occupies a broad range across Europe and Asia, but local populations are often small, isolated and genetically differentiated, particularly at the northern and western edges of the distribution where habitat fragmentation has squeezed reptile communities into remnant patches of heathland, dune grassland and forest clearing. Because sand lizards are site-faithful, easy to capture and relatively straightforward to mark and follow over time, they lend themselves to longitudinal studies that would be impossible in many other vertebrates. The experimental population at the heart of the study was founded by releasing individuals drawn from more than one genetically distinct source, creating what population geneticists call a mixed or admixed founder event, and then monitoring the genetic consequences across subsequent generations.

The central question the researchers addressed is one that divides conservation practitioners. On one side stands the worry that mixing divergent lineages can be harmful, a phenomenon sometimes called outbreeding depression, in which combinations of genes that are well adapted to local conditions are broken apart, or where chromosomes that have accumulated incompatibilities fail to work together properly in hybrids. On the other side stands the argument that the benefits of admixture, above all the restoration of heterozygosity and the masking of deleterious recessive mutations, will usually outweigh the risks, particularly for small founded populations that would otherwise march toward inbreeding depression. The study was designed to let the genomes themselves arbitrate between these competing expectations.

Using genome-wide markers, the team reconstructed the ancestry of the founded population generation by generation, quantifying how much genetic material each founding source contributed and how those contributions were reshuffled by recombination and inheritance over time. This is a powerful approach because admixture leaves characteristic signatures in the genome. In the first generations after mixing, the chromosomes of descendants exist as long blocks inherited intact from one source or the other. As generations pass, recombination slices those blocks into ever smaller segments, and the ancestry of the population converges toward a smooth, genome-wide blend. The rate and pattern of this ancestry transition reveal how selection, drift and demographic stochasticity interact in a young population.

The results provide a strikingly optimistic picture of the short-term consequences of admixture. The founded population retained a substantial share of the genetic diversity present in the combined sources, and heterozygosity, the measure most closely tied to fitness in small populations, was maintained at levels that would have been unattainable if founders had been drawn from a single source. Inbreeding, which accumulates rapidly when close relatives mate in a confined population, was diluted by the availability of genetically dissimilar partners. In practical terms, admixture acted as a genetic rescue mechanism, restoring the raw variation on which future adaptation depends.

Equally important is what the study found about the fate of ancestry blocks. Rather than showing evidence that selection was purging one lineage or another wholesale, the genomes of later generations displayed the progressive fragmentation of ancestry tracts consistent with neutral recombination, with local deviations that the authors interpret cautiously in the light of the population’s short history. This matters because it suggests that, at least on the timescale observed, mixing divergent sand lizard sources did not trigger a genomic crisis. There was no signal of wholesale hybrid breakdown, no collapse of the ancestry mixture, and no indication that the population was rejecting its mixed heritage. The admixed genome proved not merely viable but apparently robust.

These findings carry real weight for conservation practice. Reptile reintroductions are frequently criticized for being based on small numbers of founders and for source populations chosen on grounds of convenience rather than genetics. Guidelines in many countries still urge caution about mixing lineages, and managers often face an unenviable choice between founding a new population from a single, potentially inbred source or combining sources and accepting the theoretical risk of outbreeding depression. By documenting the genomic consequences of a mixed founding event in a real, monitored population, the study converts an abstract debate into empirical evidence. The message is that for species with the life history of the sand lizard, the short-term genetic cost of mixing appears small compared with the benefit of avoiding inbreeding.

The study is equally candid about the limits of its inference. Outbreeding depression often manifests only after several generations, when recombinant genotypes confront natural selection in the wild, and the timescale of the experiment, though substantial for a longitudinal reptile study, remains brief in evolutionary terms. Fitness traits, rather than genetic markers alone, are the ultimate arbiter of whether admixture is beneficial, and the authors emphasize that continued monitoring of survival, reproduction and morphology in the admixed population is essential. There is also the question of how far the results generalize. Sand lizards have moderate chromosomal uniformity across their range and no known strong intrinsic postzygotic barriers between regional lineages, whereas in other taxa deeply divergent sources may carry real incompatibilities. The appropriate lesson is not that mixing is always safe, but that the risks must be assessed taxon by taxon rather than assumed.

Methodologically, the work illustrates how genomic tools have transformed the study of founder events. Where earlier generations of conservation geneticists worked with a few dozen allozyme or microsatellite loci, genome-wide datasets now allow researchers to estimate ancestry proportions per individual, map ancestry blocks along chromosomes, track changes in effective population size and detect the erosion of diversity at fine scale. In a founded population, each generation is a natural experiment in inheritance, and high-density markers make it possible to read the record of that experiment directly from the DNA. The study thereby contributes not only to sand lizard conservation but to a broader theoretical understanding of admixture dynamics in small populations, a topic of growing urgency as assisted colonization and genetic rescue enter the mainstream conservation toolkit.

The broader significance of the work lies in its timing. Biodiversity is being reshuffled at an accelerating pace, and conservationists are increasingly asked to move organisms across landscapes that no longer resemble the ones in which those lineages evolved. Climate change is shifting the match between populations and their environments, making the question of which genetic sources to use in translocations one of the defining decisions of the coming decades. An experimentally founded sand lizard population, studied genome by genome and generation by generation, offers a rare controlled window into what actually happens when divergent lineages are deliberately brought together. The evidence so far suggests that admixture, far from being a hazard to be avoided, can be a foundation to be built upon, provided that the genomic consequences are monitored with the same care that managers devote to habitat and demography.

Subject of Research: Genomic consequences of admixture in an experimentally founded sand lizard (Lacerta agilis) population

Article Title: Genomic consequences of admixture in an experimentally founded sand lizard population

Article References: Bracamonte, S. E., Olsson, M., Wapstra, E., Lindsay, W. R., & Lillie, M. (2026). Genomic consequences of admixture in an experimentally founded sand lizard population. Heredity. https://doi.org/10.1038/s41437-026-00879-w

Image Credits: AI Generated

DOI: 10.1038/s41437-026-00879-w

Keywords: sand lizard, Lacerta agilis, admixture, genetic rescue, founder event, outbreeding depression, inbreeding, conservation genomics, reintroduction, ancestry, Heredity, population genetics

The timing of the study could hardly be more consequential. Managing authorities in Norway and Sweden have decided to reduce the Scandinavian wolf population to approximately 170 individuals, a census size they consider sustainable. That decision rests in part on recent population viability analyses commissioned by the same authorities, which concluded that a population of this magnitude could maintain its genetic viability. The new pedigree-based analysis directly challenges those conclusions, questioning whether the scientific foundation for the cull quotas is sound.

At the heart of the study is a concept that conservation biologists regard as perhaps the single most important number in population genetics: the effective population size, abbreviated Ne. Unlike the census count of animals, the effective size captures how many individuals would be needed in an idealized population to produce the same rate of genetic drift, inbreeding, and loss of diversity as the real population actually experiences. In practice, Ne is almost always far smaller than the raw headcount, because not every animal breeds, sex ratios are rarely balanced, and reproductive success is unevenly distributed. The effective size determines the pace at which genetic diversity erodes and inbreeding accumulates, and it therefore governs both short-term extinction risk through inbreeding depression and long-term viability through the loss of adaptive potential.

What makes the new analysis unusually powerful is the quality of the underlying data. Since the Scandinavian wolf population was re-established by a handful of founders, researchers have meticulously reconstructed its complete pedigree, drawing on decades of field monitoring, DNA sampling, and individual identification. Raw source data drawn from the pedigree compilation published by Åkesson and colleagues in 2023 allowed the team to calculate Ne precisely for every year since the population’s founding, using four-year cohorts along a one-year moving window. Rather than inferring genetic health from statistical snapshots of DNA samples, the researchers could trace the actual reproductive paths of every known animal, an approach that removes much of the uncertainty that plagues conventional genetic estimates.

The results reveal a population whose effective size has remained persistently and critically low throughout its history. The Scandinavian population was founded by only a few individuals and, despite growing to several hundred animals in census terms, has never achieved an effective size compatible with widely accepted conservation genetic benchmarks. The population’s history includes a well-documented severe inbreeding depression episode in the late 1990s and early 2000s, when pups from closely related pairs suffered dramatically reduced survival, and a partial genetic rescue delivered by a single immigrant male from the Finnish-Russian population. Subsequent work has documented the genomic consequences of this intensive inbreeding, including elevated levels of harmful homozygosity across the genome.

The study also scrutinizes the assumptions of the recent simulation studies that inform current management, including minimum viable population analyses prepared for the Swedish Environmental Protection Agency. According to Mergeay and colleagues, those simulations greatly overestimate the effective size of the Scandinavian wolf population, likely because they fail to adequately account for the population’s peculiar and well-documented reproductive structure. Wolves live in territorial packs in which typically only the dominant pair breeds, a social system that dramatically inflates the gap between census numbers and effective size. When this mating structure is properly represented, the same census count translates into a far smaller effective population than the commissioned analyses assumed, and the projected timelines for inbreeding and diversity loss become correspondingly more alarming.

The implications reach beyond Scandinavia. Conservation genetics has long relied on rules of thumb, notably the 50/500 rule, which suggests that an effective size of at least 50 is needed to avoid short-term inbreeding depression and 500 to retain long-term evolutionary potential, with revised recommendations pushing the long-term figure considerably higher. A population managed at a census size of roughly 170 animals, with a pack structure that suppresses Ne far below the census count, falls short of these benchmarks by a wide margin. The findings echo warnings issued in 2022 in the journal Science, when prominent conservation geneticists argued that planned culls would endanger the Swedish wolf population, and they align with a growing body of evidence from other inbred wolf populations, including the famously inbred wolves of Isle Royale, where genetic erosion contributed to a population crash.

The study arrives amid a charged political and legal debate over wolf management in Europe. Wolf culling policies in both Norway and Sweden have drawn formal complaints under the Bern Convention on the conservation of European wildlife and natural habitats, and the European Court of Justice has recently clarified the legal yardstick of favourable conservation status in a wave of wolf-related cases. Under the EU Habitats Directive, member states must maintain populations at a status where they can thrive long term without being dependent on continued conservation measures, a standard that inherently involves genetic considerations. If the effective size of the Scandinavian population is genuinely too small for long-term persistence, the legal and scientific case for reducing it further becomes considerably harder to defend.

The authors emphasize that their findings do not merely refine an academic parameter; they strike at the usefulness of the very models being used to set quotas. If simulation studies overestimate Ne, they will systematically underestimate the rate of inbreeding and the speed of diversity loss, producing optimistic projections of viability that the real population cannot match. The researchers suggest that management informed by these flawed models risks steering the population into an extinction vortex, in which shrinking numbers accelerate inbreeding, inbreeding depresses survival and reproduction, and the resulting decline further shrinks the population. Recent theoretical work suggests such vortices can be driven as much by a shortage of beneficial mutations as by the accumulation of harmful ones, underscoring how difficult a genetically impoverished population is to rescue once diversity is gone.

For the Scandinavian wolf, the path forward suggested by the genetics is clear even if politically difficult: either the population must be allowed to grow substantially, or gene flow from the larger Finnish-Russian wolf population must be facilitated at a rate sufficient to counteract drift and inbreeding. The one-migrant-per-generation rule, long a staple of conservation genetics, may need to be exceeded in this case given the population’s extreme isolation and skewed reproductive structure. What the new study makes unmistakable is that the current management trajectory, a population capped at 170 animals justified by models that overestimate its genetic buffer, is incompatible with the population’s own pedigree. As the authorities finalize their reduction plans, they will have to reckon with a complete, individual-level record of every wolf that has lived in Scandinavia, and that record tells a story of a population living dangerously close to its genetic limits.

Subject of Research: Effective population size and genetic viability of the Scandinavian wolf population

Article Title: The effective size of the Scandinavian wolf population is too small for both short- and long-term conservation

Article References: Mergeay, J., Flagstad, Ø., & Waples, R. S. (2026). The effective size of the Scandinavian wolf population is too small for both short- and long-term conservation. Heredity. https://doi.org/10.1038/s41437-026-00877-y

Image Credits: AI Generated

DOI: 10.1038/s41437-026-00877-y

Keywords: Scandinavian wolf population, effective population size, inbreeding, conservation genetics, population viability analysis, genetic drift, wolf culling, pedigree analysis, Heredity, large carnivore management, genetic rescue, extinction risk

An international research team led by Arne Bielke and Elmira Mohandesan of the University of Vienna, working with colleagues in Sweden, Wales, the United States and New Zealand, generated genome-wide single nucleotide polymorphism data from Kaimanawa Horses and integrated these results with previously generated mitochondrial and Y-chromosome markers. The comparison set comprised 22 domestic breeds, allowing the researchers to place the feral population within the broader landscape of global horse genetic diversity. The study was conducted in collaboration with the Kaimanawa Heritage Horse society and with the informed consent of horse owners who provided hair samples, photographs and background information on individual animals.

The genetic story that emerged is one of multiple founding contributions layered on top of one another. Admixture analyses showed that Kaimanawa Horses share genetic components mainly associated with British pony lineages, Thoroughbreds, Arabians and draft breeds, a pattern consistent with historical records describing the varied stock that contributed to the population during the colonial era and afterwards. Rather than descending from a single narrow source, the herd represents a living archive of the mixed breeding practices of nineteenth and twentieth century New Zealand, when working horses of many types were moved across pastoral land and often left to fend for themselves.

Yet the same analyses also documented the genetic costs of isolation. The population exhibits reduced heterozygosity, elevated inbreeding and an effective population size estimated at fewer than 50 individuals, a threshold widely regarded by conservation geneticists as dangerously low for long-term viability. The researchers traced this erosion of diversity to the population’s isolation and to recent demographic contraction, driven in part by management interventions. Kaimanawa Horses are subject to periodic musters, in which animals are rounded up and removed from the ranges to keep the population within an officially mandated ceiling, a practice that inevitably influences which genes persist in the free-ranging herd.

One of the most technically revealing aspects of the study involved runs of homozygosity, the long stretches of the genome where an individual inherits identical DNA segments from both parents, signaling recent or ancient inbreeding. By measuring the inbreeding coefficient derived from these runs and stratifying them by length class, the team could distinguish between older inbreeding events and more recent mating among relatives. The results confirmed that Kaimanawa Horses carry a substantial burden of homozygous segments relative to many domestic breeds, underscoring how founder effects, small population size and restricted gene flow have combined to shape the population’s genomic landscape.

Population structure analyses identified two distinct genetic subgroups within the herd, suggesting that geography and management history have produced detectable internal differentiation. Horses captured in different zones of the Kaimanawa Ranges, including the Argo Valley, southern zones and a designated capture zone, showed patterns consistent with limited movement between groups. Linkage disequilibrium decay, a measure of how quickly genetic variants lose their statistical association with physical distance along chromosomes, provided further evidence of the population’s demographic trajectory, while historical estimates of effective population size reconstructed over roughly the last seventeen generations revealed a declining trend that has accelerated in recent generations.

Perhaps the most striking discovery came from the paternal line. Analysis of the male-specific region of the Y-chromosome in 26 stallions revealed private paternal haplotypes that are absent from modern breed reference panels, indicating that the Kaimanawa population has retained unique paternal diversity that has disappeared, or was never present, in the registered breeds used for comparison. Mitochondrial DNA analysis of the maternal side complemented this picture, placing Kaimanawa maternal lineages within the broader spectrum of global horse diversity while highlighting the population’s distinctiveness. Together, these uniparental markers demonstrate that feral populations can serve as reservoirs of genetic variation lost from managed breeding programs.

The implications for conservation and management are considerable. An effective population size below 50 places the herd in a category where loss of genetic variation and inbreeding depression, including reduced fertility and foal survival, become realistic concerns. The authors argue that genomic approaches of the kind used in this study can directly inform management strategies, for example by guiding which animals are retained or relocated during musters to preserve the two genetic subgroups and maximize the retention of the population’s rare alleles and unique haplotypes. Without such informed intervention, the very management practices designed to control the population’s ecological footprint could inadvertently erode the genetic legacy that makes it scientifically and culturally valuable.

Beyond New Zealand, the study offers a template for understanding how founder history, demographic processes and human management jointly shape the genomes of free-ranging animal populations around the world. Feral horses occupy a contested space between pest and heritage icon, and their management is often decided in the absence of genetic data. By demonstrating that a feral population can harbor both documented vulnerability and irreplaceable diversity, the Kaimanawa work makes a compelling case that conservation genomics belongs at the center of such debates. As the researchers conclude, these findings not only illuminate the past of one of New Zealand’s most iconic wild animals but also support future research and evidence-based stewardship of feral horse populations wherever they roam.

Subject of Research: Genomic analysis of demographic history and genetic diversity in New Zealand's feral Kaimanawa Horses

Article Title: Demographic history and management practices shape the genomic landscape of New Zealand’s feral Kaimanawa Horses

Article References: Demographic history and management practices shape the genomic landscape of New Zealand’s feral Kaimanawa Horses. (n.d.). https://doi.org/10.1186/s12864-026-13345-y

Image Credits: AI Generated

DOI: 10.1186/s12864-026-13345-y

Keywords: Kaimanawa Horses, feral horses, population genomics, conservation genomics, inbreeding, New Zealand, genetic diversity, Y-chromosome, effective population size, runs of homozygosity, horse breeds, population structure