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Human‐mediated animal movement can expose wildlife populations to novel environments. Phenotypic plasticity can buffer against the challenges presented by novel environments, while adaptation to local ecosystems may limit resilience in novel ecosystems. Outbreeding depression during the mixing of disparate gene pools can also reduce reproductive success after long‐distance movement. Here, we use a ‘common‐garden’ population of gopher tortoises (Gopherus polyphemus), translocated from numerous sites across the state of Florida, USA, to a mitigation site in the north‐west (panhandle) region to assess whether geographic origin, outbreeding effects, and behavioral plasticity influence reproductive success in this threatened keystone species. We found that females from north‐east Florida produced clutches with lower hatching success than females from other regions. We detected regional differentiation in nest site selection behavior in the common environment of the translocation site, though these differences did not mediate the regional effect on hatching success. We also found evidence for outbreeding depression: hatching success declined with increasing parental geographic and genetic distances, dropping from 93% to 67% across the range of observed parental genetic distances. Together, these results suggest that newly admixed populations may suffer reproductive costs due to historical population differentiation, and that undetected outbreeding depression could significantly hamper conservation efforts for this species and others undergoing a variety of human‐mediated movements. [ABSTRACT FROM AUTHOR]

In F1 hybrids, phenotypic values are expected to be near the parental means under additive effects or close to one parent under dominance. However, F1 traits can fall outside the parental range, and outbreeding depression occurs when inferior fitness is observed in hybrids. Another possible outcome is heterosis, a phenomenon that interspecific hybrids or intraspecific crossbred F1s exhibit improved fitness compared to both parental species or strains. As an application of heterosis, hybrids between channel catfish females and blue catfish males are superior in feed conversion efficiency, carcass yield, and harvestability. Over 20 years of hybrid catfish production in experimental settings and farming practices generated abundant phenotypic data, making it an ideal system to investigate heterosis. In this study, we characterized fitness in terms of growth and survival longitudinally, revealing environment-dependent heterosis. In ponds, hybrids outgrow both parents due to an extra rapid growth phase of 2–4 months in year 2. This bimodal growth pattern is unique to F1 hybrids in pond culture environments only. In sharp contrast, the same genetic types cultured in tanks display outbreeding depression, where hybrids perform poorly, while channel catfish demonstrate superiority in growth throughout development. Our findings represent the first example, known to the authors, of opposite fitness shifts in response to environmental changes in interspecific vertebrate hybrids, suggesting a broader fitness landscape for F1 hybrids. Future genomic studies based on this experiment will help understand genome-environment interaction in shaping the F1 progeny fitness in the scenario of environment-dependent heterosis and outbreeding depression. [ABSTRACT FROM AUTHOR]

Gene flow can have rapid effects on adaptation and is an important evolutionary tool available when undertaking biological conservation and restoration. This tool is underused partly because of the perceived risk of outbreeding depression and loss of mean fitness when different populations are crossed. In this article, we briefly review some theory and empirical findings on how genetic variation is distributed across species ranges, describe known patterns of gene flow in nature with respect to environmental gradients, and highlight the effects of gene flow on adaptation in small or stressed populations in challenging environments (e.g. at species range limits). We then present a case study involving crosses at varying spatial scales among mountain populations of a trigger plant (Stylidium armeria : Stylidiaceae) in the Australian Alps to highlight how some issues around gene flow effects can be evaluated. We found evidence of outbreeding depression in seed production at greater geographic distances. Nevertheless, we found no evidence of maladaptive gene flow effects in likelihood of germination, plant performance (size), and performance variance, suggesting that gene flow at all spatial scales produces offspring with high adaptive potential. This case study demonstrates a path to evaluating how increasing sources of gene flow in managed wild and restored populations could identify some offspring with high fitness that could bolster the ability of populations to adapt to future environmental changes. We suggest further ways in which managers and researchers can act to understand and consider adaptive gene flow in natural and conservation contexts under rapidly changing conditions. [ABSTRACT FROM AUTHOR]

The fitness of immigrants and their descendants produced within recipient populations fundamentally underpins the genetic and population dynamic consequences of immigration. Immigrants can in principle induce contrasting genetic effects on fitness across generations, reflecting multifaceted additive, dominance, and epistatic effects. Yet full multigenerational and sex-specific fitness effects of regular immigration have not been quantified within naturally structured systems, precluding inference on underlying genetic architectures and population outcomes. We used four decades of song sparrow (Melospiza melodia) life history and pedigree data to quantify fitness of natural immigrants, natives, and their F1, F2, and backcross descendants and test for evidence of nonadditive genetic effects. Values of key fitness components (including adult lifetime reproductive success and zygote survival) of F1 offspring of immigrant-native matings substantially exceeded their parent mean, indicating strong heterosis. Meanwhile, F2 offspring of F1-F1 matings had notably low values, indicating surprisingly strong epistatic breakdown. Furthermore, magnitudes of effects varied among fitness components and differed between female and male descendants. These results demonstrate that strong nonadditive genetic effects on fitness can arise within weakly structured and fragmented populations experiencing frequent natural immigration. Such effects will substantially affect the net degree of effective gene flow and resulting local genetic introgression and adaptation. [ABSTRACT FROM AUTHOR]

Assisted gene flow is increasingly used to combat severe population declines. However, the associated risks, such as outbreeding depression, are often insufficiently assessed. Here, we studied the impact of assisted gene flow on the fitness of a highly endangered landlocked salmon population (Salmo salar m. sebago) from the lake Saimaa complex (Finland), using an anadromous Atlantic salmon population (S. salar) as donor. We released individuals of both parental populations and their hybrids into seminatural streams, monitoring their survival under predation risk from Northern pike (Esox lucius) and their growth on a natural diet. Before release, we exposed half of the salmon to the parasite Diplostomum pseudospathaceum to study whether assisted gene flow affects infection susceptibility, which could indirectly shape predation susceptibility and growth. The parental populations differed in both studied traits and the hybrids showed intermediate values. Relative to the target landlocked salmon population, hybrids experienced 21–26% lower survival, but 1.4–2.2% higher growth. They also carried 0.6–2.8 more parasites than the landlocked salmon, contributing to survival differences. These findings indicate that assisted gene flow can induce both negative and positive fitness effects. We propose that the overall net effects of this conservation tool need to be carefully evaluated before its implementation. [ABSTRACT FROM AUTHOR]

The ayu, Plecoglossus altivelis, with an amphidromous life migrating between the sea and streams, is a commercially important fish for the inland fisheries of Japan. Despite intensive transplantation, seedlings originating from non-native population landlocked in the Biwa Lake have been less contributable to stock recovery. In fact, no apparent evidence of introgression from the released fish to the native population has been obtained. We hypothesized that the parental property from the landlocked strain would give some disadvantages to the offspring against saline osmotic pressure during their early life in the sea. The acute performance of newly hatched larvae in 180% artificial seawater at 26°C was evaluated by the parental combinations of amphidromous and landlocked strains. As a result, hatchlings from amphidromous parents lived longer than those from landlocked parents, whereas outbred hatchlings showed no difference in their survival period between landlocked and amphidromous paternities. Hatchlings from amphidromous parents lived longer than outbred half-sibs with landlocked paternity, while hatchlings from landlocked parents lived shorter than outbred half-sibs with amphidromous paternity. Reduced salinity tolerance in the landlocked population was proved to be paternally inherited, which can explain the disappearance of hybridized lineage through the failure in the early survival. When supplementing for the purpose of stock enhancement, it is important to consider matching exotic conspecifics with native populations. [ABSTRACT FROM AUTHOR]

Abstract Assisted gene flow is increasingly used to combat severe population declines. However, the associated risks, such as outbreeding depression, are often insufficiently assessed. Here, we studied the impact of assisted gene flow on the fitness of a highly endangered landlocked salmon population (Salmo salar m. sebago) from the lake Saimaa complex (Finland), using an anadromous Atlantic salmon population (S. salar) as donor. We released individuals of both parental populations and their hybrids into seminatural streams, monitoring their survival under predation risk from Northern pike (Esox lucius) and their growth on a natural diet. Before release, we exposed half of the salmon to the parasite Diplostomum pseudospathaceum to study whether assisted gene flow affects infection susceptibility, which could indirectly shape predation susceptibility and growth. The parental populations differed in both studied traits and the hybrids showed intermediate values. Relative to the target landlocked salmon population, hybrids experienced 21–26% lower survival, but 1.4–2.2% higher growth. They also carried 0.6–2.8 more parasites than the landlocked salmon, contributing to survival differences. These findings indicate that assisted gene flow can induce both negative and positive fitness effects. We propose that the overall net effects of this conservation tool need to be carefully evaluated before its implementation.

Premise: What maintains mixed mating is an evolutionary enigma. Cleistogamy—the production of both potentially outcrossing chasmogamous and obligately selfing cleistogamous flowers on the same individual plant—is an excellent system to study the costs of selfing. Inbreeding depression can prevent the evolution of greater selfing within populations, and heterosis in crosses between populations may further tip the balance in favor of outcrossing. Few empirical estimates of inbreeding depression and heterosis in the same system exist for cleistogamous species. Methods: We investigate the potential costs of selfing by quantifying inbreeding depression and heterosis in three populations of the cleistogamous perennial Ruellia humilis Nutt (Acanthaceae). We performed three types of hand‐pollinations—self, outcross‐within, and outcross‐between populations—and measured seed number, germination, total flower production, and estimated cumulative fitness for the resulting progeny in a greenhouse experiment. Results: We found moderate inbreeding depression for cumulative fitness (<30%) in two populations, but outbreeding depression for crosses within a third population (–26%). For between‐population crosses, there was weak to modest heterosis (11–47%) in two of the population combinations, but modest to strong outbreeding depression (–21 to –71%) in the other four combinations. Conclusions: Neither inbreeding depression nor heterosis was of sufficient magnitude to explain the continued production of chasmogamous flowers given the relative energetic advantage of cleistogamous flowers previously estimated for these populations. Outbreeding depression either within or between populations makes the maintenance of chasmogamous flowers even harder to explain. More information is needed on the genetic basis of cleistogamy to resolve this conundrum. [ABSTRACT FROM AUTHOR]

Inbreeding is generally avoided in animals due to the risk of inbreeding depression following an increase in homozygous deleterious alleles and loss of heterozygosity. Species that regularly inbreed challenge our understanding of the fitness effects of these risks. We investigated the fitness consequences of extended inbreeding in the haplodiploid date stone beetle, Coccotrypes dactyliperda. We hypothesized that continuous inbreeding could result in reduced fitness, while outbreeding would either increase fitness due to heterosis or lower fitness if co-adapted gene complexes are disrupted. We established three breeding treatments with beetles from two geographically separated populations: Sib-mating (inbreeding), and outbreeding within and between populations. Between-population outbreeding groups of both populations had lower fecundity and collapsed before the experiment ended, while sib-mated and within-population breeding groups persisted for 10 generations. Sib-mated females had higher fecundity than within- and between-population outbreeding females. Inbreeding coefficients of sib-mated groups were higher than the other treatment groups, yet sib-mated beetles remained genetically polymorphic at the population level. Thus, there was no inbreeding depression, while crossing between distant populations led to outbreeding depression. Our findings are consistent with the life history of C. dactyliperda , in which sib-mating predominates within the date seed, but occasional within-population outbreeding may occur following local dispersal. [ABSTRACT FROM AUTHOR]

Restoring levels of genetic diversity in small and declining populations is increasingly being considered in biodiversity conservation. Evidence-based genetic management requires assessment of risks and benefits of crossing populations. Because risks are challenging to assess experimentally, e.g. through multi-generational crosses, decision-support approaches utilize proxy risk factors such as time since separation of lineages. However, the paucity of empirical datasets on fitness consequences of longer separation times tends to favour crossing lineages with conservatively short separations, restricting wildlife managers' options. Here, we assessed the genetic outcomes of interbreeding in the wild between lineages of a threatened Australian freshwater fish (Macquarie perch) separated by an estimated 119,000–385,000 years of evolution in distinct environments. Fish belonging to the Murray-Darling Basin (MDB) lineage escaped from Cataract Dam—into which they were translocated in ~ 1915—into the Cataract River, where they interbred with the local Hawkesbury-Nepean Basin (HNB) lineage. Analyses of reduced-representation genomic data revealed no evidence of genetic incompatibilities during interbreeding of the two lineages in the Cataract River: assignment to genotypic clusters indicated a spectrum of hybrid types including second generation hybrids and backcrosses to both parental lineages. Thus, no adverse effects were detected from genetic mixing of populations separated by > 100,000 years. We are not advocating purposely crossing the two lineages for management purposes under present cost–benefit considerations, because there are currently sufficient intra-lineage source populations to beneficially mix. Instead, this study presents a useful calibration point: two morphologically different lineages evolved in different habitats for 119,000–385,000 years can successfully interbreed. [ABSTRACT FROM AUTHOR]

Premise of research. The rarity of dioecy in plants remains a challenging issue in evolutionary biology. Two risks are likely exacerbated with dioecy: the difficulties of finding a pollen donor or recipient and heterospecific pollen transfer. These risks should be worsened in unisexual plants, relative to hermaphroditic plants, by the higher demands expected of pollinator services to compensate for their sterility. Any fitness advantage of dioecy should be conditioned to low-pollination-risk situations. Then we would expect a nonrandom distribution of dioecy and autonomously selfing hermaphrodites of sexually polymorphic species in landscapes with contrasting conspecific densities and frequencies of other pollinator-competing, coflowering species. Nevertheless, sexually polymorphic species' dwelling habitats with contrasting pollination risks are uncommon. Methodology. Using categorical data modeling, we studied associations between unisexuality and pollen risks in the rare sexually polymorphic Opuntia robusta , with male, female, and hermaphrodite individuals distributed in nine populations and with densities varying from 0.2 to 206.3 individuals per hectare. The frequencies of the nearest neighboring coflowering conspecific plants ranged from ~1% to ~90%. Pivotal results. Dioecy prevails in landscapes with abundant conspecific pollen donors or recipients, where, on the basis of previous results, sex allocation can be optimized. Hermaphroditism prevails in landscapes where conspecific pollen is expected to be low or there is abundant heterospecific pollen; in such situations selfing might assure seed set and reduce reproductive interference. Conclusions. Pollen landscapes help contextualize the reproductive environment that conditions the relative success of dioecy and hermaphroditism in a single species. Pollen landscapes can be estimated by the abundance of conspecific sexual partners and the frequency of heterospecific, coflowering, pollinator-competing nearest neighbors. This species-specific landscape property likely determines the risks of outcrossing pollen failure. Hermaphroditism is conceivably both a self-assurance mechanism and a barrier against heterospecific pollen transfer. Dioecy might outperform hermaphroditism, even if inbreeding depression is undetectable, but it is presumably conditioned in low-pollination-risk landscapes. [ABSTRACT FROM AUTHOR]

The exact fitness response resulted from a complex interplay between selection on loci with deleterious (maladaptive alleles and OD loci) and beneficial (preadaptive alleles) genetic variation and their genomic architectures. In the cases where the overall fitness benefit of preadaptive alleles was greater than or equal to the effect of maladaptation, deleterious alleles were purged and preadaptive alleles rose in frequency, increasing population fitness. We assessed the combined effect of alleles that are preadapted to a changing climatic variable (i.e., lead to local adaptation), maladaptive alleles (i.e., alleles fixed in the donor population but deleterious in the recipient population), and alleles that cause OD. At every generation, we calculated the mean allele frequency across all neutral markers to obtain a measure of the average proportion of alleles in the local population that derived from alleles introduced during AGF. [Extracted from the article]

Anthropogenically fragmented populations may have reduced fitness due to loss of genetic diversity and inbreeding. The extent of such fitness losses due to fragmentation and potential gains from conservation actions are infrequently assessed together empirically. Controlled crosses within and among populations can identify whether populations are at risk of inbreeding depression and whether interpopulation crossing alleviates fitness loss. Because fitness depends on environment and life stage, studies quantifying cumulative fitness over a large portion of the lifecycle in conditions that mimic natural environments are most informative. To assess fitness consequences of habitat fragmentation, we leveraged controlled within-family, within-population, and between-population crosses to quantify inbreeding depression and heterosis in seven populations of Echinacea angustifolia within a 6400-hectare area. We then assessed cumulative offspring fitness after 14 years of growth in a natural experimental plot (N = 1136). Mean fitness of progeny from within-population crosses varied considerably, indicating genetic differentiation among source populations, even though these sites are all less than 9 km apart. The fitness consequences of within-family and between-population crosses varied in magnitude and direction. Only one of the seven populations showed inbreeding depression of high effect, while four populations showed substantial heterosis. Outbreeding depression was rare and slight. Our findings indicate that local crossings between isolated populations yield unpredictable fitness consequences ranging from slight decreases to substantial increases. Interestingly, inbreeding depression and heterosis did not relate closely to population size, suggesting that all fragmented populations could contribute to conservation goals as either pollen recipients or donors., (© The Author(s) 2024. Published by Oxford University Press on behalf of The American Genetic Association. All rights reserved. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site—for further information please contact journals.permissions@oup.com.)

Increase of Local Plant Fitness by Genetic Rescue and Indication of Inbreeding or Outbreeding... When the plant translocation goal is genetic rescue, we expect an increase of plant fitness in reinforced, formerly inbred populations due to the introduction of new genetic variation or heterosis (increased vigor of the cross progeny) (Bell et al. 2019). Keywords: admixture; contemporary gene flow; genetic rescue; genetic restoration; inbreeding depression; outbreeding depression; quantitative genetics; reinforcement; depresión por endogamia; depresión por exogamia; flujo génico contemporáneo; genética cuantitativa; mezcla; reforzamiento; rescate genético; restauración genética EN admixture contemporary gene flow genetic rescue genetic restoration inbreeding depression outbreeding depression quantitative genetics reinforcement depresión por endogamia depresión por exogamia flujo génico contemporáneo genética cuantitativa mezcla reforzamiento rescate genético restauración genética 1 13 13 03/02/22 20220201 NES 220201 Introduction Plant translocations are intentional introductions of living organisms from one area to another, usually as spores, seeds, or plug plants (Menges 2008; Weeks et al. 2011). Changes in Genetic Diversity and Differentiation Genetic monitoring should assess whether genetic diversity (on which evolutionary resilience depends) has increased, if the translocation is a reinforcement, or is comparable to reference populations, if the translocation is an introduction or a reintroduction. Guidelines for Genetic Monitoring of Translocated Plant Populations Sampling Methods for Assessing Translocation Success To evaluate translocation success, the first step is to design appropriate sampling methods and choose the appropriate markers (Fig. [Extracted from the article]

Hybridization may drive rare taxa to extinction through genetic swamping, where the rare form is replaced by hybrids, or by demographic swamping, where population growth rates are reduced due to the wasteful production of maladaptive hybrids. Conversely, hybridization may rescue the viability of small, inbred populations. Understanding the factors that contribute to destructive versus constructive outcomes of hybridization is key to managing conservation concerns. Here, we survey the literature for studies of hybridization and extinction to identify the ecological, evolutionary, and genetic factors that critically affect extinction risk through hybridization. We find that while extinction risk is highly situation dependent, genetic swamping is much more frequent than demographic swamping. In addition, human involvement is associated with increased risk and high reproductive isolation with reduced risk. Although climate change is predicted to increase the risk of hybridization-induced extinction, we find little empirical support for this prediction. Similarly, theoretical and experimental studies imply that genetic rescue through hybridization may be equally or more probable than demographic swamping, but our literature survey failed to support this claim. We conclude that halting the introduction of hybridization-prone exotics and restoring mature and diverse habitats that are resistant to hybrid establishment should be management priorities.

Background and Aims Hybridization is increasingly recognized as an integral part of the dynamics of species range expansion and contraction. Thus, it is important to understand the reproductive barriers between co-occurring species. Extending previous studies that argued that the rare Eucalyptus risdonii was expanding into the range of the surrounding E. amygdalina by both seed and pollen dispersal, we here investigate the long-term fitness of both species and their hybrids and whether expansion is continuing. Methods We assessed the survival of phenotypes representing a continuum between the two pure species in a natural hybrid swarm after 29 years, along with seedling recruitment. The performance of pure species as well as of artificial and natural hybrids was also assessed over 28 years in a common garden trial. Key Results In the hybrid zone, E. amygdalina adults showed greater mortality than E. risdonii , and the current seedling cohort is still dominated by E. risdonii phenotypes. Morphologically intermediate individuals appeared to be the least fit. Similar results were observed after growing artificial first-generation and natural hybrids alongside pure species families in a common garden trial. Here, the survival, reproduction, health and growth of the intermediate hybrids were significantly less than those of either pure species, consistent with hybrid inferiority, although this did not manifest until later reproductive ages. Among the variable progeny of natural intermediate hybrids, the most E. risdonii -like phenotypes were the most fit. Conclusions This study contributes to the increasing number of reports of hybrid inferiority in Eucalyptus , suggesting that post-zygotic barriers contribute to the maintenance of species integrity even between closely related species. However, with fitness rapidly recovered following backcrossing, it is argued that hybridization can still be an important evolutionary process, in the present case appearing to contribute to the range expansion of the rare E. risdonii in response to climate change. [ABSTRACT FROM AUTHOR]

Abstract Pollen limitation negatively impacts endangered and endemic plants with small fragmented populations, such as Sinocalycanthus chinensis, an endangered plant endemic to China. In this study, we analyzed the pollen limitation of the S. chinensis Damingshan (DMS) population in 2006, 2009, and 2010, and crossed plants with mates separated by different distances, both within and between populations. The DMS population exhibited strong pollen limitation in fruit set, seed set, and seeds per fruit in 2006, 2009, and 2010. The average accumulated pollen limitation (for fruit set times seeds per fruit) was 0.510 ± 0.180. Progeny crossed with pollen from intermediate neighboring plants within the same population (separated by 30–50 m from pollen recipients) had the lowest fitness. No optimal outcrossing distance was found within the DMS population. Progeny from crosses with the Shunxiwu (SXW) and Daleishan (DLS) populations performed relatively better, while those from crosses with Qingliangfeng (QLF) and Longxushan (LXS) populations performed worse. Compared with average reproductive success, outbreeding depression was found in progeny from crosses with the LXS and QLF populations. Reproductive success from pure self‐pollination indicated S. chinensis is self‐compatible. Geitonogamous selfing increased reproductive success. Based on geitonogamous selfing, the proportion of selfed offspring was relatively high. These results provide basic references for the conservation of this species.

Inbreeding and outbreeding depression are dynamic forms of selection critical to mating system evolution and the efficacy of conservation biology. Most evidence on how the relative severity and timing of these forces are shaped is confined to self-fertilization, distant outcrossing, and intermediate 'optimal outcrossing' in hermaphrodites. We tested the notion that closed population demographics may reduce and delay the costs of inbreeding relative to distant outbreeding in an intertidal copepod with separate sexes and a biphasic larval / post-metamorphic life-history (Tigriopus californicus). At three lifecycle stages (fecundity, metamorphosis, and post-metamorphosis), we quantified the effects of inbreeding and outbreeding in crosses with varying degrees of recent common ancestry. Although inbreeding and outbreeding depression have distinct genetic mechanisms, both manifested the same stage-specific consequences for fitness. Inbreeding and outbreeding depression were not apparent for fecundity, post-metamorphic survival, sex ratio, or the ability to acquire mates, but inbreeding between full siblings and outbreeding between interpopulation hybrids reduced the fraction of offspring that completed metamorphosis by 32% and 47%, respectively. On average, the effects of inbreeding on metamorphic rate were weaker and nearly twice as variable among families than those of outbreeding, suggesting genetic load was less pervasive than the incompatibilities accrued between divergent populations. Overall, our results indicate the transition from larval to juvenile life stages is markedly susceptible to both inbreeding and outbreeding depression in T. californicus. We suggest stage-specific selection acting concurrently with the timing of metamorphosis may be an instrumental factor shaping reproductive optima in species with complex life-histories., (© The Author(s) 2024. Published by Oxford University Press on behalf of The American Genetic Association. All rights reserved. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site—for further information please contact journals.permissions@oup.com.)

The illegal capture and trade of wild birds have long been threats to biodiversity. The rehabilitation and release of confiscated animals may be a useful conservation tool in species management. However, differences between populations regarding health (e.g., different pathogens) and adaptation (e.g., local adaptation) must be taken into account, since both can negatively impact the recipient population. In this pilot study, we used two of the most illegally trafficked Brazilian wild passerine species, namely the red-crested cardinal (Paroaria coronata) and green-winged saltator (Saltator similis) as case studies and assessed some of the health threats that the release of confiscated passerines may pose to free-living birds. We also investigated the level of difference in mitochondrial genetic structure among populations living in different ecoregions. Blood, feces, and oropharyngeal swabs from confiscated (n = 115) and free-living (n = 120) passerines from the release sites were tested for the Newcastle disease virus, Salmonella spp., and Mycoplasma gallisepticum. These are considered major avian diseases by the Brazilian National Avian Health Program. We analyzed mtDNA to study the difference in genetic structure between populations using samples from 127 free-living passerines. We found no evidence of the Newcastle disease virus or Salmonella spp. in confiscated or free-living passerines from either species. However, the levels of infection with M. galissepticum detected in our study for red-crested cardinals and green-winged saltators calls for a high degree of caution in captive release programs. The difference in genetic structure between populations occurring in different regions was low, and was not significant between those from the Pampa/Subtropical Grasslands region. These results suggest that it may be possible to establish a cost-effective and sensitive protocol for releasing confiscated songbirds, provided that further genome-wide studies indicate that the functional genetic diversity among (at least some of the) populations is also low.

Abstract The continuing decline of many natural plant and animal populations emphasizes the importance of conservation strategies. Hybridization as a management tool has proven successful in introducing gene flow to small, inbred populations, but can be also associated with health risks. For example, hybridization can change susceptibility to infection in either direction due to heterosis (hybrid vigor) and outbreeding depression, but such health effects have rarely been considered in the genetic management of populations. Here, we investigated the effects of experimental outcrossing between the critically endangered Saimaa landlocked salmon (Salmo salar m. sebago) and the genetically more diverse Atlantic salmon (S. salar) on infection susceptibility. We exposed the parent populations and their hybrids to two parasites that commonly infect these species, the bacterium Flavobacterium columnare and the macroparasitic fluke Diplostomum pseudospathaceum. We found that landlocked salmon had lower survival during the bacterial epidemic, but higher resistance against the fluke, compared with Atlantic salmon. Hybrids showed intermediate survival and resistance, suggesting that hybridization decreased susceptibility to one parasite, but concurrently increased it to another. Our results emphasize the importance of considering health effects of different types of infections when employing hybridization for conservation.

Interactions between natural selection and population dynamics are central to both evolutionary‐ecology and biological responses to anthropogenic change. Natural selection is often thought to incur a demographic cost that, at least temporarily, reduces population growth. However, hard and soft selection clarify that the influence of natural selection on population dynamics depends on ecological context. Under hard selection, an individual's fitness is independent of the population's phenotypic composition, and substantial population declines can occur when phenotypes are mismatched with the environment. In contrast, under soft selection, an individual's fitness is influenced by its phenotype relative to other interacting conspecifics. Soft selection generally influences which, but not how many, individuals survive and reproduce, resulting in little effect on population growth. Despite these important differences, the distinction between hard and soft selection is rarely considered in ecology. Here, we review and synthesize literature on hard and soft selection, explore their ecological causes and implications and highlight their conservation relevance to climate change, inbreeding depression, outbreeding depression and harvest. Overall, these concepts emphasise that natural selection and evolution may often have negligible or counterintuitive effects on population growth—underappreciated outcomes that have major implications in a rapidly changing world. [ABSTRACT FROM AUTHOR]

Hybridization is a source of phenotypic novelty and variation because of increased additive genetic variation. Yet, the roles of nonadditive allelic interactions in shaping phenotypic mean and variance of hybrids have been underappreciated. Here, we examine the distributions of male‐mating traits in F1 hybrids via a meta‐analysis of 3208 effect sizes from 39 animal species pairs. Although additivity sets phenotypic distributions of F1s to be intermediate, F1s also showed recessivity and resemblance to maternal species. F1s expressed novel phenotypes (beyond the range of both parents) in 65% of species pairs, often associated with increased phenotypic variability. Overall, however, F1s expressed smaller variation than parents in 51% of traits. Although genetic divergence between parents did not impact phenotypic novelty, it increased phenotypic variability of F1s. By creating novel phenotypes with increased variability, nonadditivity of heterozygotic genome may play key roles in determining mating success of F1s, and their subsequent extinction or speciation. [ABSTRACT FROM AUTHOR]

Studies on insular organisms provide an important framework for investigating patterns of genetic differentiation and reproductive isolation. By focusing on populations of the same species, we have the opportunity to study the mechanisms operating during the earliest stages of speciation, as reproductive barriers can be examined among divergent lineages in a geographic context. We investigated the genetic differentiation and the evolution of early stages of intrinsic postmating reproductive isolation between continental and insular populations of Epidendrum fulgens, a neotropical orchid distributed in southeastern Brazil. Genetic diversity and structure were estimated for both nuclear and plastid markers by using genetic differentiation measures and model-based assignment test. Furthermore, two components of reproductive isolation were examined by analyzing fruit set and seed viability in interpopulation crosses. Strong plastid genetic structure (FST = 0.679) was found between insular and remaining populations, indicating that E. fulgens reduced gene flow via seed dispersal, although significant nuclear genetic structure was lower (DST = 0.179), likely due to the smaller effective population size of the plastid genome. Significant differences in seed viability between self- and cross-pollinated plants were found, indicating the occurrence of inbreeding depression in all populations. Seed viability was significantly lower in crosses between insular and remaining populations, suggesting a late postzygotic reproductive barrier due to low migration associated with genetic drift (i.e., bottleneck) and its consequences. Our results confirm the importance of islands as drivers for populational differentiation and suggest a central role for outbreeding depression during the early stages of lineage diversification. [ABSTRACT FROM AUTHOR]

The continuing decline of many natural plant and animal populations emphasizes the importance of conservation strategies. Hybridization as a management tool has proven successful in introducing gene flow to small, inbred populations, but can be also associated with health risks. For example, hybridization can change susceptibility to infection in either direction due to heterosis (hybrid vigor) and outbreeding depression, but such health effects have rarely been considered in the genetic management of populations. Here, we investigated the effects of experimental outcrossing between the critically endangered Saimaa landlocked salmon (Salmo salar m. sebago) and the genetically more diverse Atlantic salmon (S. salar) on infection susceptibility. We exposed the parent populations and their hybrids to two parasites that commonly infect these species, the bacterium Flavobacterium columnare and the macroparasitic fluke Diplostomum pseudospathaceum. We found that landlocked salmon had lower survival during the bacterial epidemic, but higher resistance against the fluke, compared with Atlantic salmon. Hybrids showed intermediate survival and resistance, suggesting that hybridization decreased susceptibility to one parasite, but concurrently increased it to another. Our results emphasize the importance of considering health effects of different types of infections when employing hybridization for conservation. [ABSTRACT FROM AUTHOR]

Augmenting gene flow is a powerful tool for the conservation of small, isolated populations. However, genetic rescue attempts have largely been limited to populations at the brink of extinction, in part due to concerns over negative outcomes (e.g., outbreeding depression). Increasing habitat fragmentation may necessitate more proactive genetic management. Broader application of augmented gene flow will, in turn, require rigorous evaluation to increase confidence and identify pitfalls in this approach. To date, there has been no assessment of best monitoring practices for genetic rescue attempts. We used genomically explicit, individual‐based simulations to examine the effectiveness of common approaches (i.e., tests for increases in fitness, migrant ancestry, heterozygosity, and abundance) for determining whether genetic rescue or outbreeding depression occurred. Statistical power to detect the effects of gene flow on fitness was high (≥0.8) when effect sizes were large, a finding consistent with those from previous studies on severely inbred populations. However, smaller effects of gene flow on fitness can appreciably affect persistence probability but current evaluation approaches fail to provide results from which reliable inferences can be drawn. The power of the metrics we examined to evaluate genetic rescue attempts depended on the time since gene flow and whether gene flow was beneficial or deleterious. Encouragingly, the use of multiple metrics provided nonredundant information and improved inference reliability, highlighting the importance of intensive monitoring efforts. Further development of best practices for evaluating genetic rescue attempts will be crucial for a responsible transition to increased use of translocations to decrease extinction risk. Article impact statement: Commonly used monitoring approaches frequently fail to detect the substantial conservation benefits of augmented gene flow. [ABSTRACT FROM AUTHOR]

Inbreeding depression strongly affects the biological fitness of organisms throughout their life cycle. These negative effects are more pronounced in species with low dispersal potentials, where mating among relatives is more likely. However, in some species, an outbreeding depression could be expressed when local adaptive and genetic beneficial interactions are disrupted when mating occurs between individuals from different localities. The amphipod Orchestoidea tuberculata inhabits the upper level of sandy beaches. This species has direct development and adults are poor swimmers, resulting in low dispersal potential. We herein evaluated potential inbreeding and outbreeding depression responses in O. tuberculata estimated in fecundity, egg size, and mate choice. Artificial families were used with individuals from different sites within a single beach and from distinct beaches. Results showed that the highest fecundity (66.7%) and the largest egg size (50.1 mm3) were recorded in females that mated with males from the same site, rather than males from other sites and beaches. We also observed potential recognition mechanisms that clearly favored inbreeding (81.8% of males chose females from the same site). These results suggest a potential outbreeding depression in O. tuberculata. Additional studies are needed to elucidate underlying adaptive mechanisms favoring inbreeding in this species. [ABSTRACT FROM AUTHOR]

Abstract Supplementing declining populations with individuals from different populations (population augmentation) is increasingly being considered an effective way to genetically rescue threatened species. However, the general value of population augmentation remains contentious, largely because few studies have experimentally tested the genetic consequences of mixing individuals from different populations. Among amphibians, which typically display fine‐scale genetic structuring, there is a high risk that population augmentation might exacerbate population decline via outbreeding depression. Here, we use artificial fertilization techniques combined with a cross‐classified breeding design (North Carolina Type II) to investigate the fitness consequences of outbreeding in the near threatened brown toadlet (Pseudophryne bibronii). Our results revealed that outcrossed families suffered from significantly lower fertilization success, hatching success and larval survival to metamorphosis. These findings suggest that inter‐population crosses might be susceptible to genetic incompatibility and outbreeding depression. However, because genetic factors specific to one of our study populations may have caused outcrossed families to have reduced fitness, an additional breeding experiment involving full factorial crosses will be required to draw firm conclusions. Nevertheless, our findings underscore the need for caution when conducting population augmentation in amphibians. Before mixing populations that might be evolutionary distinct, we encourage conservation managers to use artificial fertilization techniques and cross‐classified breeding designs to critically assess the risk of outbreeding depression. We argue that widespread adoption of this approach will enable more rapid assessment of the risks associated with population augmentation and improve the management of threatened amphibians globally.

Hybridization, defined as breeding between two distinct taxonomic units, can have an important effect on the evolutionary patterns in cross-breeding taxa. Although interspecific hybridization has frequently been considered as a maladaptive process, which threatens species genetic integrity and survival via genetic swamping and outbreeding depression, in some cases hybridization can introduce novel adaptive variation and increase fitness. Most studies to date focused on documenting hybridization events and analyzing their causes, while relatively little is known about the consequences of hybridization and its impact on the parental species. To address this knowledge gap, we conducted a systematic review of studies on hybridization in mammals published in 2010–2021, and identified 115 relevant studies. Of 13 categories of hybridization consequences described in these studies, the most common negative consequence (21% of studies) was genetic swamping and the most common positive consequence (8%) was the gain of novel adaptive variation. The total frequency of negative consequences (49%) was higher than positive (13%) and neutral (38%) consequences. These frequencies are biased by the detection possibilities of microsatellite loci, the most common genetic markers used in the papers assessed. As negative outcomes are typically easier to demonstrate than positive ones (e.g., extinction vs hybrid speciation), they may be over-represented in publications. Transition towards genomic studies involving both neutral and adaptive variation will provide a better insight into the real impacts of hybridization.

Heterozygosity-fitness correlations (HFCs) have been used to monitor the effects of inbreeding in threatened populations. HFCs can also be useful to investigate the potential effects of inbreeding in isolated relict populations of long-term persistence and to better understand the role of inbreeding and outbreeding as drivers of changes in genetic diversity. We studied a continental island population of thorn-tailed rayadito (Aphrastura spinicauda) inhabiting the relict forest of Fray Jorge National Park, north-central Chile. This population has experienced a long-term, gradual process of isolation since the end of the Tertiary. Using 10 years of field data in combination with molecular techniques, we tested for HFCs to assess the importance of inbreeding depression. If inbreeding depression is important, we predict a positive relationship between individual heterozygosity and fitness-related traits. We genotyped 183 individuals at 12 polymorphic microsatellite loci and used 7 measures of reproductive success and estimates of apparent survival to calculate HFCs. We found weak to moderate statistical support (P-values between 0.05 and 0.01) for a linear effect of female multi-locus heterozygosity (MLH) on clutch size and nonlinear effects on laying date and fledging success. While more heterozygous females laid smaller clutches, nonlinear effects indicated that females with intermediate values of MLH started laying earlier and had higher fledging success. We found no evidence for effects of MLH on annual fecundity or on apparent survival. Our results along with the long-term demographic stability of the study population contradict the hypothesis that inbreeding depression occurs in this population. [ABSTRACT FROM AUTHOR]

The evolutionary consequences of individual genetic diversity are frequently studied by assessing heterozygosity–fitness correlations (HFCs). The prevalence of positive and negative HFCs and the predominance of general versus local effects in wild populations are far from understood, partly because comprehensive studies testing for both inbreeding and outbreeding depression are lacking. We studied a genetically diverse population of blue tits in southern Germany using a genome‐wide set of 87 microsatellites to investigate the relationship between proxies of reproductive success and measures of multilocus and single‐locus individual heterozygosity (MLH and SLH). We used complimentary measures of MLH and partitioned markers into functional categories according to their position in the blue tit genome. HFCs based on MLH were consistently negative for functional loci, whereas correlations were rather inconsistent for loci found in nonfunctional areas of the genome. Clutch size was the only reproductive variable showing a general effect. We found evidence for local effects for three measures of reproductive success: arrival date at the breeding site, the probability of breeding at the study site and male reproductive success. For these, we observed consistent, and relatively strong, negative effects at one functional locus. Remarkably, this marker had a similar effect in another blue tit population from Austria (~400 km to the east). We suggest that a genetic local effect on timing of arrival might be responsible for most negative HFCs detected, with carry‐over effects on other reproductive traits. This effect could reflect individual differences in the distance between overwintering areas and breeding sites. [ABSTRACT FROM AUTHOR]

Pollen limitation negatively impacts endangered and endemic plants with small fragmented populations, such as Sinocalycanthus chinensis, an endangered plant endemic to China. In this study, we analyzed the pollen limitation of the S. chinensis Damingshan (DMS) population in 2006, 2009, and 2010, and crossed plants with mates separated by different distances, both within and between populations. The DMS population exhibited strong pollen limitation in fruit set, seed set, and seeds per fruit in 2006, 2009, and 2010. The average accumulated pollen limitation (for fruit set times seeds per fruit) was 0.510 ± 0.180. Progeny crossed with pollen from intermediate neighboring plants within the same population (separated by 30–50 m from pollen recipients) had the lowest fitness. No optimal outcrossing distance was found within the DMS population. Progeny from crosses with the Shunxiwu (SXW) and Daleishan (DLS) populations performed relatively better, while those from crosses with Qingliangfeng (QLF) and Longxushan (LXS) populations performed worse. Compared with average reproductive success, outbreeding depression was found in progeny from crosses with the LXS and QLF populations. Reproductive success from pure self‐pollination indicated S. chinensis is self‐compatible. Geitonogamous selfing increased reproductive success. Based on geitonogamous selfing, the proportion of selfed offspring was relatively high. These results provide basic references for the conservation of this species. [ABSTRACT FROM AUTHOR]

Plant species vary in the pollination distance at which negative fitness effects are expressed, and it has been proposed that optimal mating should occur at a distance large enough to diminish the risks of inbreeding, but short enough to prevent outbreeding depression. In a natural population of Pinguicula moranensis we assessed the fitness of plants (seeds per fruit) and their progeny (germination and seedling survival) as a function of pollination distance by hand‐pollinating with donors from five distances: 0 m (self‐fertilization), 1, 4 and 460 m (within‐population) and 9,900 m (between‐population) under in situ and ex situ conditions. We found that average values for fitness components were consistently lower in the in situ experiment than in the ex situ experiment. Under both conditions, the self‐fertilization treatment had the lowest values. Inbreeding depression values were high (0.94 to 0.98), corresponding with the values expected for an outcrossing species subjected to self‐fertilization. Overall, no evidence of outbreeding depression was detected. Considering that our experiment was restricted to one single donor population and one recipient population, our results indicate that despite the within‐population patchy distribution of individuals, biparental inbreeding could occur at low rates, due in part to short seed dispersal distance. However, pollen movement of at least 1 m is apparently sufficient to reverse negative effects. [ABSTRACT FROM AUTHOR]

Supplementing declining populations with individuals from different populations (population augmentation) is increasingly being considered an effective way to genetically rescue threatened species. However, the general value of population augmentation remains contentious, largely because few studies have experimentally tested the genetic consequences of mixing individuals from different populations. Among amphibians, which typically display fine‐scale genetic structuring, there is a high risk that population augmentation might exacerbate population decline via outbreeding depression. Here, we use artificial fertilization techniques combined with a cross‐classified breeding design (North Carolina Type II) to investigate the fitness consequences of outbreeding in the near threatened brown toadlet (Pseudophryne bibronii). Our results revealed that outcrossed families suffered from significantly lower fertilization success, hatching success and larval survival to metamorphosis. These findings suggest that inter‐population crosses might be susceptible to genetic incompatibility and outbreeding depression. However, because genetic factors specific to one of our study populations may have caused outcrossed families to have reduced fitness, an additional breeding experiment involving full factorial crosses will be required to draw firm conclusions. Nevertheless, our findings underscore the need for caution when conducting population augmentation in amphibians. Before mixing populations that might be evolutionary distinct, we encourage conservation managers to use artificial fertilization techniques and cross‐classified breeding designs to critically assess the risk of outbreeding depression. We argue that widespread adoption of this approach will enable more rapid assessment of the risks associated with population augmentation and improve the management of threatened amphibians globally. [ABSTRACT FROM AUTHOR]

Habitat fragmentation from urban development leaves species vulnerable to inbreeding depression and genomic erosion. Restoring gene flow can reduce inbreeding and preserve genetic diversity, but a common concern is that genomic incompatibilities may lead to outbreeding depression. The introduction of deleterious genetic load is less commonly considered. The endangered Pacific pocket mouse (Perognathus longimembris pacificus) persists in three isolated populations in southern California. Mitochondrial and microsatellite data indicated that effective population sizes were extremely small (Ne< 50), and continued declines prompted a conservation breeding program founded by individuals from each population. We tracked genetic diversity and individual fitness (measured by reproductive success) in a captive setting over six generations of admixture. Although we observed an increase in fitness in the F1 and F2 generations relative to the founding populations, inbreeding depression alone did not explain the fitness patterns, and outbreeding depression was difficult to rule out as reproductive success waned after F2. However, reproductive success was consistently higher in admixed individuals than founders from Dana Point, the smallest population with the lowest heterozygosity. Across generations, we saw a strong negative correlation between individual reproductive success and ancestry to Dana Point, leading to a rapid decrease of Dana Point alleles. Although the genomic underpinnings remain to be determined, reduced fitness associated with Dana Point ancestry is consistent with high deleterious genetic load in this population, and thus any facilitated migration should be unidirectional. Our findings highlight that, even in the absence of outbreeding depression, there may be a cost to restoration of gene flow if populations harbor high genetic load. [ABSTRACT FROM AUTHOR]

Context: Managing small, isolated populations requires conservation practitioners to weigh up the risks of inbreeding depression and outbreeding depression when assessing alternative management actions aimed at preventing species extinction. Accordingly, it is important that research intended to guide these management decisions provides the relevant evidence to inform them. Aims: To determine the extent to which studies that use genetic analyses to characterise population units for conservation consider the key theoretical concepts necessary for making sound management recommendations regarding the desirability of gene flow among units, notably the consequences and relative risks of inbreeding depression and outbreeding depression. Methods: A systematic search was conducted of peer-reviewed literature for studies that attempted to identify population units of threatened birds and mammals. Using content analysis, the theoretical framing of these studies was assessed, based on the discussion of key concepts concerning differences among populations. Key results: There has been a significant increase over time in the number of published studies that use genetics to identify population units for conservation. Many do not consider theoretical concepts relevant to the effective management of fragmented populations of threatened species. Mammals were more common than birds as focal species of studies, but the number of concepts used in the framing of the studies was similar for these two taxa, despite differences in their ecology and biology that might be expected to affect perceptions of distinctiveness. Nevertheless, species of greater conservation concern tended to have a slightly more comprehensive theoretical framing. Conclusions: There is great potential for more studies to implement theoretical guidelines and practical decision support tools when considering the best course of action for identifying appropriate population units for conservation management. Implications: The gap in the identified literature is likely to be impacting the ability of conservation practitioners to make evidence-informed decisions about how to manage the genetic health of threatened species; it would be valuable to improve this situation. Managing the genetic health of fragmented populations requires assessment of the risks and benefits of alternative management actions to prevent species extinction. This study aimed to determine the extent to which key theoretical concepts are considered when genetic analyses are used to identify population units for conservation. Our findings suggest that future studies could be better targeted to guide decisions about when to manage populations separately. Photograph by Peter Menkhorst. [ABSTRACT FROM AUTHOR]

Restoring gene flow into small, isolated populations can alleviate genetic load and decrease extinction risk (i.e., genetic rescue), yet gene flow is rarely augmented as a conservation strategy. Due to this discrepancy between opportunity and action, a recent call was made for widespread genetic rescue attempts. However, several aspects of augmenting gene flow are poorly understood, including the magnitude and duration of beneficial effects and when deleterious effects are likely to occur. We discuss the remaining uncertainties of genetic rescue in order to promote and direct future research and to hasten progress toward implementing this potentially powerful conservation strategy on a broader scale. Genetic rescue has helped prevent the extinction of several populations, yet augmented gene flow is rarely used as a conservation strategy. Recent calls have been made for a paradigm shift in the conservation of small, isolated populations away from managing populations in isolation and toward widespread restoration of gene flow. Several aspects of genetic rescue remain poorly understood. Genetic rescue is inherently an eco-evolutionary process, and successful genetic rescue attempts have been part of comprehensive conservation plans that consider habitat, life history, and genetics. Genomics is being increasingly used in the implementation and monitoring of genetic rescue attempts. [ABSTRACT FROM AUTHOR]

Both academics and practitioners consider a lack of knowledge about evolutionary theory to be a general barrier to effectively managing genetic diversity. However, it is challenging to judge practitioners' level of understanding and how this influences their management decisions. Knowledge built through experience may be difficult for practitioners to articulate, but could nonetheless result in appropriate management strategies. To date, researchers have assessed only the explicit (formal) knowledge practitioners have of evolutionary concepts. To explore practitioners' understanding of evolutionary concepts, it is necessary to consider how they might apply explicit and implicit knowledge to their management decisions. Using an online survey, we asked Australian practitioners to respond to 2 common management scenarios in which there is strong evidence that managing genetic diversity can improve outcomes: managing small, isolated populations and sourcing seeds for restoration projects. In describing their approach to these scenarios, practitioners demonstrated a stronger understanding of the effective management of genetic diversity than the definitions of the relevant concepts. However, their management of genetic diversity within small populations was closer to best practice than for restoration projects. Moreover, the risks practitioners described in implementing best practice management were more likely to affect their approach to restoration than translocation projects. These findings provide evidence that strategies to build the capacity of practitioners to manage genetic diversity should focus on realistic management scenarios. Given that practitioners recognize the importance of adapting their practices and the strong evidence for the benefits of actively managing genetic diversity, there is hope that better engagement by evolutionary biologists with practitioners could facilitate significant shifts toward evolutionarily enlightened management. Article impact statement: Better engagement with practitioners could facilitate significant shifts toward evolutionarily enlightened management. [ABSTRACT FROM AUTHOR]

Although hybridization can be used as a tool for genetic rescue, it can also generate outbreeding depression and reduce local adaptation. Improved understanding of these processes is required to better inform conservation decisions for threatened populations. Few studies, however, investigate how multiple factors influence hybridization effects. We investigated how effective population sizes (Ne), geographic distance, genetic divergence (QST, FST), and environmental stress influence hybridization effects among eight highly divergent brook trout (Salvelinus fontinalis) populations varying in Ne (range 44–589). In a common garden, we compared three fitness-related traits among hybrid and non-hybrid crosses. Contrary to expectations, we found little evidence of outbreeding depression; instead, hybridization effects were mostly neutral (60/66 non-hybrid vs. hybrid comparisons) with some support for heterosis (6/66). When controlling for maternal-family effects, several factors influenced fitness-related traits but cumulatively explained little variance in relative hybrid fitness (0–6.4%). For instance, when hybridized dams came from small Ne populations, relative fitness increased for some traits (length at hatch), suggesting heterosis, yet decreased at other traits (survival to hatch), suggesting outbreeding depression. Trait inconsistencies in relative hybrid fitness were also observed under different degrees of environmental stress. Results also differed when family variance in hybridization effects was unaccounted for. Collectively, our results suggest that, under certain conditions, current guidelines on genetic rescue and associated outbreeding risks might be too conservative. The occurrence of genetic rescue and outbreeding depression, nonetheless, remain difficult to predict because families and individual traits can express different effects from hybridization within populations. [ABSTRACT FROM AUTHOR]

The importance of genetic drift in shaping patterns of adaptive genetic variation in nature is poorly known. Genetic drift should drive partially recessive deleterious mutations to high frequency, and inter‐population crosses may therefore exhibit heterosis (increased fitness relative to intra‐population crosses). Low genetic diversity and greater genetic distance between populations should increase the magnitude of heterosis. Moreover, drift and selection should remove strongly deleterious recessive alleles from individual populations, resulting in reduced inbreeding depression. To estimate heterosis, we crossed 90 independent line pairs of Arabidopsis thaliana from 15 pairs of natural populations sampled across Fennoscandia and crossed an additional 41 line pairs from a subset of four of these populations to estimate inbreeding depression. We measured lifetime fitness of crosses relative to parents in a large outdoor common garden (8,448 plants in total) in central Sweden. To examine the effects of genetic diversity and genetic distance on heterosis, we genotyped parental lines for 869 SNPs. Overall, genetic variation within populations was low (median expected heterozygosity = 0.02), and genetic differentiation was high (median FST = 0.82). Crosses between 10 of 15 population pairs exhibited significant heterosis, with magnitudes of heterosis as high as 117%. We found no significant inbreeding depression, suggesting that the observed heterosis is due to fixation of mildly deleterious alleles within populations. Widespread and substantial heterosis indicates an important role for drift in shaping genetic variation, but there was no significant relationship between fitness of crosses relative to parents and genetic diversity or genetic distance between populations. [ABSTRACT FROM AUTHOR]

Most disturbed habitats in the tropics and subtropics harbor numerous species of invasive ants, and occasionally the same species has been introduced repeatedly from multiple geographical sources. We examined how experimental crossbreeding between sexuals from different populations affects the fitness of queens of the tramp ant Cardiocondyla itsukii, which is widely distributed in Asia and the Pacific Islands. Eggs laid by queens that mated with nestmate males had a higher hatching rate than eggs laid by queens mated to males from neighboring (Hawaii × Kauai) or distant introduced populations (Hawaii/Kauai × Okinawa). Furthermore, inbreeding queens had a longer lifespan and produced a less female‐biased offspring sex ratio than queens from allopatric mating. This suggests that the genetic divergence between different source populations may already be so large that in case of multiple invasions eventual crossbreeding might negatively affect the fitness of tramp ants. [ABSTRACT FROM AUTHOR]

Local adaptation has been demonstrated in spatially or temporally distant animal populations but seldom in proximate populations. To address the scale of local adaptation in Pacific salmon (Oncorhynchus spp.), two generations of hybrids between temporally separated spawning segments were made in a population of pink salmon (O. gorbuscha) and compared with controls to evaluate the genetic architecture underlying adult migration time and to test for declines in marine survival that resulted from outbreeding depression. Bayesian mixed‐effects models revealed that adult migration times in hybrid lines were intermediate to those of controls and that additive sources of genetic variation were significant, thereby indicating that local adaptation has acted on additive genetic variation in shaping this trait. Similarly, a line cross analysis revealed that an additive model best described the genetic architecture of adult migration time. In contrast, marine survival was generally similar between control and hybrid lines, which suggested that the effect of outbreeding upon marine survival was minimal at such a fine scale of genetic divergence. The implications of these results are that (a) local adaptation can facilitate genetic divergence of life history traits between proximate subpopulations; (b) artificial relaxation of natural barriers to gene flow can cause maladaptive shifts in life history traits; and (c) wild populations may harbour fine‐scale adaptive variation that supports productivity and sustainability. [ABSTRACT FROM AUTHOR]