Search Results

Crosses between populations within species sometimes result in reduced fitness, especially in F₂ and later generations (outbreeding depression). The primary mechanisms causing outbreeding depression in crosses between populations are fixed chromosomal differences and adaptive genetic differences, especially for long-isolated populations. Outbreeding depression is usually observed after crossing populations with ploidy differences or fixed differences for translocations, inversions or centric fusions: the magnitudes are usually ploidy > translocations and monobrachial centric fusions > inversions and simple centric fusions. Populations adapted to different environments (but with the same karyotype) often exhibit outbreeding depression when crossed, especially in the F₂ and later generations. Even if outbreeding depression occurs, it is often only temporary, as natural selection acts to remove it, especially in large populations.

Inbreeding is reduced and genetic diversity enhanced when a small isolated inbred population is crossed to another unrelated population. Crossing can have beneficial or harmful effects on fitness, but beneficial effects predominate, and the risks of harmful ones (outbreeding depression) can be predicted and avoided. For crosses with a low risk of outbreeding depression, there are large and consistent benefits on fitness that persist across generations in natural outbreeders. Benefits are greater in species that naturally outbreed than those that inbreed, and increase with the difference in inbreeding coefficient between crossed and inbred populations in mothers and zygotes. Crossing between populations also enhances the ability to evolve. Outbreeding depression result primarily from populations belonging to different taxa, having fixed chromosome differences, being genetically adapted to different environments, having a long history of isolation, or to combinations of these, and can be avoided by screening out population combinations with these characteristics.

Inbreeding is reduced and genetic diversity enhanced when a small isolated inbred population is crossed to another unrelated population. Crossing can have beneficial or harmful effects on fitness, but beneficial effects predominate, and the risks of harmful ones (outbreeding depression) can be predicted and avoided. For crosses with a low risk of outbreeding depression, there are large and consistent benefits on fitness that persist across generations in outbreeding species. Benefits are greater in species that naturally outbreed than those that inbreed, and increase with the difference in inbreeding coefficient between crossed and inbred populations in mothers and zygotes. However, benefits are similar across invertebrates, vertebrates and plants. There are also important benefits for evolutionary potential of crossing between populations.

Hybridization occurs between species or populations, and can arise from either natural or anthropogenic causes. Hybridization is important in natural evolutionary processes, but can be a harmful force reducing species identity and reproductive success. Hybridization can increase fitness through heterosis, or reduce fitness through outbreeding depression. Genetic analysis can effectively identify hybridization and has frequently used diagnostic loci that have different allele frequencies in the parents. Hybrid indices or admixture analyses use proportions of parental ancestry in individuals to identify hybrids. Hybridization contributes to decline and extinction of species through loss of reproductive potential and reduced population growth, or through genetic mixing and loss of genetically distinct populations. Determining whether hybridization is natural or anthropogenic is crucial for conservation. Protection of hybrids is often based on whether they are genetically distinct through long-term isolation or speciation, or whether they represent recent, ongoing, or anthropogenic hybridization.

Genetic management of fragmented populations is one of the major, largely unaddressed issues in biodiversity conservation. Many species across the planet have fragmented distributions with small isolated populations that are potentially suffering from inbreeding and loss of genetic diversity (genetic erosion), leading to elevated extinction risk. Fortunately, genetic deterioration can usually be remedied by augmenting gene flow (crossing between populations within species), yet this is rarely done, in part because of fears that crossing may be harmful (but it is possible to predict when this will occur). Benefits and risks of genetic problems are sometimes altered in species with diverse mating systems and modes of inheritance. Adequate genetic management depends on appropriate delineation of species. We address management of gene flow between previously isolated populations and genetic management under global climate change.

Genetic management of fragmented populations is one of the major, largely unaddressed issues in biodiversity conservation. Many species across the planet have fragmented distributions with small isolated populations that are potentially suffering from inbreeding and loss of genetic diversity (genetic erosion), leading to elevated extinction risk. Fortunately, genetic deterioration can usually be remedied by gene flow from another population (crossing between populations within species), yet this is rarely done, in part because of fears that crossing may be harmful (but we can predict when this will occur). We address management of gene flow between previously isolated populations and genetic management under global climate change.

We recommend augmentation of gene flow for isolated population fragments that are suffering inbreeding and low genetic diversity, provided that proposed population crosses have low risks of outbreeding depression, and the predicted benefits justify the financial costs.

When the decision is made to augment gene flow into an isolated population, managers must decide how to augment gene flow, when to start, from where to take the individuals or gametes to be added, how many, which individuals, how often and when to cease. Even without detailed genetic data, sound genetic management strategies for augmenting gene flow can be instituted by considering population genetics theory, and/or computer simulations. When detailed data are lacking, moving (translocating) some individuals into isolated inbred population fragments is better than moving none, as long as the risk of outbreeding depression is low.

Even without detailed genetic data, sound genetic management strategies for augmenting gene flow can be instituted by considering population genetics theory, and/or computer simulations. When detailed data are lacking, moving (translocating) some individuals into isolated inbred population fragments is better than moving none, as long as the risk of outbreeding depression is low. With more detailed genetic information, more precise genetic management of fragmented populations can be achieved. Using mean kinship within and between populations (estimated from modeling, pedigrees, genetic markers or genomes), and moving individuals among fragments with the lowest between fragment mean kinships provides the best approach to gene flow management. Populations should then be monitored to confirm that movement of individuals has resulted in the desired levels of gene flow, genetic diversity has been enhanced, and that the status of the population is improving.