Inbreeding and loss of genetic diversity increase extinction risk

Inbreeding reduces survival and reproduction (i.e. it causes inbreeding depression), and thereby increases extinction risk. Inbreeding depression is due to increased homozygosity for harmful alleles and at loci exhibiting heterozygote advantage. Inbreeding depression is nearly universal in sexually reproducing organisms that are diploid or have higher ploidies. Impacts of inbreeding are generally greater in species that naturally outbreed than those that inbreed, in stressful than benign environments, and for fitness than peripheral traits. Harmful effects accumulate across the life cycle, resulting in devastating effects on total fitness in outbreeding species.Species face ubiquitous environmental change and must adapt or they will go extinct. Genetic diversity is the raw material required for evolutionary adaptation. However, loss of genetic diversity is unavoidable in small isolated populations, diminishing their capacity to evolve in response to environmental changes, and thereby increasing extinction risk.

Managing gene flow among isolated population fragments

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.

Global climate change increases the need for genetic management

Adverse genetic impacts on fragmented populations are expected to worsen under global climate change. Many populations and species may not be able to adapt in situ, or to move unassisted to suitable habitat. Management may reduce these threats by augmenting genetic diversity to improve the ability to adapt evolutionarily, by translocation, including that outside the species’ historical range, and by ameliorating non-genetic threats. Global climate change amplifies the need for genetic management of fragmented populations.

Appropriate delineation of species for conservation purposes

The first step in conservation management is to delineate groups for separate versus combined management. However, there are many problems with species delineation, including diverse species definitions, lack of standardized protocols, and poor repeatability of delineations. Definitions that are too broad will lead to outbreeding depression if populations are crossed, while those that split excessively may preclude genetic rescue of small inbred populations with low genetic diversity. To minimize these problems, we recommend the use of species concepts based upon reproductive isolation (such as the Biological Species Concept) and advise against the use of Phylogenetic and General Lineage Species Concepts. We provide guidelines as to when taxonomy requires revision and outline protocols for robust species delineations.

Genetic rescue resulting from gene flow

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.

Are there populations suffering genetic erosion that would benefit from gene flow?

Evidence of population structure and limited gene flow often leads to the questionable conclusion that populations should be managed as separate unit. A paradigm shift is needed where evidence of genetic differentiation among populations is followed by an assessment of whether populations are suffering genetic erosion, whether there are other populations to which they could be crossed, and whether the crosses would be beneficial, or harmful, and if beneficial, whether the benefits would be large enough to justify a genetic rescue attempt. Here we address these questions based on the principles established in the preceding chapters.

Population fragmentation causes inadequate gene flow and increases extinction risk

Most species now have fragmented distributions, often with adverse genetic consequences. The genetic impacts of population fragmentation depend critically upon gene flow among fragments and their effective sizes. Fragmentation with cessation of gene flow is highly harmful in the long term, leading to greater inbreeding, increased loss of genetic diversity, decreased likelihood of evolutionary adaptation and elevated extinction risk, when compared to a single population of the same total size. The consequences of fragmentation with limited gene flow typically lie between those for a large population with random mating and isolated population fragments with no gene flow.

Evolutionary genetics of small populations

Genetic management of fragmented populations involves the application of evolutionary genetic theory and knowledge to alleviate problems due to inbreeding and loss of genetic diversity in small population fragments. Populations evolve through the effects of mutation, natural selection, chance (genetic drift), and gene flow. Large outbreeding sexually reproducing populations typically contain substantial genetic diversity, while small populations typically contain reduced levels. Genetic impacts of small population size on inbreeding, loss of genetic diversity and population differentiation are determined by the genetically effective population size, which is usually much smaller than the number of individuals.

Introduction

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.