Issues Magazine

Climate Change Adaptation: Increasing Evolutionary Resilience

Issues 94: Climate Change Adaptation

Issues 94: Climate Change Adaptation

By Ary Hoffmann

Climate change is expected to have dramatic effects on Australia’s biodiversity, and we need to allow species and natural communities to adapt. We can assist evolutionary resilience by increasing genetic variation and the exchange of genetic material.

In Australia we currently face a biodiversity crisis due to the emerging threats of climate change and expanding human population pressures. Based on the ways in which the current distributions of plants and animals associate with climate, it is expected that many species and entire communities will be threatened with extinction over the next few decades.

Many of our regions are expected to become climatically unsuitable for our native fauna and flora. Yet species and communities are not static entities. Individuals adapt by changes to their physiology and life history; populations adapt through these individual processes and through evolution and migration; and species adapt through these population processes and by further diversifying through evolution. Where species become locally extinct, others may take over in communities to maintain ecological functions.

The traditional view of evolutionary adaptation is that it is a slow process taking place over hundreds or thousands of years and involving the gradual accumulation of genetic change within species following ongoing mutation. However, we’ve learnt from changes in a variety of traits like predator avoidance, toxin resistance and dormancy that evolution can be extremely rapid, occurring over just a few generations. It has become evident from molecular data that a lot of evolutionary novelty locked up in populations can be readily accessed by natural selection. Moreover, genetic exchange between species appears to be much more common than we thought a few years ago, providing new sources of evolutionary novelty.

Genetic exchange occurs among species through hybridisation. The role of hybridisation in the formation of new species (“speciation”) has been recognised for some time in plants like the eucalypts. Hybridisation has often been suspected in many plant groups because species possess characteristics that represent a combination of characteristics of parental species. Speciation by hybridisation is particularly common in plants that are “polyploids”, possessing multiple copies of chromosomes that often come from different parent species.

Yet it is now also recognised that genetic exchange through hybridisation can be important for adaptation via evolution. Widespread gene exchange can occur among related species or subspecies that have become partly reproductively isolated through the transfer of parts of the genome. This might involve a rare hybridisation event followed by introgression of the parts of the chromosome of one species into the genome of another. The genome of the recipient species may then remain relatively intact but a part is altered, and this part can become common if it is favoured by natural selection.

Genetic exchange of this nature may be common in many of our native plant groups, including grasses. In our research at the University of Melbourne on alpine grasses, we have found common exchange between different species of grasses from the same region; parts of the genome of different species from the same region like Tasmania or the Victorian Alps are more similar to each other genetically than to populations of the same species from different regions.

The process of genetic exchange through hybridisation is also being detected in a wide range of animal species, including fish, lizards, insects and mammals. In some cases the impact of these events is quite dramatic; for instance, parts of a chromosome exchanged between different mosquito species have been critical for one species expanding into much drier and hotter areas.

Genetic exchange is not just confined to closely related species. For instance, many animal genomes contain genes or partial copies of the genomes of microbes; their role is often unclear, but it seems that they can help in the evolution of entirely new functions – like the development of defences against predators.

As well as occurring among species, genetic exchange also takes place between different populations separated in historical time. These exchanges are much more common than hybridisation between species because there are no (or only weak) reproductive barriers to mating. Dispersal patterns in plants are typically leptokurtic, where most seed and pollen movement occurs over a short distance but some movement of genes occurs over very long distances). In animals these patterns can also occur, and they are crucial for the colonisation of new habitats. For instance, rare flooding events lead to dispersal of many native freshwater fish to new areas. These very occasional movements assist in ongoing gene exchange. Thus when populations are adapted to one environment, the adapted genes might then spread quite widely through this process across a landscape.

When gene exchange occurs through crosses between populations or species that have been isolated for a long time (and particularly when these groups have become partly reproductively isolated), the immediate effects can be negative. The growth of hybrid plants or animals from crosses might be stunted, there might be problems in producing viable offspring, or the hybrids might even be sterile.

However, while some hybrids might be stunted, others can display vigour, particularly when there are no large chromosomal rearrangements in the groups being crossed. And even when there is a loss of fitness in hybrids, this can be restored quite rapidly once the hybrids are crossed to their parental species, leading to successful exchange.

This process of genetic exchange between species through the ongoing production of hybrids is quite common in nature; in fact, quite a few related animals and plants exist as adjacent species with a zone of ongoing hybridisation separating them. Well-known examples include the hybrid zones between plumage forms of the Australian magpie and between Litoria frog species in eastern Australia.

Natural selection is a highly effective process in sorting out favourable genetic combinations and maintaining genetic differences among populations – and evolution via natural selection is a very powerful mechanism that produces rapid changes in organisms. When there is a high level of gene flow in plant species along an environmental gradient – like an elevation gradient – natural selection can still maintain genetically based morphological and physiological differences along the gradient in the face of pollen movement, seed movement and other forms of dispersal. Animals are often more mobile than plants, but there is still a lot of evidence in insects and vertebrates that strong differences are maintained along gradients by natural selection.

Environmental managers tend to focus on the negative aspects of gene exchange rather than its positive effects on generating novelty for natural selection. There is a term for this process with some rather negative connotations: genetic pollution. The issue of genetic pollution is raised when the potential impacts of expanding agricultural production, forestry, weed invasions and even expanding ranges of native species are being considered. If there is enough “pollution”, the uniqueness of species and populations of species might be lost.

But what happens when “pollution” is also a creative force that, combined with natural selection, can produce novel and well-adapted combinations of genes?

The issue of genetic pollution is central to current revegetation strategies that focus on local “provenancing” – the idea that seed and plants for revegetation must have a local origin. At present, seed used for establishing nursery stock is collected within a local area often spanning only a few square kilometres and potentially involving only a small number of trees or shrubs. However, this approach does not capture leptokurtic dispersal distributions and can therefore effectively prevent the introduction of genetic novelty from distant populations and minimise variability available for natural selection, particularly in landscapes that have become fragmented because vegetation has been cleared.

Local provenancing makes sense in revegetating environments that are not changing; it captures any locally adapted genotypes that have arisen as a consequence of effective natural selection in the past. However, environments are changing much more rapidly than in the past.

As increasing periods of drought and extreme temperatures develop over the next few decades, maintaining and preferably increasing the ability of flora and fauna to adapt becomes critical. Plants and animals will need to adapt to increasing extremes of heat and to arid conditions, and also to increasing climatic variability generally along with altered patterns of ecological interactions among organisms.

Some species will be able to take advantage of the changing conditions, becoming more competitive, expanding their distributions and being able to withstand predation and parasitism pressures. Many species will face increasingly stressed conditions.

Evolutionary adaptation becomes critical in such situations. When outbreaks of pest insects or fungal diseases occur as a consequence of favourable climatic conditions, some genotypes of host plants will be better able to withstand them than others, leading to strong natural selection and evolutionary adaptation. This process can be extremely rapid and occur over a few generations. Individuals will also differ genetically in their ability to withstand heat, drought or competition from other invading species.

But effective evolutionary adaptation needs genetic variation and novelty from within a population, through an influx of genes from other populations, and/or from hybridisation.

The problem of low genetic variation is exacerbated when the local seed comes from populations that lack genetic variation. When the size of a natural population is small, as is often the case for native flora and fauna within a fragmented landscape, genetic variation will decrease quite rapidly across a few generations and inbreeding problems will also emerge. This means a loss of evolutionary potential – and an increased risk of extinction.

The increase in risk of extinction resulting from a loss of variability can be quite marked. A recent estimate in shrimp by Jeffrey Markert and colleagues suggests that the risk increases several-fold when genetic diversity is halved, particularly under stressful conditions (BMC Evolutionary Biology 2010, 10: 205).

For populations of threatened species there is currently an emphasis on preserving populations in their “current” state, which can mean (as in the case of local provenancing) avoiding any mixing of populations or more distantly related lineages like subspecies or closely related species.

But what happens when these populations have little genetic variation, or (as is increasingly likely under climate change) suffer from a rapid loss of genetic variation as environments deteriorate? Losses of genetic variation in threatened populations can occur extremely rapidly, as in the case of a mountain pygmy possum (Burramys parvus) population at Mount Buller in Victoria, which lost more than half of its genetic variation in just a few years.

When recent analyses suggest that many – perhaps the majority – of species will need to persist in areas well outside the environmental space they currently occupy, there is clearly a need to take steps that help to increase the likelihood of adaptation. It is therefore critical to maximise evolutionary adaptive potential and explore novel ways of enhancing it if possible.

Evolutionary processes have allowed the persistence of our major animal and plant groups for many millions of years across substantial periods of past climate change. What practical steps might now be taken to assist in this process?

Steps to maintain biodiversity under climate change have largely focused on maintaining ecological processes, restoring connectivity in the landscape, and identifying areas outside the national reserve system that might contain high levels of species diversity. However, additional steps can be taken when building “evolutionary resilience”.

For revegetation projects, a strong emphasis is needed on widening seed collections and nursery stock to capture genetic diversity across a wider part of a species range. While most seed and stock might still consist of local provenances, some of the material should come from further out, encompassing the long tail of the leptokurtic distribution. By having a variety of genotypes from diverse environments within an environment, it is more likely that at least some genotypes will survive, providing an insurance policy against an unpredictable set of environmental changes.

The size of an area used as a source of seed will need to vary among species. For species that are pollinated by wind or by animals moving large distances, seed should be sampled across a wide area because, historically, gene flow in such species would have been high.

This also applies to species with seed widely dispersed by animals. Such species are less likely to suffer negative effects of hybridisation when crosses take place with populations outside an area. For instance, the perennial rhizomatous plant Dianella revolute has seed widely dispersed by birds and pollen dispersed over large distances by native bees, suggesting that material can be sourced across tens of kilometres. Guidelines initially developed based on plant characteristics can be further refined as genetic data become available, providing a direct estimate of gene flow among populations.

For threatened species, the potential benefits of mixing gene pools across populations and even related species needs to be explored widely. While this might result in occasional loss of fitness in the short run, the expected benefits under a rapidly changing world are likely to be substantial.

To counter any potential negative effects of hybridisation on fitness, it may be possible to test for these effects in population or species crosses prior to mixing. For Burramys parvus, a recent program has been initiated at Mount Buller to introduce material from other populations to boost genetic variation. So far the population hybrids appear to have high fitness, pointing to an effective approach for conservation of other threatened mammals. This approach is likely to be more cost-effective than captive breeding of threatened species, which requires substantial infrastructure and might ultimately be unsuccessful given the low success of reintroduction programs based on captive material.

In situations where existing stands of vegetation occur within fragmented landscapes, the movement of seed and seedlings across fragments should be considered. An initial target might be to restore genetic exchange to levels existing in the past. But with species facing a period of environmental change likely to be more rapid than they have previously encountered, genetic exchange might need to be enhanced to boost evolutionary diversity upon which natural selection can act.

Ultimately, biodiversity is the result of evolutionary processes, and an important challenge in climate change adaptation is to find ways of maintaining and enhancing these processes. Evolutionary adaptation is a dynamic process that works on variability, and this needs to be expanded. We have already developed ways of using variability for increasing agricultural productivity in the face of climate change, involving artificial selection of genotypes within populations, across populations and across species. This has resulted in species with desirable attributes like resistance to drought, frost, insect outbreaks and diseases.

Now is the time to also reconsider variability within a biodiversity context and build evolutionary resilience.