Conservation genetics – molecular detectives at workThe new science of conservation genetics is providing important clues in the fight against the extinction of species.
Key text
Back to basics You will get more from this topic if you have mastered the basics of DNA and genes – this link will take you to an annotated list of sites with helpful background information. Key textConservation geneticists take an understanding of genetics and evolution and apply it to conservation problems (Box 1: What is genetics?). They do so in two distinct ways. The first is largely concerned with genetic problems that may arise in small populations (Box 2: The problem of small populations). The second, which is the focus of this topic, uses genetic techniques to learn more about the history, biology and ecology of threatened species.One of the principles of genetics is that the similarity between the DNA – the basic genetic material – of two individuals will depend on how closely related they are. The DNA of two baboons, for example, will be more similar than the DNA of a baboon and a chimpanzee. Conservation geneticists put this principle into practice in much of their work. Conservation geneticists can be thought of as molecular detectives. They use tiny scraps of evidence to piece together events of the past. Recently, they employed their innovative techniques to solve the strange case of the missing tammar wallabies. The intriguing case of the tammar wallaby When colonial administrator Sir George Grey released a handful of tammar wallabies onto New Zealand’s Kawau Island in 1870, he may have inadvertently performed a great service for conservation. The species was once widespread in South Australia, but in the late 1800s the population on mainland Australia was in decline, a victim of habitat destruction, fox predation and shooting. By the early 1900s, it was extinct on the mainland. Fortunately, the species itself wasn’t extinct. It still persisted on some islands, including Kangaroo Island. But these populations had been separated from the mainland population for around 10,000 years and were therefore likely to be quite different genetically. When the mainland population became extinct, the species lost a significant part of its genetic diversity. Sir George didn’t record where he obtained his animals, but it was an important question. If they were from the Australian mainland, then their descendants were the last survivors of an otherwise extinct population, and were likely to be of considerable value for biodiversity conservation. The only way to determine their origin was to compare their genetic make-up with that of the Kangaroo Island population, so this was what conservation biologists at the Cooperative Research Centre for Marsupial Conservation and Management did (Box 3: Molecular detectives at work). The evidence they uncovered was quite conclusive and great news for conservation. It showed that the Kawau Island colony had been established with animals from the mainland population. Land managers in South Australia are now considering plans to re-introduce the animals to their original range. The many uses of conservation genetics Genetic studies of plants and animals have several advantages. It often isn’t necessary to kill or even capture an organism to study it, since only small amounts of genetic material are needed. In studying the highly endangered northern hairy-nosed wombat, for example, researchers use samples of hair obtained from hair-traps located at the mouths of burrows. Genetic studies can also be carried out on tissue taken from dead organisms – this has proved useful, for example, in identifying illegally harvested whale meat. Conservation geneticists can also use genetic techniques to determine the amount of inter-breeding between different populations of the same species – do populations intermingle, or are they cut off from each other by some kind of barrier? This is often of critical importance to conservation efforts. On the one hand, small, isolated populations – with no ‘top-ups’ from other populations of the same species – are more likely to become extinct; on the other hand, it may be desirable to keep certain populations separate to minimise genetic mixing and thereby maintain maximum diversity. The potential of genetics as an aid to conservation is enormous and is only just starting to be realised. Yet, as conservation geneticists themselves point out, it cannot answer all the critical questions about species. Like any detective work, good ecological investigation requires information from a wide range of sources. Related Nova topic
A molecule of DNA is a bit like a spiral staircase, consisting of two molecular ‘strands’ twisted into a double helix. Projecting from the strands are four different kinds of bases, or molecular sub-units – adenine (A), thymine (T), cytosine (C) and guanine (G). These come together to form base ‘pairs’, C bonding with G and T bonding with A, thereby joining the two strands together. In this way, the base pairs form the steps of the ‘staircase’. What do these base pairs do? For many of the pairs in a DNA strand, the answer to this question is unknown. But for others, a sequence of three base pairs, or ‘steps’, forms a ‘codon’, which codes for one of 20 amino acids. These are the building blocks of proteins, which to a large extent determine the appearance of an organism and the way it works. A set of codons that codes for a given protein is called a gene – and gives its name to the science of genetics. Different forms, or alleles, of the same gene result in the expression of different characteristics. For example, some people have blue eyes and others have green or brown because of slight differences in the sequence of codons that code for the genes that determine eye-colour. Mutations Over generations, the DNA changes, or mutates, as a result of chemical accidents during replication. The process of mutation is an essential force in nature. Charles Darwin hypothesised in the 1800s that sometimes a mutation would improve an organism’s chance of survival – a gene that gave better night-vision to a cat, for example, may increase the cat’s hunting prowess and therefore its ability to catch the food it needed to survive. If a mutation proved beneficial, it would become more common because individuals that inherited it would be more likely to survive and to pass the mutation on to the next generation. This process of change is called ‘natural selection’ and is thought to be a major force in evolution. The importance of variation Difference between individuals – genetic variation – is thought by many conservation geneticists to hold the key to the long-term survival of species. Those species with a high degree of genetic diversity may prove more adaptable in the face of environmental change – such as exposure to a new disease or a change in climate. Some individuals will be unable to cope, but others may carry the genetic equipment they need to survive and thrive. Darwin called it ‘survival of the fittest’. Related sites
In the 1980s, conservation biologists became concerned about genetic problems that might arise within such small populations. One reason was that since small populations generally have low genetic variability, their capacity to adapt to environmental change – to evolve – is also low. Therefore, their ability to survive in the long term becomes doubtful. Another reason was the problem of inbreeding depression – the tendency of closely related organisms to produce offspring with deformities that limit their chances of survival. This is probably the reason behind the taboo in most human societies against marriage between close relatives. There are many other reasons to fear for a species when its numbers are low. In recent years, ecologists have argued that environmental pressures posed by feral animals, land clearing, fire and other threats will wipe out many species long before they succumb to problems arising from their low genetic variation. With this in mind, ecologists and conservation geneticists have attempted to combine all possible threats in the concept of ‘minimum viable population’ (MVP). This is the smallest possible population that has a good chance of surviving in the face of genetic, environmental and other pressures. Population viability analysis works out the MVP for different threatened species, which conservation managers then use in their efforts to conserve the species. Related site
The technique used by the research team involved taking DNA samples from the animals and searching for segments that contained small sequences of base pairs that are repeated numerous times. These segments are called microsatellite markers. Markers that have a different number of repeat sequences are called alleles – alternative forms of the same genetic sequence. The investigators took DNA samples from about 30 individuals in both the Kawau and Kangaroo Island populations. Back in the laboratory, they looked at the DNA of these individuals for sequences that would be useful as markers to distinguish between the two populations. Once these were obtained, the researchers used a technique called polymerase chain reaction (PCR), in conjunction with electrophoresis, to genotype – or determine the alleles of – individual wallabies. Polymerase chain reaction Scientists use PCR to make multiple copies of DNA markers so that they have sufficient quantities with which to work. The first step is to make primers – these have a sequence of bases that is complementary to the base pairs at either end of the marker. The DNA is separated into its two strands and the primers are allowed to bind to these complementary sequences. An enzyme (called polymerase) is then added, which helps a chemical reaction take place in which the marker is replicated many times – a process called amplification. Millions of copies of the DNA marker are produced in this way. These are then used in the next step, electrophoresis. Electrophoresis The technology associated with genetics is changing rapidly – a technique used widely 5 years ago might well be outdated today. But electrophoresis remains one of the most tried and true techniques. It involves placing the amplified DNA markers onto a gel (a thin slice of jelly-like material). The gel is then attached to electrodes so that an electric current flows through it. Since the DNA markers have a negative charge, they migrate along the gel towards the positive electrode. After a number of hours, the researcher removes the electrodes and analyses the position of the markers. Shorter markers (those with the base sequence repeated fewer times) move faster than longer ones and travel further in the same time, so the markers are separated by size. By comparing the distance travelled by the markers, conservation geneticists can detect differences in their length. Using a DNA sample from each wallaby, they can determine the alleles of each animal. From this, they can estimate the genetic differences between individual animals and between populations. In the case of the tammar wallaby, the results of this procedure were exciting. Of the alleles located in the Kangaroo Island population, 37 were absent from the Kawau Island population, suggesting that the two populations were quite distinct genetically and, therefore, that the Kawau Island population had originated on the mainland of Australia. Related sites
Australasian Science June 2000, pages 21-22 Thinking small is saving our mangrove forests (by Tina Maguire) Shows how genetic studies of mangrove populations can aid rehabilitation projects.
March 2000, pages 19-21 New genes rescue snake population (by Peter Pockley)
Ecos No. 130, 2006, pages 24-27 Plant detectives struggle against time Describes the efforts of botanists in Armenia to catalog and save invaluable lineages of the world’s food plants.
No. 113, 2002, pages 14-15 Patch genetics (by Steve Davidson) Describes genetic studies of isolated remnants of native vegetation that are designed to help develop effective conservation strategies.
No. 107, 2001, page 5 Squirrelling the macadamia (by Steve Davidson) Describes efforts to preserve wild macadamia varieties.
No. 106, 2001, pages 15-18 Rare qualities (by Steve Davidson) Ask whether there is a connection between the abundance of plant species and their ecological characteristics.
No. 97, 1998, pages 4-5 Silvertop study reveals healthy genetics A CSIRO study concluded that clear-fell harvesting does not reduce the genetic diversity in stands of Eucalyptus sieberi.
No. 93, 1997, pages 29-33 Teetering on the brink (by Alastair Sarre) A report on how the loss of populations of a species makes the whole species more vulnerable to extinction because genetic diversity is lost.
No. 89, 1996, pages 6-7 Gene loss and climate change – a recipe for extinction? Explains how contracted habitats resulting from climate change could lead to population decline and loss of genetic diversity.
No. 88, 1996, page 3 A few daisies short of the chain (by Tim Twaites) Studies of a native daisy are aiming to establish what size plant population is needed to avoid inbreeding.
No. 88, 1996, page 36 Woodlands meet a grave demise (by Roger Beckmann) Shows how information about woodland biodiversity and the genetics of white box trees have helped in the formulation of a management strategy for white box woodlands.
New Scientist 13 August 2005, page 41-43 Inbreeding: Keeping it in the family (by Susan Gaidos) Looks at why some species appear to benefit from inbreeding.
2 November 2004 Threatened species may spiral into oblivion (by Katherine Unger) Suggests that inbreeding puts big cats and other species at more risk from parasites and infections.
18 October 2003, page 38-41 Too close for comfort (by Lynn Dicks) Looks at the possible implications of inbreeding for captive breeding projects.
Science 1 July 2005, page 90 What determines species diversity? (by Elizabeth Pennisi) Answers the question: What determines species diversity?
A good introduction.
Australia's biodiversity – Levels of biodiversity (Australian Museum) Explains the different levels of biodiversity – genetic, species and
ecosystem diversity.
Biodiversity and endangered species (CSIRO Information Sheet)
Describes Australia's position as a 'mega diverse' country and what CSIRO is doing to conserve Australia's biodiversity.
Biodiversity and its value (Australian Government Department of the Environment and Water Resources)
Defines biodiversity and explains why it is important.
Handbook for the MSc course in conservation biology (University of Cape Town, South Africa)
A very readable series of modules on conservation biology, including 'Biodiversity and the units of conservation', 'Demography and population viability analysis', and 'Conservation genetics'.
base. Any one of four nitrogenous (nitrogen-containing) bases (adenine, thymine, guanine and cytosine). The sequence of the bases in DNA determines the sequence of amino acids in all proteins found in living things. complementary. Describes the specific matching of base pairs. This matching occurs because the structure of one base precisely fits with, and bonds to, another specific base. In DNA adenine and thymine are complementary and form a base pair, as do cytosine and guanine. When pairing occurs between DNA and RNA, adenine and uracil are complementary, and cytosine and guanine are complementary. DNA (deoxyribonucleic acid). The nucleic acid forming the genetic material of all organisms with the exception of some viruses which have RNA. DNA is present in the nucleus and other organelles such as mitochondria and chloroplasts. extinct. Occurring no more. The word is usually used for species but can apply to any level of classification. Recent extinctions are hard to prove, and an ‘official’ limit of 50 years with no recorded sightings of the species is now used. enzyme. A protein that acts as a catalyst. Every chemical reaction in living organisms is facilitated by an enzyme. gene. The basic unit of inheritance. A gene is a segment of DNA that specifies the structure of a protein or an RNA molecule. genetic diversity. The variety of different types of genes or alleles in a species or population. Genetic diversity is really a form of biodiversity. species. Living things of the same kind that are potentially capable of breeding and producing fertile offspring. Theoretically, plants or animals of different species cannot interbreed. However, occasionally this does not hold true.
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This topic is sponsored by the CRC for the Conservation and Management of Marsupials.
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