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TRENDS IN GENETIC DIVERSITY

Genetic diversity is based on variation between genes, i.e. in the functional units of heredity in organisms. Not all the genetic material has a role in heredity, and in some organisms a large proportion of DNA is highly repetitive in sequence and has no known function. The features of an organism are determined fundamentally by the genes received from the previous generation, by the way these genes are expressed and interact, and to an extent by environmental effects on the organism.

New genetic diversity arises when chromosomes are not copied with absolute accuracy (i.e. undergo mutation) and through re-assortment of genes on chromosomes when sex cells (sperm and egg cells) are being produced. In this latter process, different forms of genes at corresponding positions on maternal and paternal chromosomes can become exchanged, and entire chromosomes of maternal and paternal origin become sorted into different combinations. If the features determined by new mutations and new permutations of genetic material improve survival of individuals bearing them, the frequency of these genes in the population will increase.

Genetic diversity is important as it provides the raw material of evolution and because it enables adaptation and change in organisms. High levels of genetic diversity should allow species to be flexible in the face of environmental change, whereas low genetic diversity, for example in a small isolated population, tends to increase the risk of extinction.

Genetic diversity is also critically important for the continuing ability of human societies to derive economic and social benefits from biodiversity. The variability is an insurance policy that protects against risks that could reduce such benefits, for example widespread pest outbreaks or fluctuations in crop production from year to year.

Complex biochemical techniques can be used to measure the frequency and distribution of different gene products in species' populations, or genetic diversity may be assessed at organism level, for example in terms of the measurable production or pest resistance qualities of different crop varieties.

Bacteria differ from other organisms in that they can reproduce at a very high rate by dividing in two. They also exchange genetic material, but this sexual process is not associated with reproduction as it is in other organisms; instead, two bacteria in contact may directly exchange genetic material, or a single bacterium may take in DNA deposited in the environment by another bacterium. These exchanges, the latter in particular, are not always constrained by the kind of barriers that in higher organisms restrict exchange of genetic material to members of the same species. Genetic material can also be passed between bacteria by viral infection. The apparent frequency with which genes move between populations of bacteria means that these organisms are able to generate high levels of new genetic diversity for natural selection to act upon, and favourable genetic material can spread very quickly in rapidly reproducing populations. This explains why resistance to anti-microbial substances can arise so readily in bacteria.

Declining genetic diversity

Human activities readily lead to change in the genetic diversity of populations of wild or domestic organisms. In extreme cases, genetic diversity is reduced to zero when a species is rendered extinct. More usually, it is reduced to some extent when populations of species are reduced in abundance or distribution. Such loss of genetic diversity, particularly if brought about by human activity, is often termed genetic erosion. The extent of reduction depends on the amount of diversity in the declining population and the way it is distributed geographically. Although levels of genetic diversity within such species would in some circumstances be expected to recover over time, particular genetic material that might be important for future adaptation may well be lost completely.

It would be possible, although not practicable routinely, to demonstrate genetic erosion at gene level. More usually it is evident at the species level, when measurably distinct populations (perhaps some local variety of crop plant) are lost or reduced in abundance, or is simply assumed to have occurred when an area of species-rich habitat is cleared. Because of the prevailing high rate of habitat modification, it can safely be assumed that genetic erosion is a very common and widespread phenomenon; however, not every loss of a local population is an instance of genetic erosion, because that population may have held no elements or combinations of genetic diversity not found elsewhere.

Manipulating genetic diversity

Human activity can also increase genetic diversity in species populations. By a process of artificial selection, humans have been indirectly but purposefully promoting genetic change in species during a period of more than 10,000 years, resulting in the current world diversity of domesticated crops and livestock.

Artificial selection involves managing the reproduction and survival of individuals within populations of useful plants and animals so as to preserve and make abundant those lineages that possess particularly useful features, such as high pest resistance or milk yield. The process is indirect in that it is focused on tangible features rather than on the genetic material from which such features originate.

The activities collectively known as genetic engineering also involve manipulation of existing genetic material, creating new gene combinations with the aim of improving key features of organisms used by humans (see Table 1.2 for some examples). Genetic engineering is in one sense only an extension of traditional breeding practices, in that it relies on naturally occurring elements of diversity. However, it is fundamentally different in other important respects. Not only does it involve making direct modifications to the actual genetic material of organisms, but also genetic material from different kinds of organism, usually isolated reproductively from each other, can be brought together and perpetuated in new lineages.

Table 1.2 A selection of genetically modified living organisms

Modified organisms	Source or property of added gene	New features in GMO	Scale of field use
Soya	Gene from Salmonella bacteria for enzyme EPSP (enolpyruvyl shikimate phosphate synthase) insensitive to glyphosate.	EPSP is essential for amino acid synthesis but inhibited by glyphosate, the active ingredient in Roundup herbicide. New enzymes confer herbicide resistance by disabling inhibition.	Major commercial significance in USA.
Maize	Gene for protein toxins (Cry 1Ac and Cry 2Aa) from Bacillus thuringiensis inserted by Agrobacterium Ti-plasmid.	Toxins confer insect resistance on host plants, e.g. to Stem Borer in maize.	Major commercial significance in USA.
Tomato	DNA modified to inhibit production of enzyme polygalacturonase (PG) responsible for plant cell wall breakdown.	Fruit life prolonged by slowing natural softening and ripening, without interrupting development of desirable flavour and colour.	Sold since 1995 in Canada, Mexico and USA. Cleared for sale in UK in 1996 but withdrawn by 1999 due to consumer pressure.
Rice japponica variety T309	Two genes from a daffodil and one from a bacterium.	Develop a variety of rice rich in beta-carotene, which is most common source for Vitamin A.	Trials, much interest in developing countries in Asia.
Oilseed Rape	Gene for enzyme thio-esterase from bay laurel.	Increases level of lauric acid in oil by inhibiting synthesis of longer-chain fatty acids.	Important in detergent manufacture.
Atlantic Salmon	Gene from other fishes (flounder or ocean pout) prolongs period of hormone secretion.	Increased growth rate.	Research project in USA, other work in Canada, New Zealand, Scotland.

The more radical forms of genetic engineering have only been developed during the 1990s but already have had considerable social impact. The techniques may have great potential to improve efficiency, volume or quality in agricultural and other production processes, and these potential benefits could be of particular value to countries at risk of food insecurity. However, they also raise significant ethical and practical concerns, which have been expressed by scientists and by public opinion in both developed and developing countries.

Among the practical concerns, attention has focused on the possible effects of genetic material moving from genetically modified (GM) sources, particularly field crops, into other organisms. Regarding plants, the risk of this occurring depends partly on whether the crop is an inbreeding or outbreeding species (whether plants are self-fertile, for example rice and soya, or must be fertilised by pollen from another individual, for example oilseed rape), and whether wild relatives of the crop are grown in the area (for example maize or potatoes grown in the UK have no close wild relatives in the country). The evidence available on possible effects at other levels in the food chain, for example on plant-feeding insects, is sparse but indicates that concern may be warranted. The use of genes conferring resistance to antibiotics as marker genes (to confirm presence of target genes) has caused concern because of the potential for increasing resistance in naturally occurring bacteria.

It is widely believed that some movement of genetic material from GM sources into other organisms is inevitable in the long term; the level of risk that can be tolerated depends on the balance of benefits and costs. These questions of responsible management of GM resources and technology have given rise to the new field of `biosafety'.

Human activity has also unintentionally led to increased genetic diversity, particularly among bacteria. In such cases a form of artificial selection is imposed on bacterial populations when they are exposed to antibiotics. It has been shown, for example, that routine use of antibiotics as prophylactic or growth promoting agents in intensive agriculture, i.e. not just for therapeutic purposes, has led to emergence of bacterial strains that are resistant to antibiotics, and these strains can infect humans. Table 1.3 provides selected examples.

Table 1.3 Examples of human-induced antibiotic resistance in pathogenic bacteria

Bacterial pathogen	Resistance	Emergence
Salmonella typhimurium DT 104	multidrug resistant	Recorded 1988 in cattle in England and Wales, increased in humans during 1990s in North America and UK, drug resistance broadening.
Enterobacter & Campylobacter	Flouroquinolone resistant	Resistant strains emerged after flouroquinolone approved for veterinary use.
Escherichia coli O157:H7	multidrug resistant	Increased occurrence in humans in N America and Europe linked with therapeutic and sub-therapeutic veterinary use and phytosanitisation on fruit farms.

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