Genetic Diversity

Introduction

There are almost two million species of living beings known, ranging from mammals and birds to plants, yeast, and bacteria. Each species is characterized by a specific genetic blueprint. Therefore, there is a genetic diversity, which underpins species diversity. Genes accumulate changes over time and those that are best adapted to the environment survive and give rise to more offspring. Eventually, these genetic changes may give rise to new species.

Biodiversity, which is a broad term encompassing species and genetic diversity, is influenced by the type of biome where organisms live, with those nearer the equator, like rain forests, being more diverse. Biodiversity is important in its own right and also for the benefits that a diverse gene pool may give human populations, such as new medicines and mitigation of climate change. Human activities may have a negative impact on biodiversity, which is why conservation efforts are so important.

Historical Background and Scientific Foundations

Biodiversity is the variety of plants, animals, and microorganisms existing either in a specific biome or globally. It can be considered on two different levels. Species diversity involves the number and type of different species living in a habitat. Genetic diversity refers to the variety of DNA in the range of species within a habitat.

DNA stands for deoxyribonucleic acid, which is the chemical that genes are made of. Each gene acts as a chemical code, which directs the formation of a specific protein. These molecules define the physical characteristics of a species, such as size, color, and biochemical make up. Each species has its own particular set of genes, known as its genome. Within a species, there are a number of varieties whose genomes will differ slightly from one another. Put simply, genetic diversity comprises the molecular basis of species diversity.

Genetic diversity also drives natural selection. According to English naturalist Charles Darwin (1809–1882) in his theory of evolution, species whose genes adapt best to the environment they find themselves in are more likely to survive. They will leave more offspring and the next generation will have more individuals with that particular genetic makeup. Over time, natural selection will lead to the emergence of different species, a process called speciation. Geographical separation of populations accelerates speciation. In Darwin’s groundbreaking explorations of the Galapagos Islands, he studied species of finches on different islands which were each well adapted to their individual environments. This led him to develop the theory of natural selection.

Genetic diversity has given rise to 1.7 million known species around the world, most of which are insects. There are likely to be many more remaining to be discovered, with estimates ranging from 10 to 100 million in all. Diversity varies a great deal with the nature of a biome and its location. In general, genetic diversity decreases as latitude increases, with the tropical rain forest therefore being richest in species. This is most likely because there is less landmass at higher latitudes and marine environments are generally less biodiverse. Also, temperatures are higher at lower latitudes, which favor species diversity. Meanwhile, islands tend to have lower biodiversity than the mainland, and the larger the island, the greater the biodiversity. This is because a larger landmass generally offers more resources and a wider range of habitats.

The genetic diversity of a particular habitat depends upon the nature of the environment, including factors such as climate and availability of food and other natural resources. In an individual habitat, diversity is a balance between inward migration of species and speciation, which

WORDS TO KNOW

BIOMASS: The sum total of living and once-living matter contained within a given geographic area; or, organic matter that can be converted to fuel and is regarded as a potential energy source.

BIOME: A well-defined terrestrial environment (e.g., desert, tundra, or tropical forest) and the complex of living organisms found in that region.

ENDEMIC SPECIES: A species that is exclusively native to a certain area.

EUTROPHICATION: The process whereby a body of water becomes rich in dissolved nutrients through natural or man-made processes. This often results in a deficiency of dissolved oxygen, producing an environment that favors plant over animal life.

GENOME: The total content of genetic material in organisms.

KEYSTONE SPECIES: A species whose impact on its environment has a disproportionately large effect relative to its abundance.

NATURAL SELECTION: Also known as survival of the fittest; the natural process by which those organisms best adapted to their environment survive and pass their traits to offspring.

SPECIATION: The evolutionary development of new biological species.

increase genetic diversity, and extinction which decreases it. In the last 500 years, over 1,000 species, including more than 20 mammals, have become extinct. There are likely to be many more extinctions that have gone unrecorded, of species that were never formally identified.

Genetic diversity is important for a number of reasons. First, there is the moral argument that all species, however humble or apparently without purpose, have a right to existence. Then there is the view that any species may turn out to have a use to humans, even though that may not be immediately apparent. For instance, many bacteria, fungi, and plants manufacture organic chemicals that help defend them against invading species. On occasion, the extraction and study of these compounds has led to their development as an antibiotic or anti-cancer drug. Maybe the most famous example is penicillin, a compound secreted by the mold Penicillium crysogenum and discovered quite by chance by Alexander Fleming in 1928. Penicillin, and the antibiotics that came after it, have revolutionized the treatment of infectious disease. Loss of genetic diversity could mean missing out on some other important source of a new therapy.

A keystone species, regardless of the numbers of its population, profoundly influences the environment and the regulation of the ecosystem around it. One example is the largemouth bass, which is a top-level predator in lake ecosystems. In an experiment, nitrogen and phosphorus were added to a lake in which the bass was present and primary production was found to increase, allowing the removal of carbon from the atmosphere, through photosynthesis. In a second experiment, nitrogen and phosphorus were added, but the bass were removed. They normally feed on minnows, which are smaller fish. Without their predators, the minnow population rose dramatically. They normally feed on tiny zooplankton that, in turn, feed on algae. Therefore, zooplankton populations declined, in the absence of the largemouth bass, and the population of algae increased. The resulting algal bloom caused eutrophication of the lake, leading to an overall emission of carbon dioxide in this ecosystem. Thus, decline in biodiversity can sometimes lead to significant adverse environmental effects.

Impacts and Issues

Genetic diversity is declining for many reasons, most of which are related to human activities. Habitats are being altered, or even being lost, because of land use. For example, wetlands around the world, many of which are rich in species, are being drained for agriculture or for urban development. Overfishing is threatening many marine fish species and upsetting ocean ecosystems. Invasive species are introduced by increasing international travel and trade, and are driving some endemic species to extinction. An example of this is the brown tree snake, which first came to Guam from its native habitat (Australia and the Solomon Islands) in either military aircraft or aboard ship. Their predatory activities greatly reduced local bird populations. Meanwhile, increasing biomass, which can occur as a result of reforestation or even global warming, tends to endanger various wild plant species.

Fortunately, much can be done to help preserve genetic diversity. Governments at national or local levels can pass legislation that stops the hunting of various species or protects their habitat by not giving permission for changes in land use. However, there is no general agreement on how to prioritize the species to be protected. One can argue for criteria like the usefulness of a species, either known or potential, or for genetic uniqueness, popularity with the public, or its role in an ecosystem.

In the United States, the Endangered Species Act has several ways of protecting threatened or endangered species, such as prohibiting hunting and blocking construction that would damage their habitat. One of the act’s most notable successes has saved the American bald eagle. The use of the pesticide DDT impaired the repro-

ductive potential of the birds, while they were also threatened by hunting and destruction of their habitat so that the number of nesting pairs in the continental United States was down to fewer than 500. In 1967, they were listed under the act and by 1993, the number of bald eagle breeding pairs was up to more than 4,000. Their status then changed from endangered to threatened. By 2004, there were nearly 8,000 breeding pairs of the American bald eagle.

Advances in DNA technology have led to investigation of genetic diversity at the genomic level. Many species have now had their entire genome sequenced, a process which comprises a read-out of the DNA contained in its cells. New software tools are leading to the comparison of genomes of different species to better understand the relationship between them. There is also increasing interest in metagenomics, which is an approach involving assessing the genetic diversity of a whole ecosystem, rather than its component species.

See Also Ecosystem Diversity; Habitat Alteration; Habitat Loss; Human Impacts

BIBLIOGRAPHY

Books

Hulot, Nicholas. One Planet: A Celebration of Biodiversity. New York: Abrams, 2006.

Stein, Bruce, et al. Precious Heritage: The Status of Biodiversity in the United States. Cambridge: Oxford, 2000.

Web Sites

Environmental News Network. “Loss of Genetic Diversity Threatens Species Diversity.” September 26, 2007. http://www.enn.com/wildlife/article/23391 (accessed May 2, 2008).

Susan Aldridge