Qenetic Diversity

Genetic diversity refers to any variation in the nucleotides, genes, chromosomes, or whole genomes of organisms. This is the "fundamental currency of diversity" (Williams and Humphries, 1996) and the basis for all other organismal diversity. Approximately 1 billion different genes are recognized from all the known species on earth (World Conservation Monitoring Center, 1992). But not all species have the same number of genes. The potential genetic diversity of a species can be measured by the total number and type of genes present within its entire DNA or genome. However, a greater total number of genes might not correspond with a greater observable complexity in the anatomy and physiology of the organism (that is, greater phenotypic complexity). For example, the genome of the cultivated subspecies of rice, Oryza sativa L. ssp. indica, is estimated at 46,022 to 55,615 genes (Yu et al., 2002), and the total size of the human genome is currently predicted to be not much larger, at approximately 67,000 genes.

Genetic diversity is key for conservation efforts, since higher genetic diversity usually represents a greater capacity to adapt to environmental changes. This is, for example, an important issue in the context of changing global climate.

Genetic diversity, at its most elementary level, is represented by differences in the nucleotide sequences (adenine, cytosine, guanine, and thymine) of chromosomal DNA (deoxyribonucleic acid). This nucleotide variation is measured for particular genes. Each gene comprises a hereditary section of DNA that occupies a specific place on the chromo some and controls a particular characteristic of an organism. Differences in the nucleotide sequences of a gene can be compared for different organisms. Most organisms are diploid, having two sets of chromosomes, and therefore two copies (called alleles) of each gene. However, some organisms can be triploid or tetraploid (having three or four sets of chromosomes).

Within any single organism, there may be variation between the two (or more) alleles for each gene. This variation is introduced either through mutation of one of the alleles, or as a result of sexual reproduction. During sexual reproduction, offspring inherit alleles from both parents, and these alleles might be slightly different. Also, when the offspring's chromosomes are copied after fertilization, genes can be exchanged in a process called sexual recombination. Genetic diversity can exist between the copies of genes possessed by a single organism. Increased genetic diversity can be achieved in an organism by having multiple copies of each gene within its genome (Pen-nisi, 2001). Mutations can kill an organism. However, when an organism has two copies of the same gene, it is possible for one to mutate without harming the organism's survival. Eventually, mutations may allow the evolution of new characteristics. (In populations, genetic variation can be added through migration or hybridization.)

Each allele codes for the production of amino acids that string together to form proteins. These proteins code for the development of the anatomical, physiological, and behavioral characteristics of the organism. Differences in the nucleotide sequences of alleles result in the production of slightly different strings of amino acids or variant forms of the proteins. The variation within genes, for individual organisms and between different organisms, can be measured indirectly by measuring the biochemical variation of the proteins produced by these genes. The technique for studying protein diversity is known as protein elec-trophoresis. This was one of the most important methods for studying genetic diversity from its inception in the late 1950s until the late 1970s, when new technologies were developed that allowed direct analysis of DNA sequences.

Besides having distinct combinations of genes, species may also have variation in the shape and composition of the chromosomes carrying the genes, and in the total number of chromosomes present. Examination of these features of the chromosomes (termed karyol-ogy) provides another way of describing genetic diversity.

Analyses of genetic diversity can be applied to studies of the evolutionary ecology of populations. Genetic studies can identify alleles that might confer a selective advantage on the host organism—for example, an allele that renders the host better equipped for digesting certain foods. This selective advantage means that the organism is more likely to survive and pass its genetic traits on to its offspring. Under these circumstances, particular alleles can spread through, and become established in, a population. The spread of this genetic diversity can then affect the ecological diversity of the habitat where the organisms live. In this example, the allele might enable the organisms to feed upon certain types of plants more effectively, leading to greater predation on those plants as preferred food. This higher predation on the plant could cause related changes in other parts of the food web within that habitat.

The presence of unique genetic characteristics distinguishes members of a given population. The size of a population can be estimated by analyzing the geographic range of organisms with specific genetic characteris tics. If the population is large, and the individuals are not closely related (which is usually the case in large populations), then the overall gene pool is large, and many different alleles are likely to be present. A wide diversity of alleles indicates a greater potential for the evolution of new combinations of genes and, subsequently, a greater capacity for evolutionary adaptation to different environmental conditions. In contrast, a small population typically has a narrower diversity of alleles. The individuals are likely to be genetically, anatomically, and physiologically more homogeneous than in larger populations and less able to adapt to differing conditions. Populations with very low genetic diversity may be so susceptible to moderate environmental change or disease that they become extinct. For example, sub-Saharan populations of cheetahs show extremely low levels of genetic diversity, perhaps because their populations collapsed about 10,000 years ago when other large mammals were going extinct, creating a genetic bottleneck. Captive populations have been very susceptible to disease, suffering high mortality rates from diseases such as feline infectious peritonitis, which is not usually fatal to cats. Presumably, the virus is effective against a particular genotype that is shared by all cheetahs. Other traits apparently associated with the low genetic diversity are unusually high levels of spermatozoan abnormalities in males and a high infant mortality rate (see Hunter [2002] for discussion and references).

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