Genetic diversity ~ Conservation Biology

Some understanding of what the variety of life comprises can be obtained by distinguishing between different key elements. These are the basic building blocks of biodiversity. For convenience, they can be divided into three groups: genetic diversity, organismal diversity, and ecological diversity. Within each, the elements are organized in nested hierarchies, with those higher order elements comprising lower orderones. The three groups are intimately linked and share some elements in common.

Genetic diversity

                Genetic diversity encompasses the components of the genetic coding that structures organisms (nucleotides, genes, chromosomes) and variation in the genetic make-up between individuals within a population and between populations. This is the raw material on which evolutionary processes act. Perhaps themost basicmeasure of genetic diversity is genome size the amount of DNA (Deoxyribonucleic acid) in one copy of a species’ chromosomes (also called the C-value). This can vary enormously, with published eukaryote genome sizes ranging between 0.0023 pg (picograms) in the parasitic microsporidium Encephalitozoon intestinalis and 1400 pg in the free-living amoeba Chaos chaos (Gregory 2008).

                   These translate into estimates of 2.2 million and 1369 billion base pairs (the nucleotides on opposing DNA strands), respectively. Thus, even at this level the scale of biodiversity is daunting. Cell size tends to increase with genome size. Humans have a genome size of 3.5 pg (3.4 billion base pairs). Much of genome size comprises non-coding DNA, and there is usually no correlation between genome size and the number of genes coded. The genomes of more than 180 species have been completely sequenced and it is estimated that, for example, there are around 1750 genes for the bacteria Haemophilus influenzae and 3200 for Escherichia coli, 6000 for theyeast Saccharomyces cerevisiae, 19 000 for the nematode Caenorhabditis elegans, 13 500 for the fruit fly Drosophila melanogaster, and 25 000 for the plant Arabidopsis thaliana, themouseMusmusculus, brown rat Rattus norvegicus and human Homo sapiens.

                  There is strong conservatism of some genes acrossmuchof thediversityof life.Thedifferences in genetic composition of species giveus indications of their relatedness, andthus important informationas to how the history and variety of life developed. Genes are packaged into chromosomes. The number of chromosomes per somatic cell thus far observed varies between 2 for the jumper ant Myrmecia pilosula and 1260 for the adders-tongue fern Ophioglossum reticulatum. The ant species reproduces by haplodiploidy, in which fertilized eggs (diploid) develop into females and unfertilized eggs (haploid) become males, hence the latter have the minimal achievable single chromosome in their cells (Gould 1991). Humans have 46 chromosomes (22 pairs of autosomes, and one pair of sex chromosomes). Within a species, genetic diversity is commonly measured in terms of allelic diversity (average number of alleles per locus), gene diversity (heterozygosity across loci), or nucleotide differences.

                 Large populations tend to have more genetic diversity than small ones, more stable populations more than those that wildly fluctuate, and populations at the center of a species’ geographic range often have more genetic diversity than those at the periphery. Such variation can have a variety of population-level influences, including on productivity/ biomass, fitness components, behavior, and responses to disturbance, as well as influences on species diversity and ecosystem processes (Hughes et al. 2008).