Every extant species on Earth consists of at least one population, a group of individuals at a certain place that generally look alike and can potentially breed with each other to produce offspring. These population(s) can be very small (just a few individuals), very large (billions of individuals), or anything in-between. The individuals within each population generally differ genetically from one another to some degree. This genetic variation, a component of genetic diversity (Figure 7.5.1), exists because the genes—the functional units of hereditary information that provide the blueprint of an organism—in different individuals are made up of slightly different DNA sequences. Different forms of a gene, which arise through mutations that change DNA sequences, are known as alleles. The gene pool, in turn, consists of the total diversity of genes and alleles in a population or species. The particular mix of genes and alleles in an individual is its genotype. The expression of an individual’s genotype, as determined by the environment where an organism has developed, is its phenotype—that is, the organism’s morphology, anatomy, physiology, and biochemistry. Common characteristics to describe a person include height, hair color, and blood type, which taken together begin to describe that person’s phenotype.
In species which reproduce asexually, the potential for increased genetic diversity is limited to DNA mutations. However, sexual reproduction creates new genetic combinations by bringing together chromosomes from each parent. This process, called recombination, results in offspring that are genetically unique from their parents. Genetic mutations provide the foundation of genetic variation, but sexual reproduction dramatically increases genetic diversity by randomly mixing alleles in different combinations.
Genetic diversity enables species to adapt to environmental change.
Two factors determine a species’ genetic diversity: the number of genes that have multiple alleles (polymorphic genes) and the number of alleles present in a population for each polymorphic gene. If a gene is polymorphic, some individuals will have two different forms of the gene—that is, they will be heterozygous because they received different alleles of the same gene from their parents. Some individuals will have two of the same forms of the gene—they will be homozygous because each parent gave them the same allele. In general, the greater the genetic diversity in a population, especially the greater number of alleles present, the more capable a species will be to adapt to changing circumstances in their environment. Genetics also affect an individual organism’s development, physiology, and fitness—the relative ability of individuals to survive and reproduce. This same principle gives humans the ability to select and breed crops and domestic animals with characteristics that benefit the production and quality of food (Davis et al., 2012). Many rare species have relatively low genetic diversity, especially in populations which have dwindled to small sizes. Low genetic diversity limits small populations’ ability to adapt to changes in environmental conditions and leaves them at risk of extinction when conditions do change.