CLEP Biology w Online Practice Exams by Laurie A. Callihan

Author:Laurie A. Callihan
Language: eng
Format: epub
Publisher: Research & Education Association
Published: 2013-09-15T00:00:00+00:00

Genetic Diversity

As will be noted in the sections to follow, the diversity of species that have evolved on earth is vast. Not only is there great diversity between species, but there are also many distinguishable differences that exist between members of the same species. The genetic variation carried by members of a species for a given trait is known as a polymorphism. For example, gender is an example of dimorphism (two variations) in most mammalian species (male and female). Evolutionary mechanisms exist that tend toward a balanced polymorphism in order to keep any particular version within a species from dominating, unless that particular version is more suited for its ecological niche (aka being “fitter”). In the example of gender, it is favorable to the species to have an even mix of genders, and the tendency of the gene pool is to favor that balance. If there becomes a shortage of males, the males become more highly sought after and thus “fitter” in the selection process.

Heterozygote advantage is one mechanism whereby polymorphism is maintained. Having two different alleles for a given phenotype is a positive trait for survival in many instances. For example, a well-known situation exists with the gene inheritance of the blood trait for Sickle Cell. The allele HbS is an allele for production of a variant hemoglobin sensitive to oxygen deprival and causing misshapen red blood cells. The allele HbA is the allele for normal hemoglobin production. The HbS allele is found mainly in humans of African and Asian Indian descent. If a person has two HbA alleles, they have normal hemoglobin carrying red blood cells (most of the population). When two HbS alleles are inherited, the person has the disease known as Sickle Cell Anemia, with the accompanying debilitating symptoms and shortened life span. However, a person heterozygous for this trait, HbAHbS, is a carrier for Sickle Cell and has what is known as Sickle Cell Trait. While this trait carries some discomforting symptoms, they are far less debilitating and it also has a positive characteristic; those with Sickle Cell Trait also have a natural resistance to malaria, a deadly illness of the African/Asian regions. For this reason, while the homozygous trait is deadly, the heterozygous trait is advantageous, and therefore is favored, selected, and retained in the population.

A second mechanism of polymorphism is frequency dependent selection which occurs as the frequency of one phenotype increases relative to another.

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