Sources of Diversity

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Any population, however defined, is compounded of individuals, each genetically unique. We discuss sources of genetic difference between individuals in our module on Variation, including phenomena such as mutation and epigenetics. A different set of phenomena are responsible for genetic variants (alleles) being distributed throughout a population - or preferentially in specific subpopulations. These phenomena are intimately related to cultural, social/behavioral and environmental variables.

Geneticists classify these influences into three general categories:

  • Non-random mating - The make-up of the gene-pool is influenced by the selection of mates with specific traits. Mate selection may also be confined by cultural or geographical limitations.
  • Natural selection - A variant persists in a population because it gives individuals a survival or reproductive advantage.
  • Genetic drift and flow - External events such as migrations, social/cultural intermingling, and widespread death due to famine, epidemic, or environmental disaster can effect the gene pool of small populations.

Through mathematical modeling, geneticists have been able to demonstrate that once a variant is established in a population, it eventually reaches equilibrium. In other words, the frequency of an allele in a population does not change over time, unless evolutionary factors like those listed above are in effect. In this way, the features that distinguish populations from one another endure over time, as long as those populations remain stable and socially or geographically isolated.

Thus, the distribution of genetic variants and their associated phenotypes is a reflection of the history of modern Homo sapiens. As our human ancestors migrated out of Africa and dispersed across the continents, subpopulations were divided by geography. The small number of individuals who founded new settlements passed their unique gene pool and distinguishing features on to subsequent generations. Meanwhile, new traits arose in response to conditions in local environments. As long as these new subpopulations were stable and isolated from one another, these small genetic differences persisted. Occasionally, additional genetic diversity is introduced over time with the occurrence of sporadic new mutations. These variants might become established in a population through new migrations and social or environmental changes.

Read about Hardy-Weinberg equilibrium, the foundational mathematical model for population genetics.

Hardy-Weinberg Equilibrium (named for the mathematician and clinician who developed it in the early twentieth century) calculates the frequency of a genetic variant (allele) in a population based only on the observed frequency of an associated trait (phenotype). This is used to calculate the frequency of recessive and dominant alleles when all we know is the frequency of a recessive phenotype. One might assume that such an estimate would come simply from counting the number of individuals with the trait (phenotype) associated with the allele. However, how do we distinguish between individuals who are homozygous (with two dominant alleles) or heterozygous (with one dominant and one recessive allele), since their phenotypes appear the same? The Hardy-Weinberg formula overcomes this limitation, and is applied in situations where the gene responsible for a trait is unknown or sequencing is not available.

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Read about these gene-pool changing forces and view illustrations and examples at External Web Site Policy.this site on Understanding Evolution from the University of California Museum of Paleontology.

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