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Population Genetics and Statistics for Forensic Analysts

Genetic Drift and Natural Selection

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Allele frequencies in small populations do not generally reflect those of larger populations since too small of a set of individuals cannot represent all of the alleles for the entire population. Genetic drift occurs when the population size is limited and therefore by chance, certain alleles increase or decrease in frequency. This can result in a shift away from Hardy-Weinberg equilibrium (HWE). Unlike natural selection, genetic drift is random and rarely produces adaptations to the environment.10

Watch a video on genetic drift presented by Greggory LaBerge.

Natural Selection

Although population genetics by itself is important, one of the objectives of this field is to assess how changes in allele frequencies affect the evolution of a population. Evolution in its modern form was first explored by Charles Darwin in 1859. In his book On the Origin of Species, Darwin outlined what he called "descent with modification" and what we now refer to as evolution. He speculated that all species evolved from a common ancestor. Over time, faced with new environments and habitats, populations of species acquired modifications, which allowed them to better adapt to their environment.11

Darwin termed these changes within populations, natural selection, and he proposed the idea of "survival of the fittest." Individual variations which proved beneficial would be preserved within a population, whereas variations that were lethal to the organism would be destroyed. Under natural selection, some individuals in a population have modifications that allow them to more successfully survive and reproduce, making their adaptations more common as a whole due to their increased reproductive success. Over a long period of time, this change in the characteristics of a population can lead to the production of a new species.11

Darwin 's theory of evolution can be summarized in three main principles:

  1. Principle of variation: Among individuals within any population, there is variation in morphology, physiology, and behavior.
  2. Principle of heredity: Offspring resemble their parents more than they resemble unrelated individuals.
  3. Principle of selection: Some forms are more successful at surviving and reproducing than other forms in a given environment.12

It is important to remember that evolution occurs at the population level, not at the individual level.

To see an example of mutation and natural selection at work, consider the case of the peppered moth. Prior to the Industrial Revolution in England, the peppered moth was found almost entirely in its light-colored form. Its color provided camouflage against the lichen-covered trees, preventing the moths being seen by predators. The pollution from the industrial revolution caused much of the lichen on the trees to die. As a result, the light moths became more visible to birds, whereas the dark colored moths (which arose from a mutation) were able to blend in better with the trees and avoid being eaten. The result was that the population of dark moths, which was about 1% in 1848, increased to about 90% by 1959!13

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