Module 8: Quantitative Genetics

Genetic variation is the diversity in gene frequencies between the individual or the differences between population. Different sources of genetic variation are mutation, random mating between organisms, crossing over, and random fertilization. Various factors act to maintain the genetic variation in population such as mutation, selective neutrality, balancing selection, frequency dependent selection, and changing patterns of selection over time or space. Mutation is one of the factors which contribute to rare genetic variation in a population. Selective neutrality arises when there is only a small difference in fitness between the alternate allele of a gene. When the alleles are governed by genetic drift rather than natural selection they are selectively neutral. Balancing selection arises when the heterozygotes have higher fitness than the two homozygous genotypes. These allow both the alleles to be maintained in a population. Frequency dependent selection happen when the individuals with rare phenotypes have greater fitness compared to the individuals with more common phenotypes. An example of negative frequency-dependent selection can also be seen in the interaction between the human immune system and various infectious microbes such as pathogenic bacteria or viruses. As a particular human population is infected by a common strain of microbe, most individuals in the population become immune to it. This then selects for rarer strains of the microbe which can still infect the population because of genome mutations; these strains have greater evolutionary fitness because they are less common. Change in selection pattern over time or space can help to maintain genetic variation in a population. If the pattern of selection varies over time, different genotypes have different fitness advantages at distinct times. An example is a praying mantis and its change in color. Some scientists believe that a few of the mantis species can change their color. The European mantis is one species that is thought to have this ability. It predominantly lives in brown-grass habitats as well as green vegetation. Studies suggest that this type of mantis can transform into a green color when they live within the green grass. It is theorized that this color change occurs after they molt. The weather patterns likely play a large role in their final color. Studies that have been done in controlled environments have shown variations based on temperature and humidity levels observed. Their survival rate differs accordingly.