Blog Post : Module 8 Reflection

Selection is often assumed to eliminate genetic variation. So, why is genetic variation then not eroded over time? Based on what you have learned in the past modules, from the basic mechanisms of evolution to quantitative genetics, reflect on the diversity of mechanisms that can explain why genetic variation is maintained in populations.

àThere are many factors that plays role in preserving genetic variation within a population. We have learned from the last few modules of class that selection does not alone have an influence on genetic variation. Besides selection, diverse mechanisms such as drift, mutation, migration help to maintain genetic variation in populations.

Genetic variation can be caused by mutation. It can create entirely new alleles in a population, random mating, random fertilization, and recombination between homologous chromosomes during meiosis. (which reshuffles alleles within an organism’s offspring). There are important interactions between mutation and selection. Such as in the persistence of deleterious alleles in a population. In absence of mutation, selection keeps a recessive deleterious gene at a very low frequency. But as selection removes deleterious alleles in every generation, mutation continuously reintroduces them. When the rate of elimination of deleterious alleles is equal to the rate of mutation, the frequency of an allele is at equilibrium.

Genetic drift is another factor that impacts on maintain genetic variation within a population. When population sizes are finite and especially when they are small random chance affects evolutionary dynamics. These changes in allele frequencies across generations caused by random events are called genetic drift. While the selection is differential reproductive success caused by the differential performance of variants, genetic drift is the differential reproductive success that just happens. In contrast to the selection, which increases average fitness across populations, genetic drift does not lead to adaptation. Due to the random nature of genetic drift, populations subject to it evolve on distinct trajectories. In small populations, genetic drift affects the fate of alleles under selection. Drift can cause deleterious mutations to be more common than expected by selection alone, and it can cause beneficial alleles to disappear from the population. If the new mutation is deleterious then the likelihood of fixation becomes smaller than what is expected by chance. In contrast, if the new mutation provides a fitness advantage then the likelihood of fixation becomes greater than what is expected by chance. The likelihood of fixation is dependent on both the strength of selection and the effective population size. When populations are small, selection on novel alleles needs to be comparatively strong for them to have a high likelihood of fixation. If a mutation only provides a minor fitness benefit, drift might cause its loss from the population before it ever has a chance to become common. Conversely, when population sizes are very large, novel alleles with minute fitness benefits can have a high likelihood of fixation.

Migration is another evolutionary force. It can profoundly impact the genetic composition of populations. Migrants may carry novel alleles from one population to another, acting similar to a mutation in terms of introducing new genetic variation. Even in absence of novel alleles, migration between differentiated populations causes changes in allele frequencies. In the absence of other evolutionary forces, it homogenizes the genetic composition of different populations. Genetic variation is advantageous to a population because it enables some individuals to adapt to the environment while maintaining the survival of the population.