Fixation

Definition

Fixation refers to the process through which a specific allele becomes the only variant in a population, meaning all individuals carry that allele. This can occur due to genetic drift, natural selection, or other evolutionary forces, leading to the complete dominance of one genetic variant over others. Fixation is an important concept in understanding how populations evolve over time and how genetic diversity can be affected.

5 Must Know Facts For Your Next Test

1. Fixation occurs when the frequency of an allele reaches 100% in a population, meaning that all individuals possess that specific allele.
2. Genetic drift can lead to fixation in small populations where random events can significantly alter allele frequencies.
3. Natural selection can also drive fixation if one allele provides a reproductive advantage, leading to its dominance within the population.
4. Once an allele is fixed, it can reduce genetic diversity and potentially make the population more vulnerable to changes in the environment or disease.
5. Fixation is often measured using Wright's fixation index (F$_{ST}$), which quantifies population differentiation due to genetic structure.

Review Questions

• How does genetic drift contribute to the process of fixation in small populations?
  • Genetic drift contributes to fixation by introducing random fluctuations in allele frequencies due to chance events. In small populations, these random changes can have a significant impact, as each individual's contribution to the next generation has a larger effect on overall allele frequencies. Over time, this randomness can lead to the complete loss of some alleles and the fixation of others simply due to chance rather than natural selection.
• Discuss how natural selection can lead to the fixation of an advantageous allele within a population.
  • Natural selection can result in fixation when individuals with a specific advantageous allele have higher survival and reproductive success compared to others. This selective pressure increases the frequency of that beneficial allele over generations. As more individuals carrying the advantageous allele reproduce, it becomes more prevalent in the population, eventually reaching fixation when every individual possesses that allele. This process illustrates how adaptation can shape genetic outcomes within populations.
• Evaluate the implications of fixation on genetic diversity and population resilience in changing environments.
  • Fixation can significantly reduce genetic diversity within a population since it results in all individuals carrying the same allele. This lack of diversity limits the population's ability to adapt to environmental changes or new selective pressures, as there may be no alternative alleles available that confer different traits. Consequently, populations that experience fixation may become more susceptible to extinction due to their reduced capacity for adaptation and resilience against diseases or shifting environmental conditions.

Related terms

allele: Different forms of a gene that arise by mutation and are found at the same place on a chromosome.

genetic drift: Random changes in allele frequencies in a population, which can lead to the loss or fixation of alleles purely by chance.

natural selection: The process whereby organisms better adapted to their environment tend to survive and produce more offspring, influencing allele frequencies in a population.