Model Answer
0 min readIntroduction
The Hardy-Weinberg principle, formulated independently by G.H. Hardy and Wilhelm Weinberg in 1908, is a fundamental concept in population genetics. It describes the genetic variation within a population and predicts how allele and genotype frequencies will remain constant from generation to generation in the absence of specific evolutionary influences. This principle serves as a null hypothesis to test whether a population is evolving. Understanding the conditions required for equilibrium, and the factors that disrupt it, is vital for comprehending the mechanisms driving evolutionary change.
Hardy-Weinberg Law of Equilibrium
The Hardy-Weinberg equilibrium is mathematically expressed by two equations:
- p + q = 1: Where 'p' represents the frequency of the dominant allele and 'q' represents the frequency of the recessive allele for a particular trait.
- p2 + 2pq + q2 = 1: Where p2 is the frequency of the homozygous dominant genotype, 2pq is the frequency of the heterozygous genotype, and q2 is the frequency of the homozygous recessive genotype.
This law holds true under five key assumptions:
- No mutation
- Random mating
- No gene flow
- No genetic drift
- No natural selection
Factors Upsetting the Equilibrium
1. Mutation
Mutation is the ultimate source of genetic variation. While mutation rates are generally low, they introduce new alleles into the population, altering allele frequencies. For example, a mutation conferring antibiotic resistance in bacteria will increase the frequency of the resistance allele over time.
2. Gene Flow (Migration)
Gene flow refers to the transfer of alleles between populations. Migration of individuals (and their genes) can alter the allele frequencies in both the source and recipient populations. For instance, the introduction of a new plant species with unique alleles into a previously isolated population.
3. Genetic Drift
Genetic drift is the random fluctuation of allele frequencies due to chance events, particularly in small populations. Two important types of genetic drift are:
- Bottleneck Effect: A drastic reduction in population size due to a catastrophic event (e.g., natural disaster) can lead to a loss of genetic diversity and altered allele frequencies.
- Founder Effect: When a small group of individuals colonizes a new area, the allele frequencies in the founding population may not represent the original population.
The cheetah population, having experienced a severe bottleneck in the past, exhibits remarkably low genetic diversity.
4. Non-Random Mating
When individuals do not choose mates randomly, allele frequencies can change. Two common forms of non-random mating are:
- Assortative Mating: Individuals with similar phenotypes mate more frequently.
- Inbreeding: Mating between closely related individuals increases homozygosity.
Inbreeding depression, a reduction in fitness due to increased homozygosity of deleterious alleles, is a consequence of non-random mating.
5. Natural Selection
Natural selection is the differential survival and reproduction of individuals based on their traits. Alleles that confer a selective advantage become more common in the population over time, while disadvantageous alleles become less frequent. The evolution of pesticide resistance in insects is a classic example of natural selection.
| Factor | Mechanism of Disruption | Example |
|---|---|---|
| Mutation | Introduces new alleles | Sickle cell anemia mutation |
| Gene Flow | Transfers alleles between populations | Migration of European rabbits to Australia |
| Genetic Drift | Random allele frequency changes | Northern elephant seal bottleneck |
| Non-Random Mating | Alters genotype frequencies | Peacock feather selection |
| Natural Selection | Differential survival and reproduction | Industrial melanism in pepper moths |
Conclusion
The Hardy-Weinberg principle, while rarely perfectly met in natural populations, provides a crucial baseline for understanding evolutionary processes. By identifying the factors that disrupt equilibrium, we can gain insights into the mechanisms driving genetic change and adaptation. The principle highlights that evolution is not a directed process, but rather a consequence of the interplay between genetic variation and selective pressures. Continued monitoring of allele frequencies and understanding the impact of these disruptive forces are essential for conservation efforts and predicting evolutionary trajectories.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.