Q.Name the law that states that the sum of allelic frequencies in a population remains constant. What are the five factors that influence these values?
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Start your 14-day free trial to unlock the full solution →The Hardy–Weinberg principle states that the sum of allelic frequencies in a population remains constant from generation to generation in the absence of evolutionary influences; five factors that disturb this equilibrium are gene flow, genetic drift, mutation, recombination, and natural selection.
The idea that a population’s genetic makeup can stay perfectly stable across generations is a powerful one — and it forms the foundation of population genetics. This stability is described by the Hardy–Weinberg principle, named after the mathematician G. H. Hardy and the physician Wilhelm Weinberg, who independently formulated it in 1908. The principle says that in a large, randomly mating population with no external disturbances, the frequencies of alleles (different versions of a gene) and genotypes (the genetic combinations an individual carries) remain constant over time. In other words, the sum of allelic frequencies — often written as $p + q = 1$ for a two-allele system — does not change unless something acts upon it.
Why does this matter? Because it gives biologists a baseline. If you observe a population where allelic frequencies are shifting, you know that some evolutionary force is at work. The Hardy–Weinberg equilibrium is like a “null hypothesis” for evolution: if the population is not evolving, the frequencies stay put. If they change, something is happening.
The Hardy–Weinberg principle applies only under ideal conditions: a very large population size, random mating, no mutations, no gene flow (migration), and no natural selection. In reality, these conditions are rarely met, which is why populations evolve.
Now, the NCERT textbook explicitly lists five factors that can disturb the Hardy–Weinberg equilibrium and cause allelic frequencies to change. These are the forces that drive evolution:
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Gene flow — also called gene migration. This occurs when individuals move into a population (immigration) or leave it (emigration), bringing new alleles in or taking existing ones out. For example, if a few birds from a mainland population fly to an island and breed there, they introduce alleles that may not have been present on the island before. Gene flow tends to make populations more similar to each other.
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Genetic drift — a random change in allele frequencies, especially pronounced in small populations. Imagine a population of ten plants; by pure chance, only two of them happen to produce most of the next generation’s seeds. The alleles carried by those two plants become overrepresented, while others may disappear entirely. This is not due to any advantage or disadvantage — it’s just luck. The textbook highlights that genetic drift can lead to the founder effect (when a small group colonises a new area) or a bottleneck effect (when a population is drastically reduced by a disaster).
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Mutation — a change in the DNA sequence that creates a new allele. Mutations are rare, but they are the ultimate source of all genetic variation. A single mutation can introduce a novel allele into a population, altering the frequency of that allele relative to others. Most mutations are neutral or harmful, but occasionally one provides a survival advantage. …
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