Q.State and explain any three factors affecting allele frequency in populations.

The allele frequency in a population is not fixed; it changes over time due to several evolutionary forces. The NCERT textbook identifies three key factors that alter these frequencies.

First, natural selection acts on the phenotype. Individuals with traits better suited to the environment survive and reproduce more, passing on their alleles at a higher rate. This increases the frequency of advantageous alleles and reduces harmful ones.

Second, genetic drift refers to random changes in allele frequency, especially in small populations. A chance event — like a natural disaster — can eliminate individuals regardless of their traits, causing certain alleles to disappear or become fixed purely by luck, not by fitness. …

The concept of allele frequency lies at the heart of population genetics. An allele is simply one version of a gene, and its frequency is the proportion of that allele among all copies of the gene in a population. In a stable, ideal population — one that is infinitely large, randomly mating, and free from mutation, migration, and selection — allele frequencies remain constant from generation to generation. This is the Hardy–Weinberg equilibrium. But real populations are never ideal. Several forces constantly push and pull at these frequencies, and three of the most important are natural selection, genetic drift, and gene flow.

Natural selection is the most intuitive factor. It operates on the phenotype — the visible traits of an organism — but its effects ripple down to the allele level. Individuals with traits that give them a survival or reproductive advantage in a given environment are more likely to pass their alleles to the next generation. Over time, the alleles that confer an advantage increase in frequency, while disadvantageous alleles decrease. For example, if a particular allele makes a plant more resistant to a common pest, that allele will become more common in successive generations. The key point is that natural selection is directional — it tends to make populations better adapted to their environment, but it does not act uniformly on all alleles. It favours some and weeds out others.

Genetic drift is a very different kind of force. It is the random change in allele frequency due to chance events, especially in small populations. Imagine a population of only twenty individuals. If, by pure luck, five of them fail to reproduce one season, the alleles they carried may disappear entirely — not because they were harmful, but simply because those individuals did not leave offspring. In a large population, such random fluctuations average out and have little effect. But in a small population, genetic drift can cause alleles to become fixed (reach 100% frequency) or lost (reach 0%) purely by chance. Two special cases of genetic drift are the bottleneck effect and the founder effect. A bottleneck occurs when a population is drastically reduced by a disaster — say, a flood or a fire — and the survivors carry only a random subset of the original alleles. The founder effect happens when a small group colonises a new area, carrying only a fraction of the genetic diversity of the source population. In both cases, the resulting population has allele frequencies that are very different from the original.

Instead of describing the three factors independently, contrast them along one axis — is the frequency change driven by fitness/environment (selection), by pure chance (drift), or by movement of individuals (gene flow)? This framing makes cle …