Q.For the MN-blood group system, the frequencies of M and N alleles are 0.7 and 0.3, respectively. The expected frequency of MN-blood group bearing organisms is likely to be
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Darwinian Natural Selection
Imagine you walk into a crowded room. Some people are naturally louder, some are quieter. Some are taller, some shorter. Now imagine that, for some reason, the room is suddenly plunged into darkness, and everyone has to find the exit by touch alone. Who do you think will get out first? Probably not the loudest talker, but the person who happens to have the most sensitive fingertips or the best memory of where the door was.
That simple scenario captures the core of Darwinian natural selection. It is not about being "better" in some moral or absolute sense. It is about being a better fit for the specific situation you are in.
The Everyday Intuition: "Survival of the Fittest" — But What Does "Fittest" Mean?
The phrase "survival of the fittest" is often misunderstood. It does not mean the strongest, fastest, or most aggressive individual wins. In biology, "fitness" has a very specific meaning: the ability to survive long enough to reproduce and leave offspring.
Think of it this way: in a game of musical chairs, the "fittest" player isn't the one who dances the best. It's the one who, when the music stops, is standing on a chair. The chair is the environment. The player's ability to grab that chair is their fitness.
Darwin never used the phrase "survival of the fittest" in his first edition of On the Origin of Species. It was coined by Herbert Spencer, a philosopher, and Darwin adopted it in later editions. Darwin's own term was "natural selection," which is a more accurate description of the process.
The Precise Meaning: How Natural Selection Works
Natural selection is not a conscious force. It is a blind, automatic process that follows from three simple facts that are always true in any population of living things:
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Variation: Individuals in a population are not identical. Even within a species, there is a range of traits — different beak sizes in birds, different fur colours in rabbits, different heights in humans. This variation is the raw material.
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Inheritance: Many of these variations are passed from parents to offspring. A tall parent is more likely to have tall children. A fast-running cheetah is more likely to have fast-running cubs.
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Differential Survival and Reproduction: More individuals are born than can possibly survive. Resources (food, water, shelter, mates) are limited. This creates a "struggle for existence." Individuals with traits that give them even a slight advantage in this struggle are more likely to survive and, crucially, to reproduce.
The result: Over many generations, the traits that helped those individuals survive and reproduce become more common in the population. The traits that were less helpful become rarer. The population gradually changes to become better suited to its environment.
Natural selection does not create new traits. It only acts on the variation that already exists. It is like a sieve, not a sculptor. The sieve lets through the grains of sand that are the right size for the hole, and holds back the rest. The environment is the sieve.
Why It Matters: The Engine of Adaptation
Natural selection is the mechanism that explains how life becomes adapted to its surroundings. It is why:
- A cactus has spines instead of leaves (to reduce water loss in a desert).
- A polar bear has white fur (to blend in with snow and ice).
- A human has an opposable thumb (to grip tools).
It is not a plan or a goal. It is a consequence of simple, observable facts playing out over vast stretches of time. The NCERT textbook for Class 12 Biology (Chapter 6, "Evolution") states this clearly: "Natural selection is a process in which heritable variations enabling better survival are enabled to reproduce and leave greater number of progeny."
A Few Key Points to Remember
- It acts on individuals, but it changes populations. An individual does not evolve. The population as a whole changes over generations. …
The MN-blood group system is a classic example of codominance, where both alleles M and N are equally expressed in the heterozygote. Hardy–Weinberg principle applies here because there are only two alleles and no dominance.
Given:
- Frequency of M allele (p) = 0.7
- Frequency of N allele (q) = 0.3 …
The expected frequency of MN-blood group organisms is 42%, calculated using the Hardy-Weinberg principle for codominant alleles.
The MN-blood group system is a classic example of codominance in human genetics, where both alleles — M and N — are equally expressed in the heterozygous condition. Unlike the ABO system, there is no dominance or recessiveness here: an individual with one M and one N allele will have the MN blood type, not a blend or a hidden trait. This makes the system ideal for applying the Hardy-Weinberg principle, which predicts genotype frequencies in a population that is not evolving.
The Hardy-Weinberg equation for a two-allele system is $p^2 + 2pq + q^2 = 1$, where $p$ is the frequency of the M allele and $q$ is the frequency of the N allele. Here, $p = 0.7$ and $q = 0.3$. The frequency of the MN genotype — the heterozygote — is given by $2pq$, because there are two ways to inherit one M and one N allele (M from mother and N from father, or vice versa).
So, $2pq = 2 \times 0.7 \times 0.3 = 0.42$, or 42%. This is the expected proportion of MN-blood group individuals in the population, assuming random mating and no evolutionary forces like selection, mutation, or migration. …
Instead of memorising the 2pq shortcut, derive it from first principles: expand (p+q)^2 = p^2 + 2pq + q^2, where p^2 is MM, q^2 is NN, and 2pq is MN because there are two ways to draw one M gamete and one N gam …
- AP EAPCET 2026Set ap-2026-05-20-FN1 markMCQQ.Darwin called the macro-variations as (A) Discontinuous variation (B) Chromosomal mutations (C) Gene mutations (D) Sports of nature
›Reveal solutionSolution
Darwin's own term for large, discontinuous (macro-) variations was 'sports of nature'; De Vries later renamed this phenomenon 'mutation.'
Concept and Intuition
Darwin's theory of evolution by natural selection primarily relied on small, continuous variations accumulating gradually across generations. However, he also observed occasional large, abrupt changes appearing in a single generation, distinct from ordinary fluctuating variation — he called these unusual, discontinuous jumps 'sports.' This idea anticipated the later, more rigorous concept of mutation developed by Hugo de Vries from his breeding experiments on the evening primrose (Oenothera lamarckiana).
Step-by-Step Solution
- Recall the two categories of variation Darwin distinguished: gradual, continuous ('individual differences') vs sudden, large, discontinuous changes. …
- AP EAPCET 2025Set ap-2025-05-20-AN1 markMCQQ.Assertion (A) : In the polluted area, Birmingham, black peppered moths were abundant. In the non polluted area, Dorset, grey forms were abundant. Reason (R) : Natural selection. Identify the correct option from the following (A) (A) and (R) are true. (R) is correct explanation for (A) (B) (A) and (R) are true. But (R) is not the correct explanation for (A) (C) (A) is true, but (R) is false (D) (A) is false, but (R) is true
›Reveal solutionSolution
The classic peppered moth (industrial melanism) example shows dark moths thriving in
soot-polluted Birmingham and pale moths thriving in unpolluted Dorset — a textbook case
of natural selection acting through differential camouflage/predation, so both
statements are true and the Reason correctly explains the Assertion. Answer: (A).
Concept and Intuition
The peppered moth (Biston betularia) is the most famous illustration of natural
selection in real time. Before industrialisation, pale, lichen-camouflaged moths were
favoured because they blended into lichen-covered tree bark, escaping bird predation,
while dark (melanic) moths stood out and were eaten more often. As industrial pollution
in areas like Birmingham blackened tree trunks with soot (killing the lichen), the
camouflage advantage reversed: dark moths now blended in better and pale moths became
conspicuous. In unpolluted regions like Dorset, tree bark remained lichen-covered, so
pale moths retained their camouflage advantage there. This differential,
environment-dependent survival and reproduction is a direct demonstration of natural
selection.
Step-by-Step Solution
- Assertion: in polluted Birmingham, black moths were abundant; in unpolluted Dorset, grey (pale) moths were abundant — this matches the well-documented historical/field observations. True.
- Reason: natural selection — the differential predation based on camouflage against …
- AP EAPCET 2024Set ap-2024-05-16-FN1 markMCQQ.Match the following: List-1: A. Directional selection, B. Genetic load, C. Bottle neck effect, D. Centrifugal selection List-2: I. Polydactylic dwarfs, II. Darwin's finches, III. DDT resistant mosquitoes, IV. Tailed human baby, V. Sickle cell anaemia (A) A-III, B-IV, C-I, D-V (B) A-II, B-IV, C-V, D-I (C) A-III, B-V, C-I, D-II (D) A-III, B-IV, C-I, D-II
›Reveal solutionSolution
Four classic evolution examples map onto four selection/drift concepts: directional (DDT resistance), genetic load (sickle-cell balanced polymorphism), bottleneck/founder effect (Amish polydactylic dwarfism), and centrifugal/disruptive selection (Darwin's finches).
Concept and Intuition
- Directional selection favours one phenotypic extreme over the population mean — DDT-resistant mosquitoes are a textbook case of a population shifting toward the resistant extreme under selection pressure.
- Genetic load refers to the reduced average fitness of a population due to deleterious alleles maintained in the gene pool, often via heterozygote advantage — the sickle-cell allele persisting in malaria-endemic regions (heterozygotes resist malaria, but homozygotes suffer disease) is the classic example.
- Bottleneck/founder effect occurs when a population's gene pool is drastically reduced; the Old Order Amish community, descended from a small founder population, shows an unusually high frequency of polydactyly combined with dwarfism (Ellis–van Creveld syndrome).
- Centrifugal (disruptive) selection favours both phenotypic extremes over the intermediate form — Darwin's finches on the Galápagos show bimodal beak-size selection tied to differing food resources.
Step-by-Step Solution …
- AP EAPCET 2024Set ap-2024-05-17-AN1 markMCQQ.The driving force of evolution is (A) Reproductive isolation (B) Natural selection (C) Artificial selection (D) Mutations
›Reveal solutionSolution
This tests foundational evolutionary theory; natural selection is the primary driving force of evolution.
Concept and Intuition
Evolution requires raw material (heritable variation, arising from mutation and recombination) and a mechanism that channels that variation into adaptive change. While mutation supplies variation and reproductive isolation can lead to speciation, the actual directional "engine" that shapes populations by favouring some variants over others across generations is natural selection — Darwin's central mechanism of evolution.
Step-by-Step Solution
- Reproductive isolation (A) is a consequence/mechanism that leads to speciation, not the fundamental driving force of evolutionary change itself.
- Artificial selection (C) is a human-directed analogue of natural selection, used to illustrate the concept, but it is not the driving force operating in nature. …
- AP EAPCET 2024Set ap-2024-05-17-AN1 markMCQQ.Select the correct statement: (A) Hardy-Weinberg law is applicable to small populations. (B) Sewall Wright effect is applicable to large populations. (C) Development of resistance to DDT by mosquitoes is an example for directional selection. (D) If one species diverges to become two or more species, it is called anagenesis.
›Reveal solutionSolution
This tests population genetics concepts; DDT resistance in mosquitoes is the standard textbook example of directional selection.
Concept and Intuition
Several statements here test precise definitions in population genetics and evolution:
- The Hardy-Weinberg law describes allele frequency equilibrium and strictly applies to LARGE, randomly mating populations free of selection, mutation, migration, and drift.
- The Sewall Wright effect (genetic drift) is the random change in allele frequencies due to chance, and its effects are most pronounced in SMALL populations.
- Directional selection shifts a population's phenotype distribution consistently in one direction in response to an environmental pressure — DDT exposure selecting for resistant mosquitoes is the textbook example.
- Anagenesis refers to evolutionary change WITHIN a single lineage without splitting into new species; the SPLITTING of one species into two or more is called cladogenesis, not anagenesis.
Step-by-Step Solution
- (A) is wrong: Hardy-Weinberg applies to large populations, not small ones.
- (B) is wrong: the Sewall Wright effect (genetic drift) applies to small populations, not large ones — exactly the reverse of what's stated. …
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