In a certain population, the frequency of three genotypes is as follows: Genotypes: BB Bb bb frequency: 22% 62

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Q. In a certain population, the frequency of three genotypes is as follows: Genotypes: BB Bb bb frequency: 22% 62% 16% What is the likely frequency of B and b alleles?

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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.

Note

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:

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.
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.
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.

Important

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. …

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The frequency of an allele is calculated by summing the frequency of homozygous individuals for that allele and half the frequency of heterozygous individuals.

In population genetics, we often distinguish between genotype frequencies and allele frequencies. Genotype frequency refers to the proportion of individuals in a population that possess a specific genotype (e.g., BB, Bb, or bb). Allele frequency, on the other hand, refers to the proportion of a specific allele (e.g., B or b) within the gene pool of a population. Understanding how to derive allele frequencies from observed genotype frequencies is fundamental.

Consider a gene with two alleles, B and b. Each individual in the population carries two alleles for this gene.

An individual with genotype BB carries two B alleles.
An individual with genotype Bb carries one B allele and one b allele.
An individual with genotype bb carries two b alleles.

To find the frequency of an allele, say B, we need to count all the B alleles present in the population and divide by the total number of alleles for that gene in the population. Since each individual has two alleles, the total number of alleles is twice the number of individuals.

Let's denote the frequency of allele B as $p$ and the frequency of allele b as $q$.

Given the genotype frequencies:

Frequency of BB ($f_{BB}$) = $22% = 0.22$
Frequency of Bb ($f_{Bb}$) = $62% = 0.62$
Frequency of bb ($f_{bb}$) = $16% = 0.16$

To calculate the frequency of allele B ($p$):

Every individual with the BB genotype contributes two B alleles to the gene pool.

Every individual with the Bb genotype contributes one B allele to the gene pool.

Therefore, the total proportion of B alleles in the population can be found by taking the full frequency of BB individuals and adding half the frequency of Bb individuals (since only one of the two alleles in Bb is B).

p = f_{BB} + \frac{1}{2} f_{Bb}

Substituting the given values:

$p = 0.22 + \frac{1}{2} (0.62)$

$p = 0.22 + 0.31$

$p = 0.53$ …

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Q.In a certain population, the frequency of three genotypes is as follows: Genotypes: BB Bb bb frequency: 22% 62% 16% What is the likely frequency of B and b alleles?

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Q.In a certain population, the frequency of three genotypes is as follows:
Genotypes: BB Bb bb
frequency: 22% 62% 16%
What is the likely frequency of B and b alleles?

Unlock everything free for 14 days