CDS Biology · Genetics and Evolution
Evolution
Why a skill learnt in life is not inherited, what Lamarck, Darwin and Hardy-Weinberg said, how variation and natural selection drive evolution, and how to tell homologous from analogous organs.
Why this matters
Four CDS questions from 2020 (II) to 2026 (II), two of them in the 2026 papers. Each one tests a single idea: acquired traits are not inherited, variations do not all survive equally, Hardy-Weinberg means stable allele frequencies, and bird and bat wings are analogous.
Concept 1 of 3: Inherited and acquired traits, and the theories
A trait is passed on only if it is written in the DNA of the egg or sperm. Curly hair and freckles are written there. A skill you learn, a muscle you build or a scar you get changes your body, but not the DNA of your egg or sperm cells, so your children do not get it. Lamarck thought such acquired traits were passed on; Darwin explained evolution without that idea.
Definition
Theories of evolution (NCERT Class 10 and 12):
- Inherited traits are coded in the DNA of the germ cells: eye colour, curly hair, attached or free earlobes, freckles.
- Acquired traits are gained during life (a learnt skill, a strong muscle, a cut tail) and are not inherited, because they do not change the DNA of the germ cells.
- Lamarck: use and disuse of organs, and inheritance of acquired characters (the giraffe stretched its neck). This idea was rejected.
- Darwin (Origin of Species, 1859): branching descent and natural selection are the two key ideas.
- Hugo de Vries: the mutation theory, from work on the evening primrose.
- Hardy-Weinberg principle: allele frequencies in a population stay stable from one generation to the next. Five factors upset this balance: gene flow (migration), genetic drift, mutation, genetic recombination and natural selection.
| Person or principle | Key idea |
|---|---|
| Lamarck | Inheritance of acquired characters (rejected) |
| Darwin | Branching descent and natural selection |
| Hugo de Vries | Evolution by mutation |
| Hardy-Weinberg | Allele frequencies stay stable unless disturbedQ CDS 2026 (II): the wrong statement says Hardy-Weinberg makes allele frequencies unstable. |
| Weismann | Cut the tails of mice for generations; the young still had tails |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2026 · CDS (I) 2026 — General Knowledge · Q6Easy
Example 1 · Genetics and Evolution · Evolution
A world chess champion has curly hair, attached earlobes and freckles. Which one of the following traits is not likely to be passed on to his/her offsprings ?
Learnt skills are not in the DNA
A skill such as chess or swimming is acquired, so children do not inherit it. Hair type, earlobes and freckles are inherited because genes decide them.
Hardy-Weinberg means stable
The Hardy-Weinberg principle says allele frequencies stay constant in a population. Mutation, gene flow, drift, recombination and selection are what disturb that balance.
Concept 2 of 3: Variation and natural selection
No two members of a species are exactly alike: these differences are variations. They arise when DNA is copied with small changes, and when sexual reproduction mixes the genes of two parents. When the surroundings change, some variants happen to cope better, survive and have more young. Over time their kind becomes common. That is natural selection.
Definition
Variation and selection (NCERT Class 10 and 12):
- Variation comes from changes in the genetic material (mutation), from sexual reproduction (genes of two parents combined) and from crossing over (recombination).
- Asexual reproduction gives very little variation, only from small errors in copying DNA.
- Natural selection: the environment selects the variants best suited to it; they survive and reproduce more. So different variations have different chances of survival.
- Industrial melanism: in England, after soot darkened tree trunks, dark peppered moths became common because birds could not spot them.
- Antibiotic resistance: a few bacteria that can survive a drug multiply until most of the population resists it.
- Genetic drift is a chance change in gene frequencies, strongest in small populations; gene flow is the movement of genes between populations.
| Source or process | What happens |
|---|---|
| Mutation | A change in the DNA creates a new variant |
| Sexual reproduction | Genes of two parents mix in new combinations |
| Asexual reproduction | Very little variation |
| Natural selection | Variants best suited to the environment survive and reproduce moreQ CDS 2021 (II): the false statement is that all variations in a species have equal chances of survival. |
| Genetic drift | Chance change in gene frequency in a small population |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2021 · CDS (II) 2021 — General Knowledge · Q15Moderate
Example 2 · Genetics and Evolution · Evolution
Which one of the following statements about variations is not true ?
Variations are not equal in survival
Some variants suit the environment better than others, so they have better chances of survival. That inequality is exactly what natural selection works on.
Concept 3 of 3: Homologous and analogous organs
Two organs can look alike for two very different reasons. Some share a common ancestor and so the same basic plan of bones, even if they now do different jobs: these are homologous. Others come from different ancestors and are built differently, but do the same job because they faced the same problem, like flying: these are analogous.
Definition
Evidence from organs (NCERT Class 10 and 12):
- Homologous organs: same basic structure and origin, different functions. The forelimbs of a human, whale, bat and cheetah; the thorn of Bougainvillea and the tendril of Cucurbita (both modified stems). They show divergent evolution.
- Analogous organs: different origin and structure, same function. The wings of birds and bats (NCERT Class 10), the wings of a butterfly and a bird, the eyes of an octopus and a mammal, the potato (stem) and sweet potato (root). They show convergent evolution.
- A bird's wing is a forelimb covered in feathers; a bat's wing is skin stretched between long finger bones. Same job, different design.
- Vestigial organs are reduced and no longer used, such as the human appendix.
| Kind | Origin and function | Examples |
|---|---|---|
| Homologous | Same origin, different function | Forelimbs of human, whale and bat; thorn of Bougainvillea and tendril of Cucurbita |
| Analogous | Different origin, same function | Wings of bird and bat; wings of butterfly and bird; potato and sweet potatoQ CDS 2020 (II): bird and bat wings are analogous because they have a different origin and a common function. |
| Vestigial | Reduced, no longer used | Human appendix, wisdom teeth |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2020 · CDS (II) 2020 — General Knowledge · Q94Moderate
Example 3 · Genetics and Evolution · Evolution
Wings of birds and bats are considered analogous structures because they have
Analogous: different origin, same job
Analogous organs have a different origin and a common function. Homologous organs have a common origin and may do different jobs. Swapping the two halves is the trap in every option set.
Homologous shows divergence, analogous convergence
Homologous organs show divergent evolution (one plan, many uses). Analogous organs show convergent evolution (many plans, one use).
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (3)
Inherited and acquired traits, and the theories5 rows
| Person or principle | Key idea |
|---|---|
| Lamarck | Inheritance of acquired characters (rejected) |
| Darwin | Branching descent and natural selection |
| Hugo de Vries | Evolution by mutation |
| Hardy-Weinberg | Allele frequencies stay stable unless disturbedQ CDS 2026 (II): the wrong statement says Hardy-Weinberg makes allele frequencies unstable. |
| Weismann | Cut the tails of mice for generations; the young still had tails |
Variation and natural selection5 rows
| Source or process | What happens |
|---|---|
| Mutation | A change in the DNA creates a new variant |
| Sexual reproduction | Genes of two parents mix in new combinations |
| Asexual reproduction | Very little variation |
| Natural selection | Variants best suited to the environment survive and reproduce moreQ CDS 2021 (II): the false statement is that all variations in a species have equal chances of survival. |
| Genetic drift | Chance change in gene frequency in a small population |
Homologous and analogous organs3 rows
| Kind | Origin and function | Examples |
|---|---|---|
| Homologous | Same origin, different function | Forelimbs of human, whale and bat; thorn of Bougainvillea and tendril of Cucurbita |
| Analogous | Different origin, same function | Wings of bird and bat; wings of butterfly and bird; potato and sweet potatoQ CDS 2020 (II): bird and bat wings are analogous because they have a different origin and a common function. |
| Vestigial | Reduced, no longer used | Human appendix, wisdom teeth |
Watch out for (5)
- Learnt skills are not in the DNA→ Inherited and acquired traits, and the theories
- Hardy-Weinberg means stable→ Inherited and acquired traits, and the theories
- Variations are not equal in survival→ Variation and natural selection
- Analogous: different origin, same job→ Homologous and analogous organs
- Homologous shows divergence, analogous convergence→ Homologous and analogous organs
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