Acquired and inherited traits
An inherited trait is controlled by genes in the germ cells (egg and sperm), so it passes to the next generation: eye colour, blood group, the red colour of our beetles.
An acquired trait develops during an organism's own life because of its surroundings or habits: a thin beetle after starvation, a wrestler's muscles, a scar, a language you learn. These changes happen in body (non-reproductive) cells and do not change the DNA of germ cells, so they cannot be passed on and cannot lead to evolution.
| Inherited | Acquired |
|---|---|
| In the DNA of germ cells | In body cells only |
| Passed to offspring | Not passed on |
| Can cause evolution | Cannot cause evolution |
Natural selection and genetic drift
Variation arises during reproduction. When the environment favours some variations (red beetles hidden from crows), those individuals survive and reproduce more, and the variation spreads. This is natural selection. In a small population, a variation can also spread or vanish purely by accident (for example if an elephant tramples many beetles), which is genetic drift. Both change the gene mix of a population over time.
Speciation: how new species form
A species is a group whose members can breed with each other and produce fertile young. Speciation is the formation of a new species.
It usually starts when a population is split, for example by a river, mountain or sea (geographical isolation). With no gene flow between the groups, natural selection and genetic drift change each group differently. Over many generations the DNA or chromosome changes become so large that the groups can no longer interbreed even if they meet again. They are now separate species.
Tracing evolutionary relationships
Homologous organs
Organs with the same basic structure and origin but different functions: forelimbs of humans (grasp), bats (fly), whales (swim) and frogs (hop) all have an upper-arm bone, two forearm bones, wrist bones and digits. They point to a common ancestor.
Analogous organs
Organs with the same function but different structure and origin: a bird's wing (a modified forelimb with feathers) and an insect's wing (a thin sheet of body wall). They show that different groups can adapt to the same job, not that they are close relatives.
Fossils
Fossils are preserved remains or impressions of organisms that lived long ago, for example a leaf print in hardened mud or a shell turned to stone. Deeper rock layers are older, so fossils found deeper are older. Their age can also be found by radioactive dating (carbon dating). Fossils such as Archaeopteryx, with both reptile features (teeth, bony tail) and bird features (feathers), show links between groups.
Evolution by stages
Complex organs did not appear in one jump. They built up step by step, and each step had to be useful.
- Eye: it began as a light-sensitive patch (as in flatworms), then a cup, then a lens. Each stage helped the animal.
- Feathers: they first helped keep dinosaurs warm, and only later were used for flight. A feature that evolved for one job can later serve another.
- Artificial selection: farmers turned wild cabbage into broccoli, cauliflower, cabbage and kale in a few thousand years.
- Molecular evidence: comparing DNA of different species shows how closely they are related.
Human evolution
Fossils and DNA studies show that all living humans belong to one species, Homo sapiens. Our earliest ancestors lived in Africa. Over tens of thousands of years small groups moved out and spread across the world. Differences in skin colour or height are small variations and do not make separate 'races' in a biological sense. Evolution is not a ladder with humans at the top: every living species is the tip of its own branch of the tree of life.
Note: this topic is formative only in CBSE 2026-27, so it is assessed in class activities rather than the board paper.
Key formulas and definitions
- Inherited trait: in germ-cell DNA → passed on; acquired trait: body cells only → not passed on
- Natural selection: favourable variations survive and spread
- Genetic drift: random change in small populations
- Speciation: isolation → no gene flow → changes pile up → cannot interbreed
- Homologous organs: same origin, different function (common ancestor)
- Analogous organs: different origin, same function
- Fossil age: depth in rock layers and radioactive (carbon) dating
Worked examples
1. A person learns to play the tabla very well. Will their child be born knowing the tabla?
No. The skill is an acquired trait. It changes the brain through practice, not the DNA of egg or sperm, so it cannot be inherited.
2. Are the wing of a bat and the wing of a butterfly homologous or analogous?
Analogous. Both are used for flying, but a bat wing is a forelimb with bones and skin while a butterfly wing is a thin membrane. Different origin, same function.
3. Are the forelimb of a horse and the wing of a bird homologous or analogous?
Homologous. Both are forelimbs built from the same set of bones (upper arm, forearm, wrist, digits), though one runs and the other flies.
4. A population of snails is split by a new road across a forest. Explain how two species might form.
The two groups can no longer meet, so there is no gene flow. Different selection pressures and random drift change each group. After many generations the changes are so big that they could not interbreed even if brought together, so two species have formed.
5. How do we know that a fossil found deep in the ground is older than one near the surface?
Rock layers form one on top of another over time, so deeper layers were laid down earlier. The age can be checked by radioactive dating of the fossil or rock.
Common mistakes
- Saying acquired traits (like a cut tail or strong muscles) are passed to children. Only DNA changes in germ cells are inherited.
- Mixing up homologous and analogous: homologous = same structure, different job; analogous = same job, different structure.
- Thinking evolution means 'progress' or that humans evolved from today's chimpanzees. We share a common ancestor; both are branches.
- Believing a species changes because it 'needs' to. Variation comes first by chance; selection only keeps the useful ones.