ScienceCBSE Class 10 · Heredity

Evolution Class 10: Notes for School Assessment

Updated 14 October 2026 · Fact-checked

Assessed in school only, not in the board exam.

Evolution explains how the frequency of inherited traits in a population changes over generations through natural selection and genetic drift, why acquired traits are not passed on, how new species arise, and how homologous organs, fossils and DNA reveal relationships. It is taught from the CBSE Reading Material and assessed in school, not in the board exam.

Understand Evolution

This topic is assessed in school only, not in the board exam. It comes from the CBSE Reading Material for 2026-27 and builds on the Heredity chapter.

Reproduction always produces some variation, from errors in copying DNA and from sexual reproduction. The Reading Material follows a population of red beetles living on green bushes and eaten by crows. If a green beetle appears and passes its colour on, crows cannot spot its offspring, so green beetles become more and more common. The variation gave a survival advantage, so it was naturally selected. If instead a blue beetle appears and an elephant happens to trample the bushes, the few survivors may by chance be mostly blue. Here the change gave no advantage; it was an accident in a small population, called genetic drift. In both cases the frequency of an inherited trait, and so of certain genes, changed over generations. That is evolution.

Not every change counts. A beetle that is light because it went hungry does not change the DNA of its germ cells, so its young are not light. Cutting the tails of mice for many generations never produces tailless mice. Acquired traits are not inherited, so they cannot direct evolution.

When a large population is split into sub-populations, for example by a new river, gene flow between them falls. Drift and natural selection then make the groups more and more different, until they can no longer interbreed. Two species have formed: this is speciation.

Relationships between species are traced in several ways. Organs with the same basic plan but different uses, such as the forelimbs of mammals, birds, reptiles and amphibians, are homologous and point to a common ancestor. Organs that only look alike because they do the same job, like the wings of birds and bats, are analogous. Fossils show extinct species, and comparing DNA (molecular phylogeny) measures how far species have drifted apart.

Evolution is not 'progress'. Humans did not evolve from chimpanzees; both share an ancient common ancestor. All humans are one species whose earliest members can be traced to Africa.

Evolution: formulas and key facts

Evolution: $change in the frequency of inherited traits (genes) in a population over generations$; The core definition used throughout the topic.
Natural selection vs genetic drift: $selection: trait gives an advantage; drift: chance change in a small population$; Green beetles hidden from crows = selection; blue beetles surviving an elephant by luck = drift.
Acquired trait: $change in body (non-reproductive) tissue, not in germ-cell DNA \Rightarrow not inherited$; Starved, light beetles; mice with cut tails.
Speciation: $isolation + drift + selection \Rightarrow groups that cannot interbreed$; Gene flow falls when sub-populations are separated, for example by a river.
Homologous organs: $same basic structure, different functions$; Forelimbs of mammals, birds, reptiles and amphibians: evidence of common ancestry.
Analogous organs: $different structure and origin, similar function$; Wings of bats (skin between fingers) and birds (feathers along the arm).
Dating fossils: $relative: deeper layer = older; absolute: isotope ratios$; Fossils near the surface are more recent than those in deeper layers.

Evolution in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Acquired and inherited traits

Suppose a gym trainer in your colony has built very large arm muscles through years of exercise. Someone claims that his children will therefore be born with big muscles.

Is the claim correct? Use the idea of acquired and inherited traits to explain, and give one example from the Reading Material that supports your answer.

Try it first, then show the solutionSolution

Muscle built by exercise is a change in body tissue during the trainer's lifetime: an acquired trait.
Such a change does not alter the DNA of the germ cells, which are made in the reproductive tissue.
Only changes in germ-cell DNA can be passed to the next generation.
Example: removing the tails of mice in every generation never produces tailless young.

Answer: No. The trainer's muscles are an acquired trait: exercise changed his body tissue, not the DNA of his germ cells. Only what is in germ-cell DNA can be inherited, so his children will not be born with big muscles because of his training. In the same way, mice whose tails are cut off in every generation still have young with tails.

Memory hook: Gym muscles stay in the gym: what you acquire, you cannot pass on.

Artificial selection from wild cabbage

Imagine you visit a vegetable market and see cabbage, cauliflower, broccoli, kohlrabi and kale on one stall. The vegetable seller jokes that they are all 'one family'.

Explain how all these vegetables came from one wild plant, and why this example helps us accept that very different structures can come from a common ancestor.

Try it first, then show the solutionSolution

Over more than two thousand years, farmers selected wild cabbage plants for different features.
Short gaps between leaves gave cabbage; arrested flower growth gave broccoli; sterile flowers gave cauliflower; swollen parts gave kohlrabi; larger leaves gave kale.
This is artificial selection: humans, not nature, chose which plants bred.
It shows, in a case we have watched, that very different-looking forms can descend from one ancestor.

Answer: Farmers bred wild cabbage for different traits over more than two thousand years: close leaves gave cabbage, arrested flowers gave broccoli, sterile flowers gave cauliflower, swollen parts gave kohlrabi and bigger leaves gave kale. This is artificial selection. Because we know these very different vegetables share one ancestor, it is easier to accept that natural selection can produce very different species from a common ancestor.

Memory hook: One wild cabbage, five vegetables: selection makes variety.

Tricks to remember Evolution

Short memory aids. Say each one aloud once, then use it on a question the same day.

Green = Selected, Blue = Drift
Green beetle = natural Selection (an advantage); blue beetle = Drift (pure chance after the elephant).
Use it when: Telling natural selection from genetic drift.
Homo = same plan, Ana = same job
Homologous: same basic structure, different use. Analogous: different structure, same use.
Use it when: Classifying organ pairs.
Deeper is older
Fossils lower down in the rock layers are older than those near the surface.
Use it when: Relative dating of fossils.

Exam tips

Practise with the beetle illustration: be ready to say whether a change is selection, drift or not evolution at all.
When you classify organs, always name the basis: structure (homologous) or function (analogous).
For speciation, write the chain in order: isolation, reduced gene flow, drift and selection, inability to interbreed.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Name two sources of variation during reproduction.Show answer
Errors in DNA copying and sexual reproduction.
2.In the beetle story, who exerts natural selection on beetle colour?Show answer
The crows.
3.Why is low weight caused by starvation not inherited?Show answer
It does not change the DNA of the germ cells.
4.What reduces gene flow between two sub-populations and can lead to new species?Show answer
Geographical isolation, for example a large river forming between them.
5.Are the wings of a bird and a bat homologous or analogous?Show answer
Analogous: similar use (flight) but different structure and origin.
6.Where can the earliest members of Homo sapiens be traced?Show answer
Africa.

Evolution: frequently asked questions

Is Evolution in the Class 10 board exam?

No. For 2026-27, Evolution is taught from the CBSE Reading Material and assessed in school only, through internal or formative assessment. It does not appear in the board theory paper.

Did humans evolve from chimpanzees?

No. Humans and chimpanzees share a common ancestor that lived a long time ago and was probably neither. The two species then evolved separately into their present forms.

Is a newer species always better than an older one?

No. Evolution produces diversity shaped by the environment, not progress. Simple bacteria still thrive in hot springs, deep-sea vents and Antarctic ice, so older designs are not inefficient.

Why is a small population of tigers a worry for genetics?

In a small population, accidents can change gene frequencies by chance (genetic drift), and with few individuals there is less variation to draw on if the environment changes.