Human Evolution
Introduction
Evolution is the gradual change in living organisms over generations. In ICSE Class 10 Biology, you study the theories of evolution proposed by Lamarck and Darwin, and the evidence supporting evolution from comparative anatomy and palaeontology.
Lamarck's Theory (Theory of Inheritance of Acquired Characters)
Main Propositions
- Use and disuse of organs: Organs that are used frequently become well-developed; organs that are not used gradually degenerate.
- Inheritance of acquired characters: Changes acquired during an organism's lifetime are passed on to offspring.
Example: Giraffe's Long Neck
According to Lamarck:
- Ancestral giraffes had short necks.
- They stretched their necks to reach leaves on tall trees.
- This 'acquired' elongation was passed to offspring.
- Over generations, necks became progressively longer.
Criticism
- Acquired characters (e.g., loss of a finger, muscular development) are NOT inherited.
- Weismann's experiment: Cutting off mice tails for 22 generations did not produce tailless offspring.
Darwin's Theory (Natural Selection)
Main Propositions
- Overproduction: Organisms produce more offspring than can survive.
- Variation: Individuals in a population show variations.
- Struggle for existence: Competition for food, space, and mates.
- Survival of the fittest: Individuals with favourable variations have a better chance of survival.
- Natural selection: Nature 'selects' the best-adapted individuals to survive and reproduce.
- Inheritance of variations: Favourable variations are passed to the next generation.
Over time, natural selection leads to the evolution of new species.
Example: Industrial Melanism (Peppered Moth)
- Before the Industrial Revolution: Light-coloured moths were better camouflaged on lichen-covered trees (survived more).
- After industrialisation: Dark-coloured moths were better camouflaged on soot-darkened trees (survived more).
- This shift in population frequency is natural selection in action.
Evidence for Evolution
1. Fossil Evidence
Fossils are preserved remains of organisms from the past.
- Fossil record shows a progression from simple to complex forms.
- Transitional fossils (e.g., Archaeopteryx — reptile-bird link) show intermediate features.
- Deeper rock layers contain older, simpler fossils; upper layers contain more recent, complex fossils.
2. Homologous Organs
Organs with the same basic structure but different functions.
Example: Forelimbs of humans, cats, whales, bats, and birds.
- All have the same pattern of bones (humerus, radius, ulna, carpals, etc.).
- Used for different functions (grasping, walking, swimming, flying).
- This suggests a common ancestor.
3. Analogous Organs
Organs with different basic structures but similar functions.
Example: Wings of birds and wings of insects.
- Bird wings are modified forelimbs; insect wings are chitinous extensions.
- Different structure → different evolutionary origin.
- Similar function → convergent evolution (adaptation to similar environment).
Comparison: Homologous vs Analogous Organs
| Feature | Homologous organs | Analogous organs |
|---|---|---|
| Structure | Same basic structure | Different structure |
| Function | May be different | Similar function |
| Evolutionary origin | Common ancestor | Different ancestors |
| Evolution type | Divergent evolution | Convergent evolution |
| Example | Forelimbs of mammals | Wings of birds and insects |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| Thinking Lamarck's theory is completely wrong | It was the first comprehensive theory — the mechanism (inheritance of acquired characters) was incorrect |
| Confusing 'survival of the fittest' with physical strength | Fitness = reproductive success and adaptation to environment |
| Mixing homologous and analogous organs | Homologous = same structure, different function; Analogous = different structure, same function |
| Calling Archaeopteryx a 'missing link' | It is a transitional fossil, not a 'missing link' |
ICSE Exam Focus
This chapter carries 4–6 marks. Key topics: Lamarckism, Darwin's natural selection, homologous vs analogous organs, fossil evidence.
Marks Blueprint: Lamarck's theory — 2 marks, Darwin's theory — 2 marks, Evidences — 2 marks.
Self-Test Questions
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State the main postulates of Lamarck's theory of evolution.
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State Darwin's theory of natural selection. Explain with an example.
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Differentiate between homologous and analogous organs with examples.
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How do fossils provide evidence for evolution?
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What is industrial melanism? How does it illustrate natural selection?
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Why has Lamarck's theory been criticised?
