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medium difficulty9 min

Theories of Evolution: Lamarckism, Darwinism and Modern Synthetic Theory

Theories of evolution explain how populations of organisms change over generations, mainly through inherited variation, struggle for existence, natural selection, and changes in gene frequencies.

In one line: Darwin selects; modern theory explains the genes behind selection.

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Understand it

  • Lamarck proposed that organs used more become stronger and unused organs degenerate, and that acquired characters are inherited.
  • This idea is historically important but not accepted as the main mechanism because acquired body changes are generally not inherited genetically.
  • Darwin explained evolution through natural selection: individuals show variations, many more are born than can survive, and those with favourable heritable variations leave more offspring.
  • Neo-Darwinism and the modern synthetic theory combine Darwin’s selection with Mendelian genetics, mutation, recombination, gene flow, genetic drift, and reproductive isolation.
  • Exam answers must show that evolution acts on populations across generations, not on a single individual by need.

Example

Darwin’s finches show natural selection because beak variations helped different populations use different food sources, and favourable beak types became more common over generations.

Rule to remember

Key rule: natural selection changes the frequency of favourable heritable traits in a population.

Modern synthetic theory adds genetic sources of variation and population-level changes in allele frequencies.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Question: Explain why antibiotic resistance can be used as an example of natural selection.

  1. 1Answer: In a bacterial population, some bacteria may already possess resistance due to genetic variation.
  2. 2Antibiotic exposure kills susceptible bacteria, resistant bacteria survive and reproduce, so resistance becomes more common.

Method you can reuse

  1. 1For theory questions, start with the theory name, state its central idea, mention the role of variation, explain selection or inheritance, and give one biological example.
  2. 2For modern theory, include mutation, recombination, gene flow, drift, and isolation.

Visual tip

A comparison chart can show Lamarckism, Darwinism, and modern synthetic theory. Labels should include acquired characters, heritable variation, natural selection, mutation, recombination, gene flow, and genetic drift.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why is Darwin’s natural selection considered different from Lamarck’s inheritance of acquired characters?

Think first, then show answer
Darwin’s natural selection depends on heritable variations already present in a population, while Lamarckism assumes that acquired changes during an individual’s lifetime are inherited. Modern biology supports selection acting on genetic variation across generations.

Common wrong answer

Evolution occurs because an organism tries to change according to need and then passes that acquired change to its offspring.

Don't mix this up

A common error is writing that giraffes stretched their necks and passed the stretched neck directly to offspring as the accepted explanation of evolution.

Exam tip

Use the phrase heritable variation whenever explaining Darwinism; selection works only when useful variation can pass to offspring.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Asked as comparison of Lamarckism and Darwinism, explanation of natural selection, assertion-reason on survival of the fittest, or long answer on modern synthetic theory.

Answer plan

Name the theory, define its core idea, explain variation and inheritance, describe selection or genetic mechanism, give a standard example, and end with the population-level result.
  1. 1

    Enough for 1 mark

    Theories of evolution explain how populations of organisms change over generations, mainly through inherited variation, struggle for existence, natural selection, and changes in gene frequencies.

  2. 2

    Add for 2 marks

    Example: Darwin’s finches show natural selection because beak variations helped different populations use different food sources, and favourable beak types became more common over generations.

  3. 3

    Add for 3 marks

    Lamarck proposed that organs used more become stronger and unused organs degenerate, and that acquired characters are inherited.

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