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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.

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Student-friendly explanation

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.

How to write this in exams

  1. 1

    Start with the exact idea

    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

    Then show how to use it

    For 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. For modern theory, include mutation, recombination, gene flow, drift, and isolation.

  3. 3

    Add one concrete 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.

  4. 4

    Avoid this incomplete answer

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

Definition

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.

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.

Memory hook

Darwin selects; modern theory explains the genes behind selection.

Examples and method

Worked example

Question: Explain why antibiotic resistance can be used as an example of natural selection. Answer: In a bacterial population, some bacteria may already possess resistance due to genetic variation. Antibiotic exposure kills susceptible bacteria, resistant bacteria survive and reproduce, so resistance becomes more common.

Method to apply

For 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. For modern theory, include mutation, recombination, gene flow, drift, and isolation.

Diagram support

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.

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.

Avoid common mistakes

Common confusion

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

Common wrong answer

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

Exam tip

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

Quick check

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

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.

Answer writing and exam use

1-mark answer

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.

2-mark answer

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.

3-mark answer

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.
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