Class 12th CBSE | Lamarckism and Darwinism

What this covers at a glance 📚

• Lamarckism: key ideas, the use–disuse principle, and the claim that acquired traits are inherited.
• Darwinism: what Darwin observed, what he inferred, and how natural selection works in practice.
• Evidence: Weismann’s germplasm concept, industrial melanism, and classic case studies you are expected to know.
• Who’s who and their landmark works: Lamarck, Darwin, and Weismann.
• Core contrasts: mechanism of change, the origin of variation, and how traits move across generations.
• Exam tools: concise definitions, crisp comparisons, and application-focused answers.
• The working chain: variation → selection → adaptation → inheritance.

Core concepts — quick clarity 🧠

Evolution proceeds only when traits can be passed on. In other words, variation must be heritable; otherwise, selection has nothing to act upon.

Variation arises through recombination and mutation, which the modern synthesis later integrated into Darwin’s framework. Heritability links the visible phenotype to the underlying genotype, and selection shifts trait frequencies across generations.

💡 Exam tip: Define “heritable variation” before explaining natural selection for a clean setup.

Lamarckism: use–disuse and acquired traits 🧬

Lamarck proposed that organisms adapt because they need to, with frequently used parts becoming stronger or larger while unused parts regress. Traits acquired during life were, in his view, transmitted to offspring, which he used to explain long-term change such as the giraffe’s neck.

This framework emphasizes direct environmental effects on body structures.

💡 Exam tip: State the four pillars: new needs, use–disuse, inheritance of acquired traits, and a trend toward increasing complexity.

Darwinism: natural selection and fitness 🐦

Darwin observed that organisms produce more offspring than the environment can support. With limited resources, a struggle for existence follows. Individuals vary, and if a heritable difference confers even a small advantage, its carriers leave more offspring.

Over time, populations shift as advantageous variants accumulate. “Survival of the fittest” is best read as survival and reproduction of those best suited to the current environment.

💡 Exam tip: Lay out the observation → inference chain to demonstrate the logic clearly.

Weismann’s germplasm: limits of acquired inheritance 🔬

Weismann distinguished germ cells from somatic cells, arguing that heredity passes through the germline and that alterations to the body do not modify offspring. His tail-cutting mouse experiments repeatedly yielded normal-tailed pups, reinforcing the point.

This supports a strict separation—often summarized as the germline–somatic barrier—which counters Lamarck’s inheritance of acquired characters.

💡 Exam tip: Include the phrase “germline–somatic barrier” once to signal precision.

Evidence for selection: industrial melanism 🏭🦋

During heavy pollution, soot-darkened trees favored darker peppered moths through improved camouflage and higher survival. As air quality recovered and bark lightened, lighter morphs regained their advantage.

This is selection in motion—context-dependent and reversible—linking environment, variation, and differential survival.

💡 Exam tip: Explicitly mention “differential survival of morphs” when applying this example.

Functional link 🔗

Evolution gets moving only when a population has heritable variation; without traits that pass from parents to offspring, selection has nothing to shape. Natural selection then works like a sieve, favoring variants that boost survival or reproduction, so adaptive traits grow more common over generations.

Lamarck put the environment front and center, suggesting that use and disuse could build traits that offspring would inherit. Weismann drew a firm line instead: heredity runs through germ cells, not body cells, which neatly supports Darwin’s selection-driven mechanism rather than the inheritance of acquired characters.

Important features to remember (Quick facts) ⚡

• You should state Lamarck’s propositions in order: new needs → use–disuse → acquired traits → inheritance.
• Darwin’s logic runs as follows: overproduction plus limited resources leads to struggle, then selection, then adaptation.
• Remember that acquired characters are somatic, and germline inheritance excludes them.
• Industrial melanism exemplifies environment-driven selection acting on pre-existing variation.
• The phrase “survival of the fittest” means higher reproductive success, not mere survival.
• Define fitness as relative, environment-specific reproductive output.
• Use the giraffe’s neck as a quick illustration of Lamarck’s explanatory style.
• Name key scientists with correct spellings and works, for example, On the Origin of Species.
• Contrast mechanisms and inheritance pathways clearly in comparison answers.
• Keep definitions to one line, and write mechanisms in three to four lines for exam efficiency.

How the 15 quiz questions map to the topic 🎯

• The question “Who proposed the theory of inheritance of acquired characters?” targets Lamarckism fundamentals and authorship.
• The question “Which concept explains the giraffe’s long neck in Lamarckism?” highlights the use–disuse principle.
• The statement “According to Darwin, favourable heritable variations are preserved by natural selection” tests the core selection mechanism.
• The question “Which classic study challenged inheritance of acquired characters?” points to Weismann’s evidence against Lamarckism.
• The prompt “‘Survival of the fittest’ is central to which theory?” checks keyword accuracy for Darwinism.
• The question “Which is NOT a postulate of Lamarckism?” asks you to distinguish Lamarckian and Darwinian mechanisms.
• The statement “Neo-Lamarckism retains the idea that acquired characters can be inherited” probes concept refinement and terminology.
• The question “Which is NOT among Darwin’s key observations leading to natural selection?” checks the observation–inference chain.
• The phrase “Differential reproductive success of variants describes which mechanism?” defines natural selection.
• The statement “Darwin explained evolution mainly via mutations as the primary source of variation” tests the historical limits of Darwin’s theory.
• The question “Which book by Darwin outlined natural selection as the mechanism of evolution?” asks for source work identification.
• The question “Who proposed the germplasm theory opposing inheritance of acquired traits?” contrasts germline and soma in heredity.
• The prompt “Which statement best distinguishes Lamarckism from Darwinism?” demands a high-yield comparison statement.
• The example “Industrial melanism in peppered moths supports natural selection” applies evidence of selection.
• The question “In Darwin’s theory, the struggle for existence mainly arises due to which factor?” emphasizes resource limitation as the driver.

Revision checklist & exam tips ✅

• Write Lamarck’s four propositions in order and add one example such as the giraffe’s neck.
• List Darwin’s observations and inferences in order to show the logic clearly.
• Define natural selection and fitness precisely in one line each.
• Explain Weismann’s mouse experiment and state the conclusion plainly.
• Use industrial melanism as a clean application example of selection.
• Contrast Lamarckism and Darwinism with at least three crisp points.
• Memorize key scientists and publication titles for accuracy.
• Practice three to five mark comparison answers using tabular contrasts.
• Avoid equating “fittest” with “strongest,” and define it in terms of reproductive success.
• Draw quick flowcharts for Darwin’s logic to structure longer answers.
• Anchor comparison answers with one textbook example in each case.
• Underline key terms such as selection, variation, and germplasm for visibility.
• Manage time by writing definitions first, examples second, and evaluation last.
• Review common pitfalls, especially mixing up somatic and germline inheritance.

Key terms (glossary) 🗂️

• Lamarckism is the theory proposing use–disuse and the inheritance of acquired characters, contrasted with selection-based change.
• Use–disuse states that frequent use strengthens organs and disuse weakens them, as proposed by Lamarck.
• An acquired character is a somatic trait developed during life due to environment or behavior.
• Darwinism describes evolutionary change via natural selection acting on heritable variation.
• Natural selection is differential survival and reproduction that alters trait frequencies over time.
• The struggle for existence is competition that arises from limited resources and overproduction.
• Fitness is relative, environment-specific reproductive success.
• The germplasm theory holds that heredity passes through germ cells and that somatic changes are not inherited.
• Industrial melanism is a selection-driven shift in moth coloration caused by pollution.
• Variation consists of heritable differences among individuals that enable selection.
• Adaptation is a trait that increases fitness under specific environmental conditions.
• Neo-Lamarckism is a set of reinterpretations that retain elements of acquired-character inheritance.
• Orthogenesis is the historical idea of directed evolution independent of selection.
• Mutation is a source of new genetic variation incorporated into the modern synthesis.
• Inheritance is the transmission of genetic information from parents to offspring.
• Selection pressure is any environmental factor that influences survival and reproduction.