What traumatic event does Zitkala-Sa describe at the school?

The forcible cutting of her long hair

Why was the cutting of her hair so humiliating to Zitkala-Sa?

Among her people, shingled hair was worn by cowards and mourners

What did the Miller-Urey experiment demonstrate?

It demonstrated that organic molecules such as amino acids could be formed abiotically from simple gases (methane, ammonia, hydrogen, water vapour) under conditions simulating early Earth.

Differentiate between homologous and analogous organs.

Homologous organs have a common origin but different functions (divergent evolution), whereas analogous organs have different origins but similar functions (convergent evolution).

What is adaptive radiation? Give an example.

It is the diversification of a single ancestral species into many forms adapted to different habitats; Darwin's finches of the Galapagos Islands are a classic example.

State the Hardy-Weinberg principle.

In a large, randomly mating population, allele and genotype frequencies remain constant from generation to generation in the absence of mutation, migration, genetic drift and selection.

Name the factors that affect Hardy-Weinberg equilibrium.

Gene mutation, gene flow (migration), genetic drift, genetic recombination and natural selection.

What is industrial melanism?

It is the example of natural selection in peppered moths, where dark-coloured moths increased in industrial areas due to soot-darkened surfaces favouring their survival.

Who disproved spontaneous generation and how?

Louis Pasteur disproved it through experiments with flasks showing that life arises only from pre-existing life.

What are vestigial organs? Give a human example.

They are reduced, non-functional structures that were functional in ancestors; examples in humans include the vermiform appendix and the tail bone (coccyx).

Arrange the human ancestors in evolutionary sequence.

Dryopithecus and Ramapithecus, then Australopithecus, Homo habilis, Homo erectus, Neanderthal man and finally Homo sapiens (modern humans).

Define genetic drift.

Genetic drift is the random change in allele frequencies that occurs by chance, especially in small populations, sometimes leading to the founder effect.

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Chapter 6: Evolution — Class 12 Biology

Biology · 13 chapters

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Chapter 6: Evolution

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The Miller-Urey experiment demonstrated the abiotic formation of:

Summary

This chapter traces the origin and evolution of life on Earth and the diversity of organisms. It begins with theories of the origin of the universe (Big Bang) and life, including the chemical evolution theory of Oparin and Haldane and its experimental support from the Miller-Urey experiment, contrasted with panspermia and the discarded theory of spontaneous generation, disproved by Louis Pasteur. Evidence for evolution is drawn from palaeontology (fossils), comparative anatomy (homologous and analogous organs, divergent and convergent evolution, vestigial organs), embryology and molecular biology. Adaptive radiation, illustrated by Darwin's finches and Australian marsupials, shows diversification from a common ancestor. The mechanisms of evolution include Lamarckism, Darwin's theory of natural selection (descent with modification, survival of the fittest), and the modern synthetic theory incorporating mutation, recombination, gene flow, genetic drift and natural selection. The Hardy-Weinberg principle describes allele frequency stability and the factors that disturb it. Types of natural selection (stabilising, directional, disruptive) are explained, with examples like industrial melanism in peppered moth and antibiotic and pesticide resistance. The chapter concludes with human evolution, tracing the lineage from Dryopithecus and Ramapithecus through Australopithecus, Homo habilis, Homo erectus and Neanderthals to modern Homo sapiens.

Origin of life and chemical evolutionEvidences of evolutionAdaptive radiationTheories of evolution: Lamarck, Darwin, modern synthesisHardy-Weinberg principle and mechanismsHuman evolution

Key terms

Natural selection

The differential survival and reproduction of individuals best adapted to their environment, proposed by Darwin and Wallace.

Homologous organs

Organs with similar structure and origin but different functions, indicating common ancestry (divergent evolution).

Analogous organs

Organs with different structure and origin but similar function, indicating convergent evolution.

Adaptive radiation

The evolution of different species from a common ancestor in a geographical area, as in Darwin's finches.

Genetic drift

Random changes in allele frequencies in a small population, an evolutionary force independent of natural selection.

Hardy-Weinberg principle

The principle stating that allele frequencies in a population remain constant across generations in the absence of evolutionary influences.

Important questions

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Natural selection

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The differential survival and reproduction of individuals best adapted to their environment, proposed by Darwin and Wallace.

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