Evolution
Class 12 Biology · Biology · Chapter 6
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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.
Extra questions & answers
Try one from this chapter
The Miller-Urey experiment demonstrated the abiotic formation of: