Evolution — Class 12 Biology

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.